What are carboxylic acids and how are they classified in organic chemistry?

Carboxylic acids are a class of organic compounds characterized by the presence of a carboxyl group (COOH). They are classified as organic acids and are known for their acidic properties due to the ability to donate a hydrogen ion (H+). In the Edexcel IGCSE Chemistry curriculum, carboxylic acids are studied as part of organic chemistry, focusing on their structure, properties, and reactions.

How do carboxylic acids react with alcohols in organic chemistry?

Carboxylic acids react with alcohols in a process known as esterification, forming esters and water. This reaction is catalyzed by an acid, typically sulfuric acid, and is an important topic in the Edexcel IGCSE Chemistry syllabus. For example, ethanoic acid reacts with ethanol to form ethyl ethanoate and water.

What are the common properties of carboxylic acids covered in the Edexcel IGCSE Chemistry syllabus?

Carboxylic acids have several common properties: they are typically weak acids, have higher boiling points than similar alcohols due to hydrogen bonding, and are soluble in water. These properties are essential for understanding their behavior in chemical reactions and are emphasized in the Edexcel IGCSE Chemistry curriculum.

Why are carboxylic acids considered weak acids in chemistry?

Carboxylic acids are considered weak acids because they only partially dissociate in water to release hydrogen ions (H+). This partial ionization results in a higher pH compared to strong acids. Understanding the concept of weak acids is crucial for students studying organic chemistry under the Edexcel IGCSE syllabus.

What is the significance of the carboxyl group in carboxylic acids?

The carboxyl group (COOH) is the functional group that defines carboxylic acids. It consists of a carbonyl group (C=O) and a hydroxyl group (OH) attached to the same carbon atom. This group is responsible for the acidic properties and reactivity of carboxylic acids, making it a key focus in the study of organic chemistry within the Edexcel IGCSE curriculum.

Why is "Confusing 'weak' acid with 'dilute' acid" a common mistake in Carboxylic Acids?

Why it happens: Both result in fewer H⁺ ions in solution — appears similar. How to avoid it: **Weak** = partially ionised (extent of dissociation — e.g. ethanoic acid). **Dilute** = low concentration (amount per volume — e.g. 0.1 mol/dm³ HCl). You can have a CONCENTRATED WEAK acid (glacial ethanoic, ~ 17 mol/dm³) and a DILUTE STRONG acid (0.01 mol/dm³ HCl). Memorise: 'strong/weak = extent of ionisation; concentrated/dilute = amount per volume'. Source: 4CH1 Examiner Reports 2022-2024

Why is "Using → instead of ⇌ for the ionisation of a weak acid" a common mistake in Carboxylic Acids?

Why it happens: Habit from balancing equations. How to avoid it: Weak acid ionisation is REVERSIBLE — must use ⇌. CH₃COOH ⇌ CH₃COO⁻ + H⁺. Strong acid ionisation is essentially complete → use →. HCl → H⁺ + Cl⁻. Source: 4CH1 Examiner Reports 2023-2024

Why is "Writing 'sodium acetate' or 'magnesium acetate' (the old common names)" a common mistake in Carboxylic Acids?

Why it happens: Old/common names appear in some textbooks. How to avoid it: Use the IUPAC name: -oate (from -oic acid). Acetate → ethanoate. Acetic acid → ethanoic acid. Formic acid → methanoic acid. Edexcel mark schemes use the IUPAC names. Source: 4CH1 Examiner Reports 2022-2024

Why is "Writing CH₃COOH + Mg → CH₃COOMg + H₂ (unbalanced)" a common mistake in Carboxylic Acids?

Why it happens: Forgetting that Mg is 2+ so needs TWO carboxylate anions. How to avoid it: Mg²⁺ + 2 carboxylate⁻ → (CH₃COO)₂Mg. So 1 Mg needs 2 CH₃COOH. The balanced equation is: 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂. Check both Mg and H balance. Source: 4CH1 Examiner Reports 2022-2024

Why is "Saying weak acids 'don't fully ionise because they are dilute'" a common mistake in Carboxylic Acids?

Why it happens: Trying to find a cause. How to avoid it: Weak acids don't fully ionise because the O–H bond in –COOH is strong relative to the energy released by hydration. It's a property of the MOLECULE, not the concentration. Even pure (100%) ethanoic acid (glacial) is still a weak acid — it just has higher concentration than a dilute solution.

Why is "Writing CO₂ as a product of acid + metal" a common mistake in Carboxylic Acids?

Why it happens: Confusion with acid + carbonate reaction. How to avoid it: Acid + reactive metal → SALT + HYDROGEN (H₂, not CO₂). Acid + metal carbonate → salt + water + CO₂. Acid + base → salt + water. Memorise the three patterns. Source: 4CH1 Examiner Reports 2023-2024

Organic ChemistryPearson Edexcel IGCSE Chemistry 4CH1 Higher

Carboxylic Acids

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Carboxylic Acids — Pearson Edexcel International GCSE Chemistry 4CH1 Study Notes (2026 onwards, Spec Issue 4)

Homologous series with –COOH (carboxyl) functional group. General formula CₙH₂ₙ₊₁COOH (n ≥ 0). First three: methanoic acid HCOOH (ant stings); ethanoic acid CH₃COOH (vinegar ~5%); propanoic acid C₂H₅COOH. Short-chain liquids, soluble in water, pungent smells. WEAK ACIDS — partially ionised: CH₃COOH ⇌ CH₃COO⁻ + H⁺ → pH ~3-4. React like other acids but slower / less vigorous than strong acids of same concentration: with reactive metals (→ salt + H₂); with carbonates (→ salt + H₂O + CO₂); with bases (→ salt + H₂O); with alcohols (→ ester + H₂O — Topic 4.7). Salts end in -OATE (ethanoate, methanoate). Vinegar = household descaler for limescale (CaCO₃).

At a glance

Carboxylic acid = organic compound with –COOH (carboxyl) group. General formula CₙH₂ₙ₊₁COOH (n = 0, 1, 2,...).
First three: methanoic HCOOH (ant stings); ethanoic CH₃COOH (vinegar ~5%); propanoic C₂H₅COOH (preservative).
Short-chain ones are colourless LIQUIDS, soluble in water, SHARP/PUNGENT smells.
WEAK ACIDS — partially ionised in water: RCOOH ⇌ RCOO⁻ + H⁺ (pH ~ 3-4).
Compare: strong acids (HCl, H₂SO₄, HNO₃) FULLY ionised → lower pH at same concentration.
Reactions (typical acid reactions, but slower than strong acids):
• + reactive METAL → salt + H₂ (fizzing, gentler than HCl).
• + CARBONATE → salt + H₂O + CO₂ (fizzing, limewater turns cloudy).
• + BASE → salt + H₂O (neutralisation).
• + ALCOHOL → ester + H₂O (esterification, conc. H₂SO₄, Topic 4.7).
Salt names end in -oate (sodium ethanoate, magnesium ethanoate).
Vinegar = dilute ethanoic acid; descales kettles (reacts with CaCO₃ limescale).

What you’ll learn

Mapped to the Pearson Edexcel IGCSE 4CH1 syllabus (2026 onwards).

4.32 — Recall the general formula CₙH₂ₙ₊₁COOH and state the functional group –COOH of carboxylic acids.
4.33 — Describe the reactions of carboxylic acids (methanoic, ethanoic, propanoic) with: (a) reactive metals (e.g. magnesium), (b) metal carbonates, (c) metal oxides / hydroxides (bases). Write balanced equations and name the products (carboxylate salts ending in -oate).
4.34 — Understand that carboxylic acids are weak acids — only partially ionise in water; compare with the strong acids (HCl, H₂SO₄) of the same molar concentration in terms of pH and reaction rate.

Carboxylic acid structure and the –COOH functional group (spec 4.32)

CₙH₂ₙ₊₁COOH. Methanoic, ethanoic, propanoic. Sharp / pungent smells. Soluble in water.

Carboxylic acids are a homologous series of organic compounds characterised by the CARBOXYL group –COOH at the end of an alkyl chain.

General formula: $C_n H_{2n+1} COOH$ (where n = 0, 1, 2,...).

For methanoic acid n = 0 (the alkyl group is just H — HCOOH). For ethanoic acid n = 1 (CH₃COOH). For propanoic acid n = 2 (C₂H₅COOH or CH₃CH₂COOH).

The carboxyl group: –COOH.

The –COOH group consists of TWO functional groups merged into one:

     O
     ||
  ─ C ─ O ─ H
     │
    (rest of molecule)

A C=O double bond (carbonyl).
An O–H group bonded to the same carbon (hydroxyl).

Together this combination makes the H atom of the –OH part ACIDIC — it can dissociate as H⁺. This is why carboxylic acids are acidic.

The −COOH group combines a C=O and an O−H; the O−H hydrogen is the acidic proton.

First three carboxylic acids.

n	Name	Structural formula	Common name	Source / use
0	Methanoic acid	HCOOH	Formic acid	Ant stings, nettle hairs
1	Ethanoic acid	CH₃COOH (or HC₂H₃O₂)	Acetic acid	VINEGAR (~ 5% aqueous solution); food preservative
2	Propanoic acid	CH₃CH₂COOH or C₂H₅COOH	Propionic acid	Mould inhibitor in bread
3	Butanoic acid	C₃H₇COOH	Butyric acid	Rancid butter, body odour

Naming pattern. Take the alkane name (methane, ethane, propane) → drop '-e' → add -oic acid suffix. The carboxyl C is counted as carbon 1. So 'propanoic acid' has 3 carbons total (including the COOH carbon), with the carboxyl group on C1.

Displayed formula of ethanoic acid.

    H        O
    |        ||
H — C ── C ── O ── H
    |
    H

Three carbons total? Wait — only TWO carbons in ethanoic acid! Let me re-draw:

    H       O
    |       ||
H — C ──── C
    |       |
    H       O
            |
            H

Two carbons. The –CH₃ is the methyl group (from the alkane stem); the –COOH on C1 is the carboxyl group. So CH₃ + COOH = CH₃COOH.

Physical properties.

Property	Value (for ethanoic acid)	Reason
State at room T	Liquid (mp 16 °C, bp 118 °C)	H-bonding between molecules
Smell	Sharp / pungent (vinegar)	Volatile + irritant
Solubility in water	Fully soluble (miscible)	–COOH H-bonds with water
Density	1.05 g/cm³	Slightly denser than water

Short-chain carboxylic acids (C₁–C₄) are colourless LIQUIDS with sharp, vinegar-like smells. Higher members are oily liquids; very long-chain ones (fatty acids C₁₂+) are waxy solids.

Why higher boiling points than alkanes / alcohols of similar size.

Compare:

Compound	bp (°C)	Reason
Ethanoic acid (CH₃COOH, M = 60)	118	Strong H-bonding via –COOH; forms cyclic dimers
Ethanol (CH₃CH₂OH, M = 46)	78	H-bonding via –OH (weaker than –COOH)
Propane (C₃H₈, M = 44)	–42	Only van der Waals forces

Carboxylic acids have the HIGHEST bp among the small organic molecules because:

The –COOH group has BOTH the C=O (acceptor) AND O-H (donor) for hydrogen bonding.
In pure liquid, carboxylic acid molecules form DIMERS — two molecules joined by two H-bonds in a ring:

        O ─ ─ H ─ O
        ||         ||
  CH₃ ─ C          C ─ CH₃
        |          |
        O ─ H ─ ─ O

Breaking dimers requires extra energy → higher bp.

Why soluble in water.

The –COOH group is polar and H-bonds strongly with water. Methanoic, ethanoic, propanoic, and butanoic acids are FULLY miscible with water in any ratio. Longer chains have non-polar regions that dominate → solubility decreases. C₁₀+ fatty acids are essentially insoluble in water (but soluble in non-polar solvents).

