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 a type of functional group in organic chemistry. Carboxylic acids are known for their acidic properties, which arise from the ability of the carboxyl group to donate a hydrogen ion (H+). In the Cambridge IGCSE Chemistry curriculum, understanding the structure and properties of carboxylic acids is essential for studying organic reactions and synthesis.

How do carboxylic acids react with bases, and what are the products of such reactions?

Carboxylic acids react with bases to form salts and water in a neutralization reaction. For example, when ethanoic acid (a common carboxylic acid) reacts with sodium hydroxide (a base), sodium ethanoate (a salt) and water are produced. This reaction is an important concept in the Cambridge IGCSE Chemistry syllabus as it illustrates the acid-base behavior of carboxylic acids and their ability to form ionic compounds.

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

The carboxyl group (–COOH) is the defining feature of carboxylic acids and is responsible for their chemical properties. It consists of a carbonyl group (C=O) and a hydroxyl group (–OH) attached to the same carbon atom. This group is polar and can form hydrogen bonds, which contributes to the relatively high boiling points of carboxylic acids compared to other organic compounds. Understanding the carboxyl group is crucial for Cambridge IGCSE students studying organic chemistry, as it influences the reactivity and solubility of carboxylic acids.

Why do carboxylic acids have higher boiling points than alcohols of similar molecular weight?

Carboxylic acids have higher boiling points than alcohols of similar molecular weight due to the presence of strong hydrogen bonding between the carboxyl groups. The ability to form dimers through hydrogen bonding significantly increases the intermolecular forces, requiring more energy to break these interactions during boiling. This concept is important in the Cambridge IGCSE Chemistry curriculum as it helps students understand the physical properties of organic compounds and the role of intermolecular forces.

How are carboxylic acids named according to IUPAC nomenclature?

In IUPAC nomenclature, carboxylic acids are named by identifying the longest carbon chain that includes the carboxyl group and replacing the '-e' ending of the corresponding alkane with '-oic acid'. For example, the simplest carboxylic acid, methanoic acid, is derived from methane. This naming convention is part of the Cambridge IGCSE Chemistry syllabus, where students learn to systematically name organic compounds, ensuring clear communication in scientific contexts.

Why is "Treating ethanoic acid as a strong acid" a common mistake in Carboxylic Acids?

Why it happens: All acids look the same. How to avoid it: Carboxylic acids are WEAK — only partially ionised. Reactions slower than HCl. Source: 0620/42 — recurring

Why is "Calling the salt 'sodium acetic'" a common mistake in Carboxylic Acids?

Why it happens: Mixing common with IUPAC name. How to avoid it: IUPAC: 'ethanoate' (from ethanoic). 'Acetate' is the older common name.

Why is "Forgetting CO₂ in carbonate reaction" a common mistake in Carboxylic Acids?

Why it happens: Speed. How to avoid it: Carboxylic acid + carbonate → salt + water + CO₂. Three products.

Organic ChemistryCambridge IGCSE Chemistry (0620)

Carboxylic Acids

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Carboxylic Acids — Cambridge IGCSE 0620 Chemistry Extended (2026)

Functional group -COOH. Ethanoic acid (vinegar) is the famous one. Weak acids — react with metals, bases, carbonates. Plus esterification with alcohols.

What you’ll learn

Mapped to the Cambridge IGCSE 0620 syllabus (2026-2028).

13.7 — Describe carboxylic acids: structure, formula, naming.
13.7 — Describe reactions of carboxylic acids with metals, bases, carbonates.
13.7 — Describe esterification.

Structure and the -COOH group

-COOH = carbonyl + hydroxyl. The H ionises in water giving the acidic H⁺.

Carboxylic acid = compound containing a -COOH (carboxyl) group.

The -COOH group is:

C double-bonded to one O.
C single-bonded to another OH.

Drawn:

    O
    ||
R - C - O - H

The H attached to that second O is acidic — ionises in water.

The -COOH group: C=O plus O-H. The hydroxyl hydrogen is the acidic proton.

General formula: $\text{C}_{n}\text{H}_{2n+1}\text{COOH}$ (so a one-carbon acid, methanoic acid HCOOH, has $n = 0$ ).

First three.

Name	Structural	Common name
Methanoic acid	HCOOH	Formic acid
Ethanoic acid	CH₃COOH	Acetic acid
Propanoic acid	CH₃CH₂COOH	(none common)

Vinegar is dilute ethanoic acid in water (about $5\%$ ).

Properties.

Weak acid (partially ionises): $\text{CH}_{3}\text{COOH} \rightleftharpoons \text{CH}_{3}\text{COO}^{-} + \text{H}^{+}$ .
Soluble in water (small chains).
Corrosive in concentrated form.
Distinctive smell — vinegary / sharp.

Cambridge tip. Memorise both the IUPAC names AND common names. Cambridge usually accepts either, but you should recognise both.

-COOH functional group.
$\text{C}_{n}\text{H}_{2n+1}\text{COOH}$ .
Weak acid (partial ionisation).
Vinegar = $\sim 5\%$ ethanoic acid in water.
First three: methanoic, ethanoic, propanoic.

Reactions of carboxylic acids

Same four classic acid reactions as inorganic acids: with metals, bases, carbonates, ammonia.

Carboxylic acids react like other (weak) acids.

1. With reactive metals. $\text{CH}_{3}\text{COOH} + \text{Mg} \to \text{Mg(CH}_{3}\text{COO})_{2} + \text{H}_{2}.$

(Magnesium ethanoate forms; H₂ effervesces.)

2. With bases / alkalis. $\text{CH}_{3}\text{COOH} + \text{NaOH} \to \text{CH}_{3}\text{COONa} + \text{H}_{2}\text{O}.$

(Sodium ethanoate + water — a neutralisation.)

3. With carbonates. $2\text{CH}_{3}