Smell + safety.

Methanoic acid — sharp, ant-sting smell. Causes painful skin irritation (the burning sensation of an ant bite).
Ethanoic acid — pungent, vinegar smell. At 5% (vinegar) it's safe for food; at 100% ('glacial acetic acid') it's corrosive — can burn skin.
Propanoic acid — fruity / cheesy smell. Found in some cheeses (Swiss cheese gets its distinctive smell from propanoic acid + other carboxylic acids).
Butanoic acid — RANCID / sweaty smell. Found in rancid butter, body odour, vomit. Powerful — detectable at very low concentrations.

The smells of foods and fragrances are heavily influenced by short-chain carboxylic acids and their esters.

Industrial production of ethanoic acid.

Two main routes:

Bacterial oxidation of ethanol (vinegar route) — slow, low concentration, used for food vinegar.
Catalytic carbonylation of methanol — modern industrial route. Methanol + CO → ethanoic acid, over Rh / Ir catalysts. Most pure industrial ethanoic acid is made this way.

Global ethanoic acid production: ~ 17 million tonnes/year. Major uses:

Vinyl acetate monomer → poly(vinyl acetate) plastic (paints, adhesives).
Cellulose acetate → photographic film, cigarette filters.
Ethyl ethanoate solvent → nail polish, paints.
Food additive (E260 — preservative + flavouring).

Carboxylic acids: CₙH₂ₙ₊₁COOH, –COOH functional group.
First three: methanoic HCOOH (ant stings), ethanoic CH₃COOH (vinegar), propanoic CH₃CH₂COOH.
Liquids at room T, soluble in water, pungent smells.
Higher bp than alcohols of similar mass — strong H-bonding via –COOH (dimers).
Naming: drop '-e' from alkane stem; add '-oic acid'. Carboxyl C is counted as C1.
Industrial uses: solvents, plastics (PVA), film, cigarette filters, food preservatives.
Glacial ethanoic acid (100%) is CORROSIVE; vinegar (5%) is safe for food.

See the full worked example for carboxylic acids →

Carboxylic acids as WEAK ACIDS (spec 4.34)

Partial ionisation in water: RCOOH ⇌ RCOO⁻ + H⁺. Lower [H⁺] than strong acids → higher pH, slower reactions.

Carboxylic acids are classified as WEAK ACIDS — they only PARTIALLY ionise in water. This is a key concept that comes up repeatedly in 4CH1.

Ionisation in water.

When ethanoic acid is dissolved in water, an equilibrium is established between the undissociated molecule and the ions:

$\text{CH}_3\text{COOH(aq)} \rightleftharpoons \text{CH}_3\text{COO}^-(\text{aq)} + \text{H}^+(\text{aq})$

At equilibrium, the position of the equilibrium is far to the LEFT — most of the molecules remain as CH₃COOH (undissociated). Only a small fraction (~ 1% at 1 mol/dm³, depending on concentration) has split into CH₃COO⁻ + H⁺.

A strong acid is fully ionised; a weak acid is only partially ionised, so it has a lower [H⁺] at the same concentration.

The reversible arrow (⇌) is essential — it indicates partial ionisation. Compare with strong acids:

$\text{HCl(g)} \xrightarrow{\text{water}} \text{H}^+(\text{aq)} + \text{Cl}^-(\text{aq)}$

(Strong acids use → — full / one-way ionisation.)

Strong vs weak acid — extent of ionisation.

Acid	Type	% ionisation at 1 mol/dm³	[H⁺] at 1 mol/dm³	pH
HCl	Strong	100%	1 mol/dm³	~ 0
HNO₃	Strong	100%	1 mol/dm³	~ 0
H₂SO₄	Strong	100% (1st H⁺); 99% (2nd H⁺)	~ 2 mol/dm³ effective	< 0
Methanoic acid (HCOOH)	Weak	~ 1.3%	~ 0.013 mol/dm³	~ 1.9
Ethanoic acid (CH₃COOH)	Weak	~ 0.4%	~ 0.004 mol/dm³	~ 2.4
Propanoic acid	Weak	~ 0.4%	~ 0.004 mol/dm³	~ 2.5
Carbonic acid (H₂CO₃)	Weak	< 0.1%	< 0.001 mol/dm³	~ 4-5

Why are some acids strong and others weak?

The extent of ionisation depends on bond strengths and energy considerations:

In HCl, the H–Cl bond is highly polar but relatively weak; and Cl⁻ is well-stabilised in water (large, diffuse). Full ionisation is energetically favourable.
In carboxylic acids, the O–H bond is strong, but the carboxylate ion RCOO⁻ is stabilised by resonance (the negative charge is shared between two oxygen atoms). Still, the energy 'cost' of breaking the O–H is high enough that only a small fraction ionises.

The DETAILED reason involves bond energies, hydration energies, and electron density — beyond 4CH1. For the exam, just remember: carboxylic acids are weak; HCl/HNO₃/H₂SO₄ are strong.

pH consequence.

pH = −log₁₀[H⁺]. Lower [H⁺] → higher pH. So a 1 mol/dm³ weak acid (low [H⁺]) has higher pH than a 1 mol/dm³ strong acid (high [H⁺]) — even though they have the same molar concentration of acid molecules.

Specifically:

1 mol/dm³ HCl: pH ≈ 0.
1 mol/dm³ CH₃COOH: pH ≈ 2.4.

A pH difference of 2.4 means about a 250-fold difference in [H⁺] concentration.

Reaction rate consequence.

The rate of acid reactions (with metals, carbonates, bases) depends on the [H⁺] concentration. Higher [H⁺] → more frequent collisions with the reactant → faster rate.

Reaction	1 M HCl rate	1 M CH₃COOH rate
Mg + acid	VIGOROUS fizzing, ribbon gone in seconds	Gentle fizzing, ribbon gone in minutes
Na₂CO₃ + acid	VIGOROUS fizzing	Slow fizzing
NaOH + acid	Rapid neutralisation	Slightly slower but still 'rapid' (NaOH-driven)

Important: BOTH acids give the SAME TOTAL amount of product (CO₂, H₂, H₂O — depending on the reaction) — they just differ in RATE. Given enough time, the weak acid completes the reaction.

This is because as the [H⁺] is consumed in the reaction, MORE ethanoic acid ionises (Le Chatelier — the equilibrium shifts right) → continually supplies fresh H⁺. So eventually all the ethanoic acid molecules give up their H⁺ → same total result.

Confusing — strong / weak vs concentrated / dilute.

These are TWO DIFFERENT axes of acid character. Don't confuse them.

Term	Meaning	Example
Strong	Fully ionised	HCl, HNO₃, H₂SO₄
Weak	Partially ionised	CH₃COOH, HCOOH, H₂CO₃
Concentrated	High mol/dm³	18 M H₂SO₄ ('conc. H₂SO₄')
Dilute	Low mol/dm³	0.1 M HCl

You can have:

Dilute strong acid: 0.01 M HCl. Low concentration, but fully ionised. pH ~ 2.
Concentrated weak acid: glacial (100%) ethanoic acid. High concentration, but only partially ionised. Strange to think about — even at 100% concentration, only a fraction of the molecules are in the ionic form.
Dilute weak acid: 0.01 M ethanoic acid. Low concentration AND partial ionisation. pH ~ 3.4.
Concentrated strong acid: 12 M HCl. High concentration + fully ionised. pH negative!

Common error to avoid. Don't write 'weak' when you mean 'dilute'. 'Weak' refers to the EXTENT of ionisation (a property of the molecule); 'dilute' refers to the AMOUNT per volume (a property of the solution).

Practical implications.

Vinegar (dilute ethanoic acid) is safe to use in cooking, on skin, on surfaces — because it's both weak (only ~1% ionised) AND dilute (~5%). Low [H⁺] → won't burn skin or damage food.
Concentrated HCl is very dangerous — fully ionised + high concentration → very high [H⁺] → highly corrosive.
Battery acid (~ 6 M H₂SO₄) is very strong AND concentrated → extremely corrosive. Strong acid + high concentration = high [H⁺] = high danger.

Common weak acids you'll meet in 4CH1.

Carboxylic acids (methanoic, ethanoic, propanoic).
Carbonic acid (H₂CO₃) — dissolved CO₂ in water; in fizzy drinks.
Sulfurous acid (H₂SO₃) — dissolved SO₂ + H₂O; in acid rain.

Common strong acids you'll meet in 4CH1.

Hydrochloric acid (HCl) — gastric acid, lab reagent.
Nitric acid (HNO₃) — explosive manufacturing, fertilisers.
Sulfuric acid (H₂SO₄) — battery acid, industrial chemical.

Edexcel exam tip. A common 4CH1 question asks you to compare a strong and weak acid at the same concentration. The MARK is given for stating:

'Weak acid is partially / not fully ionised'.
'Lower [H⁺] in weak acid solution'.
'Lower rate of reaction / higher pH'.

Use these exact phrases.

Carboxylic acids are WEAK acids — partially ionised in water.
Equation: RCOOH ⇌ RCOO⁻ + H⁺ (note the ⇌, not →).
At 1 mol/dm³, only ~1% of CH₃COOH molecules are ionised.
Strong acids (HCl, HNO₃, H₂SO₄) fully ionise → high [H⁺] → low pH → fast reactions.
Weak acids (CH₃COOH, HCOOH) partially ionise → lower [H⁺] → higher pH → slower reactions.
Same total amount of product given enough time (weak acid ionises further as H⁺ consumed).
Strong/weak = EXTENT of ionisation. Concentrated/dilute = AMOUNT per volume. Don't confuse.

See the full worked example for carboxylic acids →

Reactions of carboxylic acids (spec 4.33)

Same patterns as other acids — metals → salt + H₂; carbonates → salt + H₂O + CO₂; bases → salt + H₂O. But slower than strong acids of same concentration.

Carboxylic acids show ALL the typical reactions of acids — but slower / less vigorous than strong acids of the same concentration because they are only partially ionised.

Carboxylic acids show the three standard acid reactions; the salts formed are named with the −oate ending.

Reaction 1: With REACTIVE METALS (e.g. Mg, Zn, Fe).

General: acid + metal → salt + H₂.

Specific examples:

$2\text{CH}_3\text{COOH(aq)} + \text{Mg(s)} \rightarrow (\text{CH}_3\text{COO})_2\text{Mg(aq)} + \text{H}_2(\text{g})$

(magnesium + ethanoic acid → magnesium ethanoate + hydrogen)

$2\text{HCOOH(aq)} + \text{Zn(s)} \rightarrow (\text{HCOO})_2\text{Zn(aq)} + \text{H}_2(\text{g})$

(zinc + methanoic acid → zinc methanoate + hydrogen)

Observations:

Fizzing / effervescence (H₂ gas evolved).
Metal gradually dissolves.
Solution may stay colourless (Mg²⁺, Zn²⁺ colourless) or have a faint blue/green (Cu²⁺ — but Cu doesn't react with weak acids at room T; need stronger acid).
Mildly exothermic — slight warming.
Test for H₂: collect gas in test tube; lit splint → 'squeaky pop'.

Compared with HCl + Mg: much SLOWER fizzing with ethanoic acid because [H⁺] is lower (weak acid, partially ionised). Same total amount of H₂ produced eventually if both acids are at same moles.

Reaction 2: With METAL CARBONATES (or hydrogen carbonates).

General: acid + carbonate → salt + water + CO₂.

Specific examples:

$2\text{CH}_3\text{COOH(aq)} + \text{Na}_2\text{CO}_3(\text{s}) \rightarrow 2\text{CH}_3\text{COONa(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2(\text{g})$

(sodium carbonate + ethanoic acid → sodium ethanoate + water + CO₂)

$2\text{CH}_3\text{COOH(aq)} + \text{CaCO}_3(\text{s}) \rightarrow (\text{CH}_3\text{COO})_2\text{Ca(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2(\text{g})$

(limescale + vinegar → calcium ethanoate + water + CO₂ — descaling kettles!)

Observations:

Fizzing (CO₂ gas).
Carbonate solid gradually disappears.
Test for CO₂: bubble through LIMEWATER (Ca(OH)₂ solution) → turns CLOUDY / MILKY (white precipitate of CaCO₃ forms): Ca(OH)₂ + CO₂ → CaCO₃ + H₂O.

This reaction is the basis of using vinegar as a household descaler (CaCO₃ limescale + vinegar → soluble Ca ethanoate + H₂O + CO₂).

Reaction 3: With BASES (metal oxides, hydroxides) — NEUTRALISATION.

General: acid + base → salt + water.

Specific examples:

$\text{CH}_3\text{COOH(aq)} + \text{NaOH(aq)} \rightarrow \text{CH}_3\text{COONa(aq)} + \text{H}_2\text{O(l)}$

(ethanoic acid + sodium hydroxide → sodium ethanoate + water)

$2\text{CH}_3\text{COOH(aq)} + \text{CuO(s)} \rightarrow (\text{CH}_3\text{COO})_2\text{Cu(aq)} + \text{H}_2\text{O(l)}$

(copper oxide + ethanoic acid → blue copper(II) ethanoate + water)

Observations:

NaOH: no visible reaction (both colourless). Use indicator (phenolphthalein pink → colourless) or pH change to detect endpoint.
CuO: black CuO dissolves; solution turns blue (Cu²⁺).

Method for making salt (similar to other neutralisations): TITRATE with indicator to find correct volume of acid; repeat WITHOUT indicator to make the actual salt; EVAPORATE to crystallise.

Reaction 4: With ALCOHOLS → ESTERIFICATION (Topic 4.7).

General: carboxylic acid + alcohol ⇌ ester + water.

$\text{CH}_3\text{COOH} + \text{C}_2\text{H}_5\text{OH} \xrightleftharpoons[\text{}]{\text{H}_2\text{SO}_4} \text{CH}_3\text{COOC}_2\text{H}_5 + \text{H}_2\text{O}$

Catalyst: concentrated H₂SO₄ (also a dehydrating agent). Heat under reflux.

Product: an ESTER (sweet/fruity smell). The H of the acid's –OH + the OH of the alcohol's –OH = water; the remaining –O– bonds to the alcohol's alkyl group.

Covered in detail in Topic 4.7 (next subtopic).

Naming carboxylate SALTS.

The salt is named by replacing the acid's '-oic acid' suffix with '-oate':

Carboxylic acid	Sodium salt	Magnesium salt
Methanoic acid (HCOOH)	Sodium methanoate (HCOONa)	Magnesium methanoate ((HCOO)₂Mg)
Ethanoic acid (CH₃COOH)	Sodium ethanoate (CH₃COONa)	Magnesium ethanoate ((CH₃COO)₂Mg)
Propanoic acid (C₂H₅COOH)	Sodium propanoate (C₂H₅COONa)	Magnesium propanoate ((C₂H₅COO)₂Mg)
Butanoic acid (C₃H₇COOH)	Sodium butanoate	Magnesium butanoate

Cation pairing. The carboxylate anion (RCOO⁻) pairs with a cation:

1+ cation (Na⁺, K⁺, NH₄⁺): one carboxylate per cation → RCOONa, RCOOK.
2+ cation (Mg²⁺, Ca²⁺, Cu²⁺, Zn²⁺): TWO carboxylates per cation → (RCOO)₂Mg, (RCOO)₂Ca.
3+ cation (Al³⁺, Fe³⁺): THREE carboxylates per cation → (RCOO)₃Al.

Mark scheme keyword: use IUPAC names (e.g. 'sodium ethanoate'), NOT common names ('sodium acetate'). Edexcel uses IUPAC nomenclature.

Compared with strong acid reactions.

The key difference is RATE — carboxylic acid reactions are slower:

Reaction	Strong acid (HCl)	Weak acid (CH₃COOH) at same concentration
Mg	Vigorous fizz; ribbon gone in seconds	Gentle fizz; ribbon gone in minutes
Na₂CO₃	Vigorous CO₂	Slow CO₂
NaOH	Rapid neutralisation	Rapid (NaOH drives it forward — equilibrium shifts)
CuO + heat	Quick dissolution	Slower dissolution

In all cases, same TOTAL amount of product if same moles of acid are used. Just slower for the weak acid (lower [H⁺]).

The 'wine to vinegar' transformation.

Wine (~ 12% ethanol in water + acids + other compounds) can be turned into vinegar by aerobic bacterial oxidation:

$\text{C}_2\text{H}_5\text{OH} + \text{O}_2 \xrightarrow{\text{Acetobacter}} \text{CH}_3\text{COOH} + \text{H}_2\text{O}$

This happens spontaneously if wine is exposed to air for several weeks. The result is wine vinegar (~ 6-8% ethanoic acid). This is also why wine bottles are SEALED — to keep O₂ + bacteria out.

Why this matters for 4CH1.

A typical 4CH1 question gives you a carboxylic acid and asks you to:

Identify the salt formed in a reaction.
Write the balanced equation.
Compare the rate with a strong acid.
Suggest a household / industrial use.

Practise these patterns — they appear in nearly every paper.

Carboxylic acid + metal → salt + H₂. e.g. 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂.
Carboxylic acid + carbonate → salt + H₂O + CO₂. e.g. 2CH₃COOH + CaCO₃ → (CH₃COO)₂Ca + H₂O + CO₂.
Carboxylic acid + base → salt + H₂O. e.g. CH₃COOH + NaOH → CH₃COONa + H₂O.
Carboxylic acid + alcohol → ester + H₂O (Topic 4.7).
Salts named with -oate suffix (replacing -oic acid).
All reactions SLOWER than strong acid of same concentration (because weak acid has lower [H⁺]).
Limewater test for CO₂: turns cloudy/milky (white CaCO₃ precipitate).
Vinegar (dilute ethanoic acid) = household descaler — reacts with CaCO₃ limescale.

See the full worked example for carboxylic acids →

Ethanoic acid: vinegar, uses, and industrial importance

Vinegar (dilute ethanoic acid) is food + descaler. Industrial ethanoic acid is feedstock for plastics, esters, dyes.

Ethanoic acid is the most important carboxylic acid commercially. Its dilute form is VINEGAR (food/household); concentrated forms are industrial feedstocks.

Vinegar — the household form.

Vinegar is a dilute aqueous solution of ethanoic acid, typically 4-8% by volume. Made by:

Wine vinegar: aerobic bacterial oxidation of wine.
Malt vinegar: from beer / malt liquor.
Cider vinegar: from apple cider.
Distilled white vinegar: from synthetic ethanol or industrial fermentation.
Balsamic vinegar: from aged grape must (Italy).

Each has the same chemistry (mostly ethanoic acid + water) but different organic compounds → different flavours.

Uses of vinegar.

Cooking / salad dressing — sour flavour; preservative (low pH inhibits bacteria).
Pickling — preserves vegetables (cucumbers, onions) by lowering pH.
Household cleaning — mildly acidic; cuts through grease.
Descaling — dissolves limescale (CaCO₃) in kettles, coffee makers, shower heads.
Window cleaner — diluted vinegar gives streak-free glass (no soap residue).
Mild antiseptic — has antibacterial properties at low pH.
Removes rust stains — reacts with iron oxide.

The descaler reaction.

$2\text{CH}_3\text{COOH(aq)} + \text{CaCO}_3(\text{s}) \rightarrow (\text{CH}_3\text{COO})_2\text{Ca(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2(\text{g})$

The hardness in tap water (Ca²⁺) precipitates as CaCO₃ scale when heated. Vinegar reacts to give soluble calcium ethanoate, which can be rinsed away. Common household chemistry!

Why vinegar (and not HCl) for household use?

HCl would descale faster but is dangerous:

Vinegar is FOOD-SAFE (trace residues are harmless).
Vinegar is GENTLE on metal surfaces (slow reaction).
Vinegar is CHEAP and widely available.
HCl would burn skin + irritate lungs.

The weak / dilute / slow nature of vinegar is exactly what makes it suitable.

Industrial ethanoic acid (concentrated, pure).

Glacial (~ 100% pure) ethanoic acid is a corrosive industrial chemical. Major uses:

Vinyl acetate monomer (VAM) — combine ethanoic acid with ethyne or ethene → vinyl acetate (CH₂=CH–O–CO–CH₃) → polymerises to poly(vinyl acetate) (PVA) → emulsion paints, glue, chewing gum.
Cellulose acetate — react ethanoic acid (or acetic anhydride) with cellulose → cellulose acetate plastic → photographic film, cigarette filters, sunglasses, fibres.
Esters — ethyl ethanoate, butyl ethanoate, isopentyl ethanoate are major industrial solvents (paints, nail polish, glue) and food flavourings.
Aspirin — ethanoic acid (or acetic anhydride) acetylates salicylic acid → acetylsalicylic acid = aspirin.
Dyes + pigments — many synthetic dyes use ethanoic acid in their manufacture.
Food additive (E260) — direct use as preservative and flavouring.

Production of industrial ethanoic acid.

Two main routes:

Catalytic carbonylation of methanol (the modern route): $\text{CH}_3\text{OH} + \text{CO} \xrightarrow{\text{Rh or Ir catalyst}} \text{CH}_3\text{COOH}$ Continuous, fast, high purity. Most industrial ethanoic acid is made this way (Monsanto / Cativa processes).
Aerobic oxidation of butane / naphtha (older route, less used): $2\text{C}_4\text{H}_{10} + 5\text{O}_2 \rightarrow 4\text{CH}_3\text{COOH} + 2\text{H}_2\text{O}$ At high T + P with manganese / cobalt catalyst.
Aerobic oxidation of ethanol (vinegar route — bacterial) for FOOD-grade vinegar: $\text{C}_2\text{H}_5\text{OH} + \text{O}_2 \xrightarrow{\text{bacteria}} \text{CH}_3\text{COOH} + \text{H}_2\text{O}$ Slower but FOOD-SAFE (no industrial contaminants).

Global ethanoic acid production: ~ 17 million tonnes per year. Majority (~ 65%) goes to vinyl acetate / PVA plastics.

Methanoic acid — uses and importance.

Methanoic acid (HCOOH) is less abundant industrially but has interesting uses:

Textile dyeing — used in fabric processing.
Leather tanning — preserves and softens leather.
Antibacterial / preservative in livestock feed.
Reduction reagent in chemistry (HCOOH → H₂ + CO₂ — can act as a hydrogen source).
Found naturally in ant venom (the burning sensation of an ant bite is methanoic acid + alkaloids), bee stings, nettle hairs.

Propanoic acid — uses.

Bread preservative (E280) — prevents mould growth in baked goods.
Animal feed additive — antifungal.
Industrial intermediate — in dyes, fragrances.

The bigger picture.

Carboxylic acids span:

Food: vinegar, citric acid (in lemons), lactic acid (sour milk).
Industry: huge tonnages of ethanoic acid for plastics + esters.
Biology: fatty acids (long-chain carboxylic acids) are major energy stores in fats / oils.
Pharmacy: aspirin, ibuprofen, many drugs contain carboxylic acid groups.
Cleaning: descalers + cleaners.

The –COOH functional group is one of the most important in organic chemistry, second only to –OH.

Vinegar = 4-8% ethanoic acid in water. Made by aerobic bacterial fermentation of ethanol.
Vinegar uses: food, pickling, household cleaning, descaling, window cleaner.
Descaling reaction: 2CH₃COOH + CaCO₃ → (CH₃COO)₂Ca + H₂O + CO₂.
Industrial ethanoic acid: vinyl acetate (PVA paints), cellulose acetate (film), esters, aspirin, dyes.
Methanoic acid in ant stings; propanoic acid as bread preservative.
Industrial ethanoic acid made by methanol + CO catalytic carbonylation (Monsanto / Cativa process).
Vinegar's gentle / weak / dilute nature is why it's safe for household use vs HCl.

Quick recap

Carboxylic acids: CₙH₂ₙ₊₁COOH, –COOH functional group, sharp smells, liquids, soluble.
First three: methanoic HCOOH (ant stings), ethanoic CH₃COOH (vinegar), propanoic C₂H₅COOH.
WEAK ACIDS — partially ionised: RCOOH ⇌ RCOO⁻ + H⁺. pH ~ 3-4 at 1 mol/dm³.
Compare with STRONG acids (HCl, H₂SO₄, HNO₃): fully ionised → pH ~ 0 → faster reactions.
Same total amount of product eventually, just slower for weak acid.
Reactions: + metal → salt + H₂; + carbonate → salt + H₂O + CO₂; + base → salt + H₂O; + alcohol → ester + H₂O (Topic 4.7).
Salts end in -oate: sodium ethanoate (CH₃COONa), magnesium ethanoate ((CH₃COO)₂Mg).
Vinegar uses: cooking, pickling, descaling, window cleaning. Industrial ethanoic acid: VAM/PVA, esters, aspirin, film.
Use IUPAC names: ethanoate (not acetate), methanoate (not formate). 'Strong/weak' = extent of ionisation; 'concentrated/dilute' = amount per volume.

Memorise this

Verbatim phrases and definitions Edexcel mark schemes credit.

Carboxylic acid general formula: CₙH₂ₙ₊₁COOH (n ≥ 0).
First three: methanoic HCOOH, ethanoic CH₃COOH, propanoic C₂H₅COOH (= CH₃CH₂COOH).
Weak acid ionisation: RCOOH ⇌ RCOO⁻ + H⁺ (use ⇌, not →).
Strong acid ionisation: HCl → H⁺ + Cl⁻ (use →, fully ionised).
Acid + metal: 2RCOOH + Mg → (RCOO)₂Mg + H₂.
Acid + carbonate: 2RCOOH + Na₂CO₃ → 2RCOONa + H₂O + CO₂.
Acid + base: RCOOH + NaOH → RCOONa + H₂O.
Limewater test for CO₂: 'turns cloudy/milky' (CaCO₃ precipitate).
Vinegar descaling: 2CH₃COOH + CaCO₃ → (CH₃COO)₂Ca + H₂O + CO₂.
Why weak acid reacts slower than strong acid at same concentration: 'weak acid only partially ionised → lower [H⁺] → slower rate'.
Salt naming: 'ethanoate' (from ethanoic acid), 'methanoate' (from methanoic acid). NOT 'acetate / formate' (old common names).

How it’s examined

Carboxylic acids appear on every 4CH1 paper, typically worth 5-8 marks per session. Paper 1 (4CH1/1C) items: (a) define + first three names (3 marks); (b) reaction with metal/carbonate/base (4-5 marks each); (c) name the salt produced (1 mark each). Paper 2 (4CH1/2C) items: deeper analysis of strong vs weak (4-5 marks — must include 'partially ionised'); comparison of reaction rates with explanation; industrial uses; vinegar as descaler. The key examined concept is 'weak acid partially ionised → lower [H⁺] → slower rate but same total product'. Mark scheme keywords: 'partially ionised', 'partially dissociated', 'weak acid', 'lower concentration of H⁺'. Common errors: confusing 'weak' with 'dilute'; using old common names ('acetate' instead of 'ethanoate'); writing → instead of ⇌ for weak-acid ionisation.

Sources: Pearson Edexcel International GCSE Chemistry 4CH1 specification — Issue 4, September 2024 (valid for 2026 onwards); Pearson Edexcel 4CH1 Examiner Reports 2022-2024; 4CH1/1C May/Jun 2024 question paper and mark scheme; 4CH1/1C January 2024 question paper and mark scheme; 4CH1/2C May/Jun 2024 question paper and mark scheme; 4CH1/2C January 2024 question paper and mark scheme. Last reviewed 2026-05-12.

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Step-by-step worked examples — Carboxylic Acids

Step-by-step solutions to past-paper-style questions on carboxylic acids, written exactly the way a tutor would explain them at the board.

1Define the carboxylic acid homologous series and give first three members (4 marks, Paper 1)

Core• Adapted from 4CH1/1C May/Jun 2024 Q3• carboxylic acids, general formula, spec-4.32

Question

(a) State the functional group of a carboxylic acid. (b) State the general formula of the homologous series. (c) Name and give the structural formula of the first THREE members. (d) State and explain ONE chemical property that all three share. (4 marks)

Step-by-step solution

Step 1
(a) Functional group (1 M). The –COOH group (carboxyl group) is shared by all carboxylic acids. It consists of a C double-bonded to one O and single-bonded to an –OH group:
```
   O
   ||
   C — O — H
```
This is what gives carboxylic acids their characteristic ACIDIC chemistry (the H of the –OH can dissociate as H⁺).
Step 2
(b) General formula (1 A). Carboxylic acids follow:
$C_n H_{2n+1} COOH$
Step 3
(b) Note on n. For methanoic acid n = 0 (no alkyl carbons, just HCOOH). Be careful — some textbooks write the general formula as CₙH₂ₙO₂ (n ≥ 1), which counts the carboxyl carbon. Both are equivalent.

Step 4

(c) First three carboxylic acids (1 A). Named using the alkane stem + -oic acid suffix.

n	Name	Structural formula	Common name / source
0	Methanoic acid	HCOOH	Formic acid (ant stings, nettle hairs)
1	Ethanoic acid	CH₃COOH	Acetic acid (vinegar — ~ 5%)
2	Propanoic acid	CH₃CH₂COOH or C₂H₅COOH	Propionic acid (food preservative)

Step 5
(d) Shared chemical property + reason (1 B). All three are WEAK ACIDS — they partially dissociate in water:

$\text{RCOOH(aq)} \rightleftharpoons \text{RCOO}^-(\text{aq}) + \text{H}^+(\text{aq})$

The H⁺ ion in solution makes them ACIDIC (pH ~ 3-4 in dilute solution). They turn blue litmus red and react with reactive metals, carbonates, and bases — like other acids, but more slowly (because only a small fraction of molecules dissociate at any time).

Answer

(a) –COOH (carboxyl group). (b) CₙH₂ₙ₊₁COOH. (c) Methanoic HCOOH; ethanoic CH₃COOH; propanoic C₂H₅COOH. (d) All WEAK ACIDS — partially dissociate: RCOOH ⇌ RCOO⁻ + H⁺ → pH 3-4 → react with metals, carbonates, bases (slower than strong acids).

Examiner tip

Edexcel 4-mark mark scheme: 1 for –COOH; 1 for CₙH₂ₙ₊₁COOH; 1 for all three names + structures; 1 for property + reason. Mark scheme keyword: 'carboxyl group' and 'weak acid / partially dissociated / partially ionised'.

2Reaction of ethanoic acid with magnesium — observations and equation (5 marks, Paper 1)
Core• Adapted from 4CH1/1C Jan 2024 Q5• weak acid, metal, spec-4.33
Question
Magnesium ribbon is added to dilute aqueous ethanoic acid. (a) Describe what you would observe. (b) Write the balanced equation. (c) Name the salt produced. (d) Compare the rate of this reaction with the equivalent reaction of magnesium with dilute hydrochloric acid (same concentration). Explain any difference. (5 marks)
Step-by-step solution
1. Step 1
  (a) Observations (1 M). • EFFERVESCENCE (fizzing / bubbles) — colourless gas evolved. • Magnesium ribbon GRADUALLY DISSOLVES / disappears. • Solution remains colourless. • Slight WARMING of the test tube (mildly exothermic). • The bubbling is SLOW compared with the same reaction using dilute HCl.
2. Step 2
  (b) Balanced equation (1 A). Acid + metal → salt + hydrogen gas. The salt formed is magnesium ethanoate (Mg(CH₃COO)₂).
  $2\text{CH}_3\text{COOH(aq)} + \text{Mg(s)} \rightarrow (\text{CH}_3\text{COO})_2\text{Mg(aq)} + \text{H}_2(\text{g})$
3. Step 3
  (c) Name of salt (1 A). Magnesium ethanoate — Mg(CH₃COO)₂ or written (CH₃COO)₂Mg. The carboxylate suffix is -oate (replacing -oic acid). So ethanoic acid → ethanoate; methanoic → methanoate; propanoic → propanoate.
4. Step 4
  (d) Comparison with HCl (1 B). With dilute HCl of the SAME molar concentration: the reaction is MUCH FASTER, with VIGOROUS bubbling. Equation: Mg + 2HCl → MgCl₂ + H₂.
5. Step 5
  (d) Explanation of difference (1 B). HCl is a STRONG ACID — it is FULLY ionised in solution: HCl → H⁺ + Cl⁻. So a 1 mol/dm³ HCl solution contains 1 mol/dm³ of H⁺ ions. Ethanoic acid is a WEAK ACID — it is only PARTIALLY ionised: CH₃COOH ⇌ CH₃COO⁻ + H⁺. So a 1 mol/dm³ ethanoic acid solution contains a much LOWER concentration of H⁺ ions (about 0.004 mol/dm³). The reaction rate depends on the H⁺ concentration → HCl gives a much faster rate, even though both solutions have the same concentration of acid molecules.
Answer
(a) Fizzing / colourless gas (hydrogen) evolved; magnesium gradually dissolves; solution colourless; slightly warm; SLOWER than with HCl. (b) 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂. (c) Magnesium ethanoate. (d) HCl reaction much faster because HCl is fully ionised (high H⁺); ethanoic acid only partially ionised (low H⁺) → slower rate.
Examiner tip
Edexcel 5-mark mark scheme: 1 for fizzing / H₂; 1 for Mg dissolving; 1 for balanced equation; 1 for salt name (magnesium ethanoate); 1 for explaining rate difference in terms of partial vs full ionisation. Mark-scheme keywords: 'weak acid partially ionised → lower H⁺ concentration → slower rate'.
3Ethanoic acid + sodium carbonate (4 marks, Paper 1)
Core• Adapted from 4CH1/1C May/Jun 2024 Q5• weak acid, carbonate, spec-4.33
Question
Aqueous ethanoic acid is added to solid sodium carbonate. (a) Describe what you would observe. (b) Write the balanced equation. (c) State a test you could use to confirm the identity of the gas produced. (d) Suggest one household / commercial use of this reaction. (4 marks)
Step-by-step solution
1. Step 1
  (a) Observations (1 M). • EFFERVESCENCE — bubbles of colourless gas. • Solid sodium carbonate dissolves / disappears. • Solution remains colourless. • Test tube cools very slightly (or stays at room T).
2. Step 2
  (b) Balanced equation (1 A). Acid + carbonate → salt + water + CO₂.
  $2\text{CH}_3\text{COOH(aq)} + \text{Na}_2\text{CO}_3(\text{s}) \rightarrow 2\text{CH}_3\text{COONa(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2(\text{g})$
3. Step 3
  (b) Salt name. Sodium ethanoate (CH₃COONa). The salt name comes from: cation (Na) + anion (ethanoate, from ethanoic acid → -oate).
4. Step 4
  (c) Test for CO₂ (1 A). Bubble the gas through LIMEWATER (a saturated solution of calcium hydroxide, Ca(OH)₂). If CO₂ is present, the limewater turns CLOUDY / MILKY because a white precipitate of calcium carbonate forms:
  
  $\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}$
  
  (With excess CO₂, the cloudiness disappears as soluble calcium hydrogencarbonate forms — but the initial cloudy/milky observation is the standard test.)
5. Step 5
  (d) Use (1 B). Vinegar (dilute ethanoic acid) reacts with limescale (calcium carbonate, CaCO₃) in kettles and pipes:
  
  $2\text{CH}_3\text{COOH} + \text{CaCO}_3 \rightarrow (\text{CH}_3\text{COO})_2\text{Ca} + \text{H}_2\text{O} + \text{CO}_2$
  
  → vinegar is used as a household descaler to dissolve limescale (which is mostly CaCO₃). The calcium ethanoate formed is soluble in water and can be rinsed away. Another household use: dissolving rust stains or 'metal cleaning'.
Answer
(a) Fizzing — colourless CO₂ gas; carbonate dissolves; solution colourless. (b) 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂. (c) Bubble through limewater → turns cloudy / milky (white precipitate of CaCO₃). (d) Vinegar used as kettle descaler to remove limescale (CaCO₃).
Examiner tip
Edexcel 4-mark mark scheme: 1 for fizzing / CO₂ evolution; 1 for balanced equation with all three products; 1 for limewater test; 1 for any valid household use. Mark-scheme keywords: 'limewater turns cloudy/milky'; common error: 'turns chalky' is acceptable but vague — use 'cloudy' or 'milky'.
4Ethanoic acid + sodium hydroxide — neutralisation (3 marks, Paper 1)
Core• Adapted from 4CH1/1C Jan 2024 Q4• neutralisation, salt, spec-4.33
Question
Aqueous ethanoic acid reacts with aqueous sodium hydroxide. (a) Write the balanced equation. (b) Name the salt produced. (c) State the type of reaction. (3 marks)
Step-by-step solution
1. Step 1
  (a) Balanced equation (1 M). Acid + alkali → salt + water. NaOH is the base; ethanoic acid is the acid. The Na⁺ pairs with the ethanoate ion; the OH⁻ combines with H⁺ to make water.
  $\text{CH}_3\text{COOH(aq)} + \text{NaOH(aq)} \rightarrow \text{CH}_3\text{COONa(aq)} + \text{H}_2\text{O(l)}$
2. Step 2
  (b) Name of salt (1 A). Sodium ethanoate (CH₃COONa or NaCH₃COO). The carboxylate anion CH₃COO⁻ ('ethanoate') pairs with the sodium cation.
3. Step 3
  (c) Type of reaction (1 B). NEUTRALISATION — an acid + a base → a salt + water. The H⁺ of the acid combines with the OH⁻ of the base. The product solution is approximately neutral (pH 7 — though for a weak acid + strong base salt like sodium ethanoate, the solution is slightly alkaline due to hydrolysis of the ethanoate ion; this is beyond 4CH1).
Answer
(a) CH₃COOH + NaOH → CH₃COONa + H₂O. (b) Sodium ethanoate. (c) Neutralisation (acid + base → salt + water).
Examiner tip
Edexcel 3-mark mark scheme: 1 for balanced equation; 1 for salt name; 1 for 'neutralisation'. Common error: writing the salt as 'sodium acetate' (the old common name) — the IUPAC mark-scheme name is 'sodium ETHANOATE'.
5Why ethanoic acid is a WEAK acid (4 marks, Paper 2)
Higher• Adapted from 4CH1/2C Jan 2024 Q6• weak acid, ionisation, spec-4.33
Question
Ethanoic acid is classified as a WEAK acid. (a) Write an equation showing the dissociation of ethanoic acid in water. (b) Explain what is meant by 'weak acid'. (c) The pH of 0.1 mol/dm³ HCl is 1.0 and the pH of 0.1 mol/dm³ ethanoic acid is 2.9. Explain why these pH values are different. (4 marks)
Step-by-step solution
1. Step 1
  (a) Dissociation equation (1 M). Ethanoic acid PARTIALLY dissociates in water. Use the reversible arrow ⇌ to show partial ionisation.
  $\text{CH}_3\text{COOH(aq)} \rightleftharpoons \text{CH}_3\text{COO}^-(\text{aq}) + \text{H}^+(\text{aq})$
2. Step 2
  (b) Definition of weak acid (1 A). A WEAK ACID is one that only PARTIALLY DISSOCIATES (or partially ionises) into ions in water. Most of the molecules remain as the undissociated, uncharged acid molecule; only a small fraction at a time are in the ionised form.
  
  Compare with a STRONG ACID (e.g. HCl, H₂SO₄, HNO₃): a strong acid FULLY (100%) DISSOCIATES in water — every molecule splits into H⁺ + the anion.
3. Step 3
  (c) pH explanation (1 B). pH = −log₁₀[H⁺]. So pH 1 corresponds to [H⁺] = 0.1 mol/dm³ (matches the HCl concentration — fully ionised). pH 2.9 corresponds to [H⁺] ≈ 1.3 × 10⁻³ mol/dm³ — only about 1.3% of the ethanoic acid molecules have ionised.
4. Step 4
  (c) Reason for difference (1 B). Both solutions have the same concentration of acid MOLECULES (0.1 mol/dm³), but they have very different concentrations of H⁺ IONS:
  
  • 0.1 mol/dm³ HCl: ALL the HCl molecules ionise → [H⁺] = 0.1 mol/dm³ → pH = 1.0. • 0.1 mol/dm³ ethanoic acid: ONLY ~ 1.3% of the molecules ionise → [H⁺] ≈ 1.3 × 10⁻³ mol/dm³ → pH ≈ 2.9.
  
  Higher [H⁺] → lower pH. So HCl is more acidic (lower pH) than ethanoic acid of the same concentration. This is because HCl is a STRONG acid (fully ionised) and ethanoic acid is a WEAK acid (partially ionised).
  
  Note: 'strong' and 'weak' refer to the EXTENT of ionisation, NOT the concentration. A dilute strong acid (low concentration of HCl) can have a higher pH than a concentrated weak acid (high concentration of ethanoic acid).
Answer
(a) CH₃COOH ⇌ CH₃COO⁻ + H⁺. (b) Weak acid = partially ionised / dissociated in water. (c) HCl fully ionises → [H⁺] = 0.1 = full concentration → pH 1. Ethanoic acid only ~ 1% ionises → [H⁺] ≈ 0.001 → pH ~ 3. Same concentration of acid molecules but very different [H⁺].
Examiner tip
Edexcel 4-mark mark scheme: 1 for ⇌ in equation; 1 for definition (partially ionised); 1 for relating pH to [H⁺]; 1 for comparing the two acids. Mark-scheme keywords: 'partially ionised / partially dissociated' for weak; 'fully / completely ionised' for strong. Common error: confusing 'weak' with 'dilute' — 'weak' refers to extent of ionisation; 'dilute' refers to concentration.

Model Answers — Carboxylic Acids

High-scoring sample answers for carboxylic acids on the Pearson Edexcel IGCSE 4CH1 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.

Question 1

4CH1/1C-style — ethanoic acid + magnesium5 marks

Describe and explain in detail the reaction between aqueous ethanoic acid and magnesium ribbon. Include observations, equation, salt name, and a comparison with the reaction of magnesium with dilute hydrochloric acid of the same concentration. (5 marks)

Model answer

The reaction.

Magnesium reacts with dilute ethanoic acid in the same way as it does with any acid:

$2\text{CH}_3\text{COOH(aq)} + \text{Mg(s)} \rightarrow (\text{CH}_3\text{COO})_2\text{Mg(aq)} + \text{H}_2(\text{g})$

(acid + reactive metal → salt + hydrogen)

The salt produced is magnesium ethanoate, Mg(CH₃COO)₂. The two ethanoate (acetate) ions each carry a 1− charge, balancing the 2+ charge of the magnesium ion.

Observations.

When magnesium ribbon is added to dilute ethanoic acid (e.g. 1 mol/dm³):

Observation	Explanation
EFFERVESCENCE / bubbling	Hydrogen gas is being released
Colourless gas	H₂ is colourless
Magnesium ribbon gradually disappears / dissolves	Mg is being oxidised to Mg²⁺ and goes into solution
Solution remains colourless	Mg²⁺ and CH₃COO⁻ are both colourless in solution
Slight warming of the test tube	Mildly exothermic (less energy released than with strong acids)
Bubbling stops eventually	Either all Mg is consumed or all acid is consumed

Compared with HCl of same concentration — observations.

With 1 mol/dm³ HCl: VIGOROUS effervescence, ribbon disappears quickly (seconds), more pronounced warming. Equation: Mg + 2HCl → MgCl₂ + H₂.

With 1 mol/dm³ ethanoic acid: GENTLE / SLOW effervescence, ribbon takes minutes to dissolve, less warming. Equation: 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂.

Why the rates are different — the key insight.

Both equations show the same stoichiometry (1 Mg + 2 H⁺ → Mg²⁺ + H₂). The rate of any acid reaction depends on the concentration of H⁺ ions in the solution.

HCl is a STRONG acid: it ionises COMPLETELY in water:

$\text{HCl(g)} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+(\text{aq}) + \text{Cl}^-(\text{aq})$

(simplified to HCl → H⁺ + Cl⁻ in 4CH1). So 1 mol/dm³ HCl provides 1 mol/dm³ of H⁺ ions.

Ethanoic acid is a WEAK acid: it ionises only PARTIALLY:

$\text{CH}_3\text{COOH(aq)} \rightleftharpoons \text{CH}_3\text{COO}^-(\text{aq}) + \text{H}^+(\text{aq})$

At equilibrium, only about 1-2% of the molecules are ionised. So 1 mol/dm³ ethanoic acid provides only about 0.004 mol/dm³ of H⁺ ions — over 200 times less than HCl of the same concentration.

Because the H⁺ concentration is so much lower:

Fewer H⁺ ions per unit volume → fewer successful collisions with the magnesium surface per second → SLOWER rate of reaction.
The reaction still goes to completion (all the magnesium can be reacted, given enough time), but the rate is much lower.

Quantitative pH evidence.

1 mol/dm³ HCl: pH ≈ 0 ([H⁺] = 1 mol/dm³).
1 mol/dm³ ethanoic acid: pH ≈ 2.4 ([H⁺] ≈ 0.004 mol/dm³).

The HCl is about 200× more acidic by [H⁺], even though both have the same concentration of acid molecules.

Why are they still both 'acidic'?

Both:

Have pH < 7.
Turn blue litmus paper red.
React with reactive metals to give H₂.
React with carbonates to give CO₂.
React with bases to give salt + water.

The DIFFERENCE is the RATE / EXTENT of the reaction, not whether or not it happens. Ethanoic acid is still a 'genuine' acid — it just reacts more slowly.

Important clarification — 'weak' is NOT the same as 'dilute'.

Term	Meaning
Strong acid	Fully ionised (HCl, H₂SO₄, HNO₃)
Weak acid	Partially ionised (CH₃COOH, HCOOH, H₂CO₃)
Concentrated acid	High concentration (e.g. 18 mol/dm³ H₂SO₄)
Dilute acid	Low concentration (e.g. 0.1 mol/dm³ HCl)

You can have:

A dilute strong acid (e.g. 0.01 mol/dm³ HCl) — low concentration but fully ionised.
A concentrated weak acid (e.g. glacial / 100% pure ethanoic acid) — high concentration but partially ionised.

Don't confuse the two. 'Strong / weak' refers to the EXTENT of ionisation; 'concentrated / dilute' refers to the AMOUNT of substance per unit volume.

Real-world echo.

This explains why vinegar (a dilute weak acid) is safe to use as a salad dressing — even though it's chemically an acid (pH ~ 2.4), the low H⁺ concentration means it's not corrosive to skin or food. Whereas dilute HCl of similar molarity would be much more aggressive.

Why this scores

Edexcel 5-mark mark scheme: 1 for fizzing / H₂ + Mg dissolves; 1 for balanced equation; 1 for salt name (magnesium ethanoate); 1 for HCl reacts faster; 1 for explanation in terms of partial vs full ionisation. Mark-scheme keywords: 'partially ionised → lower [H⁺] → slower rate'.

Question 2

4CH1/2C-style — strong vs weak acid5 marks

Compare the reactivity of dilute hydrochloric acid (1 mol/dm³) and dilute ethanoic acid (1 mol/dm³) with sodium carbonate. Include equations and explain any difference in rate. (5 marks)

Model answer

Both reactions occur — same general pattern.

Both acids react with sodium carbonate to give a salt + water + carbon dioxide. This is the typical 'acid + carbonate' reaction.

Equation with HCl:

$2\text{HCl(aq)} + \text{Na}_2\text{CO}_3(\text{s}) \rightarrow 2\text{NaCl(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2(\text{g})$

Salt produced: sodium chloride (NaCl).

Equation with ethanoic acid:

$2\text{CH}_3\text{COOH(aq)} + \text{Na}_2\text{CO}_3(\text{s}) \rightarrow 2\text{CH}_3\text{COONa(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2(\text{g})$

Salt produced: sodium ethanoate (CH₃COONa).

Observations.

In both cases:

Effervescence (bubbles of colourless gas — CO₂).
Sodium carbonate solid dissolves into the solution.
Solution remains colourless.
Slight cooling (or no change in T — these are only slightly exothermic).

The DIFFERENCE: rate of reaction.

With 1 mol/dm³ HCl + Na₂CO₃: the fizzing is VIGOROUS and rapid. Bubbles form quickly; the carbonate dissolves within seconds. Loud and visible.

With 1 mol/dm³ ethanoic acid + Na₂CO₃: the fizzing is GENTLE / SLOW. Bubbles form steadily but slowly; the carbonate may take minutes to dissolve fully. Quieter and less dramatic.

Why is the rate different?

The rate of reaction depends on the concentration of H⁺ ions in solution (the reactive species that attacks the carbonate). At a given molar concentration of acid:

Acid	Ionisation behaviour	[H⁺] at 1 mol/dm³ acid
HCl (strong)	FULL ionisation: HCl → H⁺ + Cl⁻	~ 1 mol/dm³
CH₃COOH (weak)	PARTIAL: CH₃COOH ⇌ CH₃COO⁻ + H⁺	~ 0.004 mol/dm³ (~ 1%)

HCl gives ~ 250 times more H⁺ ions per unit volume than ethanoic acid of the same concentration → far more frequent successful collisions of H⁺ with Na₂CO₃ → much faster CO₂ production.

This is consistent with the pH:

1 mol/dm³ HCl: pH ≈ 0.
1 mol/dm³ CH₃COOH: pH ≈ 2.4.

Will the same TOTAL volume of CO₂ be produced eventually?

YES — given enough time. The MOLES of acid are the same (1 mol of acid in each), and both follow the same stoichiometry (2 mol acid + 1 mol Na₂CO₃ → 1 mol CO₂). So the total volume of CO₂ produced will be the same once the reaction goes to completion.

The DIFFERENCE is just the RATE: HCl gets there in seconds; ethanoic acid takes minutes (or longer). The 'as the reaction proceeds, more CH₃COOH ionises' principle (Le Chatelier) means that the weak-acid reaction continues until all the molecules have eventually given up their H⁺.

Graphical comparison — rate curves.

If you plotted volume of CO₂ produced against time, both curves would level off at the same final volume (same total moles of CO₂). But the HCl curve would rise steeply and level off quickly; the ethanoic acid curve would rise gently and level off later.

   CO₂
    ↑
    |
[Both curves reach same max]
    |     _______ HCl (fast)
    |    /
    |   /
    |  /  ____ CH₃COOH (slow)
    | / /
    |//
    +-------------→ time

Real-world implications.

Vinegar (dilute ethanoic acid) is used safely as a household cleaner / descaler / cooking ingredient because its slow reaction with carbonates (e.g. limescale, dead skin) is gentle enough not to damage skin or surfaces.
Industrial / lab HCl is much more aggressive — fast reaction with carbonates would be hazardous to handle without protection.
Limewater test (used to detect CO₂) works with EITHER acid + carbonate — the cloudiness with Ca(OH)₂ is the same; only the rate of producing CO₂ differs.

Bottom line.

Feature	1 M HCl + Na₂CO₃	1 M CH₃COOH + Na₂CO₃
Acid type	Strong	Weak
% ionisation	100%	~ 1%
[H⁺]	1 M	~ 0.004 M
pH	~ 0	~ 2.4
Reaction rate	FAST	SLOW
Equation	2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂	2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
Final CO₂ volume	Same	Same

The rate difference is the KEY observable difference between strong and weak acids in this kind of reaction.

Why this scores

Edexcel 5-mark mark scheme: 1 for both equations balanced; 1 for both produce CO₂ + observations; 1 for stating ethanoic acid reacts SLOWER; 1 for explaining 'weak acid partially ionised → lower [H⁺]'; 1 for noting that the TOTAL CO₂ is the same (same moles of acid). Mark-scheme keyword: 'partially ionised' or 'partially dissociated'.

Question 3
4CH1/2C-style — neutralisation + salt preparation5 marks
Write the balanced equation for the reaction between aqueous ethanoic acid and aqueous sodium hydroxide. Name the salt produced and describe how you would obtain a pure dry sample of this salt from the reaction mixture. (5 marks)
Model answer
The neutralisation reaction.

Ethanoic acid (a weak acid) reacts with sodium hydroxide (a strong base) in a standard acid + base neutralisation:

$\text{CH}_3\text{COOH(aq)} + \text{NaOH(aq)} \rightarrow \text{CH}_3\text{COONa(aq)} + \text{H}_2\text{O(l)}$

The H⁺ from the acid combines with the OH⁻ from the base to make water; the Na⁺ and CH₃COO⁻ form the soluble salt.

Salt produced: sodium ethanoate (CH₃COONa, sometimes written NaCH₃COO or sodium acetate in the older common name).

In the equation, sodium hydroxide is fully ionised (it's a strong base), so the underlying ionic process is:

$\text{CH}_3\text{COOH(aq)} + \text{OH}^-(\text{aq}) \rightarrow \text{CH}_3\text{COO}^-(\text{aq}) + \text{H}_2\text{O(l)}$

The reaction goes effectively to completion because the OH⁻ pulls the H⁺ off the ethanoic acid, driving the dissociation equilibrium fully to the right.

Obtaining a pure dry sample of sodium ethanoate.

Because both reactants (CH₃COOH and NaOH) are SOLUBLE and have NO visible reaction (no fizz, no colour change, no precipitate), you must use TITRATION to find the exact equivalence point, then mix without indicator to make the actual salt.

Step 1 — Find the volume of ethanoic acid needed by titration.

Use a pipette to put a known volume (e.g. 25.0 cm³) of NaOH(aq) of known concentration into a conical flask.

Add 2-3 drops of indicator (e.g. phenolphthalein — pink in alkali, colourless in acid; or methyl orange).

Fill a burette with the ethanoic acid solution. Record the starting reading.

Add the acid from the burette to the flask, swirling, until the indicator JUST changes colour (phenolphthalein pink → colourless). Record the end reading.

Calculate the volume of acid used (end − start). Repeat for concordant results (within 0.10 cm³).

Step 2 — Repeat WITHOUT indicator to make the pure salt.

Pipette the same 25.0 cm³ of NaOH(aq) into a clean conical flask.

Add the calculated volume of ethanoic acid (from Step 1) from the burette. Do NOT add any indicator — it would contaminate the product.

Stir / swirl to ensure thorough mixing. The mixture is now a solution of sodium ethanoate in water.

Step 3 — Crystallisation.

Pour the solution into an evaporating basin.

Heat gently (over a water bath, or with a low Bunsen flame) to evaporate most of the water. Stop when crystals first start to form on the surface (or when a glass rod dipped in shows crystals on cooling). This is the 'point of crystallisation'.

Leave the basin to cool slowly to room temperature so crystals of sodium ethanoate grow.

Filter the crystals using a Buchner funnel + filter paper to separate them from the remaining liquid (mother liquor).

Step 4 — Drying.

Wash the crystals with a small amount of cold water or cold ethanol to remove soluble impurities (without dissolving the salt).

Dry the crystals by pressing between filter paper, or place in a warm oven (~ 60 °C) until dry.

Result.

You will have a pure dry sample of sodium ethanoate (CH₃COONa) — a white crystalline solid, soluble in water.

Why this works — chemistry behind the method.

NaOH and CH₃COOH are both soluble — no insoluble product forms, so simple filtration of a precipitate isn't possible.

A pH indicator can't be left in the final product (it would colour the salt).

Titration to a colour change is the only way to know exactly when neutralisation is complete.

Once you know the volume, repeat without indicator to make the actual salt.

Sodium ethanoate is soluble in water → crystallisation by evaporation is needed to recover it as a solid.

Why titration matters.

If you ADD TOO MUCH acid → product contaminated with leftover acid (and crystallised salt would be acidic). If you ADD TOO LITTLE acid → product contaminated with leftover NaOH (and crystallised salt would be alkaline). Titration with an indicator gives a precise endpoint.

Equation summary.

$\boxed{\text{CH}_3\text{COOH(aq)} + \text{NaOH(aq)} \rightarrow \text{CH}_3\text{COONa(aq)} + \text{H}_2\text{O(l)}}$

Salt: sodium ethanoate. Method: TITRATION to find correct volume, then repeat WITHOUT indicator, EVAPORATE to crystallise, FILTER and DRY.
Why this scores
Edexcel 5-mark mark scheme: 1 for balanced equation; 1 for salt name (sodium ethanoate); 1 for titration with indicator to find volume; 1 for repeating without indicator; 1 for evaporation / crystallisation / filtration / drying.

Question 4

4CH1/2C-style — household use of ethanoic acid4 marks

Suggest why vinegar (dilute aqueous ethanoic acid) is widely used as a household descaler for kettles. Include an equation for the descaling reaction. (4 marks)

Model answer

The problem: limescale.

In hard-water areas, hot water contains dissolved calcium hydrogencarbonate (Ca(HCO₃)₂). When heated (as in a kettle), this decomposes:

$\text{Ca(HCO}_3)_2(\text{aq}) \xrightarrow{\text{heat}} \text{CaCO}_3(\text{s}) + \text{H}_2\text{O(l)} + \text{CO}_2(\text{g})$

The insoluble calcium carbonate (CaCO₃) is deposited on the heating element and inside surfaces of the kettle as LIMESCALE — a chalky white scaly layer. Limescale:

Reduces heating efficiency (insulates the element).
Looks ugly inside the kettle.
Can damage the element / break it over time.
Falls into your tea / coffee as flakes.

So we want to dissolve / remove it.

Why ethanoic acid (vinegar) works.

Calcium carbonate is a BASE (technically a carbonate of a metal hydroxide). It reacts with ACIDS:

$\text{CaCO}_3(\text{s}) + 2\text{CH}_3\text{COOH(aq)} \rightarrow (\text{CH}_3\text{COO})_2\text{Ca(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2(\text{g})$

(acid + carbonate → salt + water + CO₂)

The products are:

Calcium ethanoate (soluble in water → rinses away).
Water (no problem).
CO₂ gas (escapes as effervescence — you see bubbles).

So adding warm vinegar to the kettle DISSOLVES the limescale. You then rinse out the calcium ethanoate solution + any unreacted vinegar with clean water. Limescale gone.

Why vinegar specifically (vs HCl)?

You COULD use dilute HCl to descale a kettle — it would work much FASTER (HCl is a strong acid; ethanoic is weak). But several considerations make vinegar preferable for household use:

Feature	Vinegar (dilute ethanoic acid)	Dilute HCl
Strength	Weak acid (slow, gentle)	Strong acid (fast, aggressive)
Safety	Safe to handle, low risk of burns	Corrosive — burns skin + eyes
Smell	Pungent but familiar (vinegar)	Sharp / chemical (releases HCl fumes)
Cost	Cheap, widely available in supermarkets	Available but not in food sections
Food-safe	Yes — used in cooking	NO — toxic residues if rinsed poorly
Damage to kettle?	Minimal — slow gentle reaction	Could attack metal parts if left too long
Smell of tea afterwards?	Slightly vinegary but quickly fades	Would leave chloride residue → spoils taste

So vinegar is:

Cheap (a bottle of supermarket vinegar costs < $2).
Safe to handle (won't burn skin).
Food-safe (won't poison you if traces remain — vinegar is in your food anyway).
Gentle on metal kettles (the slow reaction means the kettle isn't damaged).
Effective enough for the job (given an hour or so).

How to use vinegar to descale a kettle (4CH1 application).

Empty the kettle.
Half-fill with white vinegar diluted 1:1 with water (or pure vinegar for stubborn scale).
Heat to boil, then turn off and let stand for 30 min - 1 hour. Fizzing as CO₂ is released.
Pour out the solution (now containing soluble calcium ethanoate).
Rinse the kettle 2-3 times with clean water to remove vinegar smell.
Boil and discard one or two more kettles of clean water before using for drinks.

The equation matters in 4CH1.

The Edexcel-friendly equation (memorise!):

$\boxed{\text{CaCO}_3(\text{s}) + 2\text{CH}_3\text{COOH(aq)} \rightarrow (\text{CH}_3\text{COO})_2\text{Ca(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2(\text{g})}$

Acid + carbonate → salt + water + CO₂ — the same pattern you'd use for any acid + carbonate reaction. The 'salt' here is calcium ethanoate.

Why this is a 'weak' acid story.

Ethanoic acid is a weak acid → only ~ 1% of the molecules are ionised at any time → reaction is SLOW (over half an hour for full descaling). If it reacted faster (strong acid speed), it would be too aggressive for a household chemical — could damage the kettle, irritate skin, or be unsafe around food. The SLOW kinetics of weak acids is exactly why they are widely used in food and cleaning products.

So in summary, vinegar dissolves limescale because:

Limescale is CaCO₃ (a base/carbonate).
Ethanoic acid is an acid (provides H⁺ that reacts with CO₃²⁻).
The reaction produces SOLUBLE calcium ethanoate (rinses away) + water + CO₂ (escapes).
Vinegar is cheap, safe, food-grade, and gentle — ideal for household use.

Why this scores

Edexcel 4-mark mark scheme: 1 for limescale = CaCO₃; 1 for ethanoic acid reacts with CaCO₃; 1 for balanced equation; 1 for product calcium ethanoate is SOLUBLE → rinses away (or for any valid 'safety / food-grade / cheap' justification of choosing vinegar over HCl).

Key Definitions and Keywords — Carboxylic Acids

Definitions to memorise and the exact keywords mark schemes credit for carboxylic acids answers — sharpened from recent examiner reports for the 2026 Pearson Edexcel IGCSE 4CH1 sitting.

Carboxylic acid
Examiner keyword
A homologous series of organic compounds containing the –COOH (carboxyl) functional group. General formula CₙH₂ₙ₊₁COOH (n ≥ 0). All are weak acids, partially ionised in water. Examples: methanoic HCOOH, ethanoic CH₃COOH, propanoic C₂H₅COOH.
Carboxyl group (–COOH)
Examiner keyword
The functional group of carboxylic acids. A carbon double-bonded to one O and single-bonded to an –OH group. The H of the –OH dissociates as H⁺ in water → makes the compound acidic. Written as –COOH or –CO₂H.
Weak acid
Examiner keyword
An acid that only PARTIALLY ionises (dissociates) into ions in aqueous solution. The dissociation is reversible: CH₃COOH ⇌ CH₃COO⁻ + H⁺. Most molecules remain undissociated. Examples: ethanoic, methanoic, citric, carbonic acid. Compared with strong acids (HCl, HNO₃, H₂SO₄ — fully ionised), weak acids have higher pH and react more slowly with metals / carbonates of the same molar concentration.
Ethanoic acid (CH₃COOH)
Examiner keyword
The C₂ carboxylic acid. Old name: acetic acid. Main acid in vinegar (~ 5% aqueous solution). Weak acid; pungent smell. Reacts with metals, carbonates, bases, and alcohols (to form esters). Produced industrially by bacterial oxidation of ethanol (vinegar route) or by catalytic carbonylation of methanol.
Carboxylate salt
Examiner keyword
The salt of a carboxylic acid. Names end in -oate (e.g. ethanoate from ethanoic; methanoate from methanoic; propanoate from propanoic). Examples: sodium ethanoate CH₃COONa; calcium ethanoate (CH₃COO)₂Ca; magnesium methanoate (HCOO)₂Mg.
Vinegar
Examiner keyword
A dilute aqueous solution of ethanoic acid (typically 4-8%), produced by aerobic bacterial oxidation of dilute ethanol (wine, cider, malt). Uses: food (salad dressing, pickling), cleaning, household descaler. Common name 'acetic acid' refers to pure ethanoic acid; 'vinegar' refers to the dilute solution.
Partial ionisation / dissociation
Examiner keyword
When only a small fraction of acid (or base) molecules split into ions in water. The remaining molecules stay undissociated. Equation uses the reversible ⇌ arrow. Hallmark of WEAK acids and bases. For 1 mol/dm³ ethanoic acid only about 1% is in the ionised form at equilibrium.
Strong acid
Examiner keyword
An acid that ionises COMPLETELY (100%) in aqueous solution. Examples: HCl, H₂SO₄, HNO₃. Equation uses → not ⇌. A 1 mol/dm³ strong acid provides 1 mol/dm³ of H⁺. pH ≈ 0-1 for ~ 1 mol/dm³.
Esterification
Examiner keyword
Reaction between a carboxylic acid and an alcohol, catalysed by concentrated sulfuric acid (H₂SO₄), to form an ESTER and water. Reversible. Example: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ (ethyl ethanoate) + H₂O. Covered in detail in Topic 4.7.

Common Mistakes and Misconceptions — Carboxylic Acids

The traps other students keep falling into on carboxylic acids questions — taken from recent Pearson Edexcel IGCSE 4CH1 examiner reports and mark schemes — and how to avoid them.

✕Confusing 'weak' acid with 'dilute' acid
4CH1 Examiner Reports 2022-2024
Why it happens
Both result in fewer H⁺ ions in solution — appears similar.
How to avoid it
Weak = partially ionised (extent of dissociation — e.g. ethanoic acid). Dilute = low concentration (amount per volume — e.g. 0.1 mol/dm³ HCl). You can have a CONCENTRATED WEAK acid (glacial ethanoic, ~ 17 mol/dm³) and a DILUTE STRONG acid (0.01 mol/dm³ HCl). Memorise: 'strong/weak = extent of ionisation; concentrated/dilute = amount per volume'.
✕Using → instead of ⇌ for the ionisation of a weak acid
4CH1 Examiner Reports 2023-2024
Why it happens
Habit from balancing equations.
How to avoid it
Weak acid ionisation is REVERSIBLE — must use ⇌. CH₃COOH ⇌ CH₃COO⁻ + H⁺. Strong acid ionisation is essentially complete → use →. HCl → H⁺ + Cl⁻.
✕Writing 'sodium acetate' or 'magnesium acetate' (the old common names)
4CH1 Examiner Reports 2022-2024
Why it happens
Old/common names appear in some textbooks.
How to avoid it
Use the IUPAC name: -oate (from -oic acid). Acetate → ethanoate. Acetic acid → ethanoic acid. Formic acid → methanoic acid. Edexcel mark schemes use the IUPAC names.
✕Writing CH₃COOH + Mg → CH₃COOMg + H₂ (unbalanced)
4CH1 Examiner Reports 2022-2024
Why it happens
Forgetting that Mg is 2+ so needs TWO carboxylate anions.
How to avoid it
Mg²⁺ + 2 carboxylate⁻ → (CH₃COO)₂Mg. So 1 Mg needs 2 CH₃COOH. The balanced equation is: 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂. Check both Mg and H balance.
✕Saying weak acids 'don't fully ionise because they are dilute'
Why it happens
Trying to find a cause.
How to avoid it
Weak acids don't fully ionise because the O–H bond in –COOH is strong relative to the energy released by hydration. It's a property of the MOLECULE, not the concentration. Even pure (100%) ethanoic acid (glacial) is still a weak acid — it just has higher concentration than a dilute solution.
✕Writing CO₂ as a product of acid + metal
4CH1 Examiner Reports 2023-2024
Why it happens
Confusion with acid + carbonate reaction.
How to avoid it
Acid + reactive metal → SALT + HYDROGEN (H₂, not CO₂). Acid + metal carbonate → salt + water + CO₂. Acid + base → salt + water. Memorise the three patterns.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Video lesson

Short walkthrough of the concepts students most often get stuck on.

Carboxylic Acids — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Carboxylic Acids — Pearson Edexcel International GCSE Chemistry 4CH1 Study Notes (2026 onwards, Spec Issue 4)

Name

Structural formula

Common name

Source / use

Methanoic acid

HCOOH

Formic acid

Ant stings, nettle hairs

Ethanoic acid

CH₃COOH (or HC₂H₃O₂)

Acetic acid

VINEGAR (~ 5% aqueous solution); food preservative

Propanoic acid

CH₃CH₂COOH or C₂H₅COOH

Propionic acid

Mould inhibitor in bread

Butanoic acid

C₃H₇COOH

Butyric acid

Rancid butter, body odour

Property

Value (for ethanoic acid)

Reason

State at room T

Liquid (mp 16 °C, bp 118 °C)

H-bonding between molecules

Smell

Sharp / pungent (vinegar)

Volatile + irritant

Solubility in water

Fully soluble (miscible)

–COOH H-bonds with water

Density

1.05 g/cm³

Slightly denser than water

Compound

bp (°C)

Reason

Ethanoic acid (CH₃COOH, M = 60)

118

Strong H-bonding via –COOH; forms cyclic dimers

Ethanol (CH₃CH₂OH, M = 46)

H-bonding via –OH (weaker than –COOH)

Propane (C₃H₈, M = 44)

–42

Only van der Waals forces

Acid

Type

% ionisation at 1 mol/dm³

[H⁺] at 1 mol/dm³

HCl

Strong

100%

1 mol/dm³

~ 0

HNO₃

Strong

100%

1 mol/dm³

~ 0

H₂SO₄

Strong

100% (1st H⁺); 99% (2nd H⁺)

~ 2 mol/dm³ effective

< 0

Methanoic acid (HCOOH)

Weak

~ 1.3%

~ 0.013 mol/dm³

~ 1.9

Ethanoic acid (CH₃COOH)

Weak

~ 0.4%

~ 0.004 mol/dm³

~ 2.4

Propanoic acid

Weak

~ 0.4%

~ 0.004 mol/dm³

~ 2.5

Carbonic acid (H₂CO₃)

Weak

< 0.1%

< 0.001 mol/dm³

~ 4-5

Reaction

1 M HCl rate

1 M CH₃COOH rate

Mg + acid

VIGOROUS fizzing, ribbon gone in seconds

Gentle fizzing, ribbon gone in minutes

Na₂CO₃ + acid

VIGOROUS fizzing

Slow fizzing

NaOH + acid

Rapid neutralisation

Slightly slower but still 'rapid' (NaOH-driven)

Term

Meaning

Example

Strong

Fully ionised

HCl, HNO₃, H₂SO₄

Weak

Partially ionised

CH₃COOH, HCOOH, H₂CO₃

Concentrated

High mol/dm³

18 M H₂SO₄ ('conc. H₂SO₄')

Dilute

Low mol/dm³

0.1 M HCl

Carboxylic acid

Sodium salt

Magnesium salt

Methanoic acid (HCOOH)

Sodium methanoate (HCOONa)

Magnesium methanoate ((HCOO)₂Mg)

Ethanoic acid (CH₃COOH)

Sodium ethanoate (CH₃COONa)

Magnesium ethanoate ((CH₃COO)₂Mg)

Propanoic acid (C₂H₅COOH)

Sodium propanoate (C₂H₅COONa)

Magnesium propanoate ((C₂H₅COO)₂Mg)

Butanoic acid (C₃H₇COOH)

Sodium butanoate

Magnesium butanoate

Reaction

Strong acid (HCl)

Weak acid (CH₃COOH) at same concentration

Vigorous fizz; ribbon gone in seconds

Gentle fizz; ribbon gone in minutes

Na₂CO₃

Vigorous CO₂

Slow CO₂

NaOH

Rapid neutralisation

Rapid (NaOH drives it forward — equilibrium shifts)

CuO + heat

Quick dissolution

Slower dissolution

Ethanoic acid is the most important carboxylic acid commercially. Its dilute form is VINEGAR (food/household); concentrated forms are industrial feedstocks.

Vinegar — the household form.

Vinegar is a dilute aqueous solution of ethanoic acid, typically 4-8% by volume. Made by:

Wine vinegar: aerobic bacterial oxidation of wine.
Malt vinegar: from beer / malt liquor.
Cider vinegar: from apple cider.
Distilled white vinegar: from synthetic ethanol or industrial fermentation.
Balsamic vinegar: from aged grape must (Italy).

Each has the same chemistry (mostly ethanoic acid + water) but different organic compounds → different flavours.

Uses of vinegar.

Cooking / salad dressing — sour flavour; preservative (low pH inhibits bacteria).
Pickling — preserves vegetables (cucumbers, onions) by lowering pH.
Household cleaning — mildly acidic; cuts through grease.
Descaling — dissolves limescale (CaCO₃) in kettles, coffee makers, shower heads.
Window cleaner — diluted vinegar gives streak-free glass (no soap residue).
Mild antiseptic — has antibacterial properties at low pH.
Removes rust stains — reacts with iron oxide.

The descaler reaction.

$2\text{CH}_3\text{COOH(aq)} + \text{CaCO}_3(\text{s}) \rightarrow (\text{CH}_3\text{COO})_2\text{Ca(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2(\text{g})$

The hardness in tap water (Ca²⁺) precipitates as CaCO₃ scale when heated. Vinegar reacts to give soluble calcium ethanoate, which can be rinsed away. Common household chemistry!

Why vinegar (and not HCl) for household use?

HCl would descale faster but is dangerous:

Vinegar is FOOD-SAFE (trace residues are harmless).
Vinegar is GENTLE on metal surfaces (slow reaction).
Vinegar is CHEAP and widely available.
HCl would burn skin + irritate lungs.

The weak / dilute / slow nature of vinegar is exactly what makes it suitable.

Industrial ethanoic acid (concentrated, pure).

Glacial (~ 100% pure) ethanoic acid is a corrosive industrial chemical. Major uses:

Vinyl acetate monomer (VAM) — combine ethanoic acid with ethyne or ethene → vinyl acetate (CH₂=CH–O–CO–CH₃) → polymerises to poly(vinyl acetate) (PVA) → emulsion paints, glue, chewing gum.
Cellulose acetate — react ethanoic acid (or acetic anhydride) with cellulose → cellulose acetate plastic → photographic film, cigarette filters, sunglasses, fibres.
Esters — ethyl ethanoate, butyl ethanoate, isopentyl ethanoate are major industrial solvents (paints, nail polish, glue) and food flavourings.
Aspirin — ethanoic acid (or acetic anhydride) acetylates salicylic acid → acetylsalicylic acid = aspirin.
Dyes + pigments — many synthetic dyes use ethanoic acid in their manufacture.
Food additive (E260) — direct use as preservative and flavouring.

Production of industrial ethanoic acid.

Two main routes:

Catalytic carbonylation of methanol (the modern route): $\text{CH}_3\text{OH} + \text{CO} \xrightarrow{\text{Rh or Ir catalyst}} \text{CH}_3\text{COOH}$ Continuous, fast, high purity. Most industrial ethanoic acid is made this way (Monsanto / Cativa processes).
Aerobic oxidation of butane / naphtha (older route, less used): $2\text{C}_4\text{H}_{10} + 5\text{O}_2 \rightarrow 4\text{CH}_3\text{COOH} + 2\text{H}_2\text{O}$ At high T + P with manganese / cobalt catalyst.
Aerobic oxidation of ethanol (vinegar route — bacterial) for FOOD-grade vinegar: $\text{C}_2\text{H}_5\text{OH} + \text{O}_2 \xrightarrow{\text{bacteria}} \text{CH}_3\text{COOH} + \text{H}_2\text{O}$ Slower but FOOD-SAFE (no industrial contaminants).

Global ethanoic acid production: ~ 17 million tonnes per year. Majority (~ 65%) goes to vinyl acetate / PVA plastics.

Methanoic acid — uses and importance.

Methanoic acid (HCOOH) is less abundant industrially but has interesting uses:

Textile dyeing — used in fabric processing.
Leather tanning — preserves and softens leather.
Antibacterial / preservative in livestock feed.
Reduction reagent in chemistry (HCOOH → H₂ + CO₂ — can act as a hydrogen source).
Found naturally in ant venom (the burning sensation of an ant bite is methanoic acid + alkaloids), bee stings, nettle hairs.

Propanoic acid — uses.

Bread preservative (E280) — prevents mould growth in baked goods.
Animal feed additive — antifungal.
Industrial intermediate — in dyes, fragrances.

The bigger picture.

Carboxylic acids span:

Food: vinegar, citric acid (in lemons), lactic acid (sour milk).
Industry: huge tonnages of ethanoic acid for plastics + esters.
Biology: fatty acids (long-chain carboxylic acids) are major energy stores in fats / oils.
Pharmacy: aspirin, ibuprofen, many drugs contain carboxylic acid groups.
Cleaning: descalers + cleaners.

The –COOH functional group is one of the most important in organic chemistry, second only to –OH.

Name

Structural formula

Common name / source

Methanoic acid

HCOOH

Formic acid (ant stings, nettle hairs)

Ethanoic acid

CH₃COOH

Acetic acid (vinegar — ~ 5%)

Propanoic acid

CH₃CH₂COOH or C₂H₅COOH

Propionic acid (food preservative)

Observation

Explanation

EFFERVESCENCE / bubbling

Hydrogen gas is being released

Colourless gas

H₂ is colourless

Magnesium ribbon gradually disappears / dissolves

Mg is being oxidised to Mg²⁺ and goes into solution

Solution remains colourless

Mg²⁺ and CH₃COO⁻ are both colourless in solution

Slight warming of the test tube

Mildly exothermic (less energy released than with strong acids)

Bubbling stops eventually

Either all Mg is consumed or all acid is consumed

Term

Meaning

Strong acid

Fully ionised (HCl, H₂SO₄, HNO₃)

Weak acid

Partially ionised (CH₃COOH, HCOOH, H₂CO₃)

Concentrated acid

High concentration (e.g. 18 mol/dm³ H₂SO₄)

Dilute acid

Low concentration (e.g. 0.1 mol/dm³ HCl)

Acid

Ionisation behaviour

[H⁺] at 1 mol/dm³ acid

HCl (strong)

FULL ionisation: HCl → H⁺ + Cl⁻

~ 1 mol/dm³

CH₃COOH (weak)

PARTIAL: CH₃COOH ⇌ CH₃COO⁻ + H⁺

~ 0.004 mol/dm³ (~ 1%)

Feature

1 M HCl + Na₂CO₃

1 M CH₃COOH + Na₂CO₃

Acid type

Strong

Weak

% ionisation

100%

~ 1%

[H⁺]

1 M

~ 0.004 M

~ 0

~ 2.4

Reaction rate

FAST

SLOW

Equation

2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂

2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂

Final CO₂ volume

Same

Feature

Vinegar (dilute ethanoic acid)

Dilute HCl

Strength

Weak acid (slow, gentle)

Strong acid (fast, aggressive)

Safety

Safe to handle, low risk of burns

Corrosive — burns skin + eyes

Smell

Pungent but familiar (vinegar)

Sharp / chemical (releases HCl fumes)

Cost

Cheap, widely available in supermarkets

Available but not in food sections

Food-safe

Yes — used in cooking

NO — toxic residues if rinsed poorly

Damage to kettle?

Minimal — slow gentle reaction

Could attack metal parts if left too long

Smell of tea afterwards?

Slightly vinegary but quickly fades

Would leave chloride residue → spoils taste

Carboxylic Acids

Short Study Notes in the form of Slides

Short Study Notes — Carboxylic Acids

Detailed Notes

What you’ll learn

How it’s examined

1Define the carboxylic acid homologous series and give first three members (4 marks, Paper 1)

2Reaction of ethanoic acid with magnesium — observations and equation (5 marks, Paper 1)

3Ethanoic acid + sodium carbonate (4 marks, Paper 1)

4Ethanoic acid + sodium hydroxide — neutralisation (3 marks, Paper 1)

5Why ethanoic acid is a WEAK acid (4 marks, Paper 2)

Carboxylic acid

Carboxyl group (–COOH)

Weak acid

Ethanoic acid (CH₃COOH)

Carboxylate salt

Vinegar

Partial ionisation / dissociation

Strong acid

Esterification

✕Confusing 'weak' acid with 'dilute' acid

✕Using → instead of ⇌ for the ionisation of a weak acid

✕Writing 'sodium acetate' or 'magnesium acetate' (the old common names)

✕Writing CH₃COOH + Mg → CH₃COOMg + H₂ (unbalanced)

✕Saying weak acids 'don't fully ionise because they are dilute'

✕Writing CO₂ as a product of acid + metal

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Carboxylic Acids — frequently asked questions

Carboxylic Acids

Short Study Notes in the form of Slides

Short Study Notes — Carboxylic Acids

Detailed Notes

What you’ll learn

How it’s examined

1Define the carboxylic acid homologous series and give first three members (4 marks, Paper 1)

2Reaction of ethanoic acid with magnesium — observations and equation (5 marks, Paper 1)

3Ethanoic acid + sodium carbonate (4 marks, Paper 1)

4Ethanoic acid + sodium hydroxide — neutralisation (3 marks, Paper 1)

5Why ethanoic acid is a WEAK acid (4 marks, Paper 2)

Carboxylic acid

Carboxyl group (–COOH)

Weak acid

Ethanoic acid (CH₃COOH)

Carboxylate salt

Vinegar

Partial ionisation / dissociation

Strong acid

Esterification

✕Confusing 'weak' acid with 'dilute' acid

✕Using → instead of ⇌ for the ionisation of a weak acid

✕Writing 'sodium acetate' or 'magnesium acetate' (the old common names)

✕Writing CH₃COOH + Mg → CH₃COOMg + H₂ (unbalanced)

✕Saying weak acids 'don't fully ionise because they are dilute'

✕Writing CO₂ as a product of acid + metal

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Carboxylic Acids — frequently asked questions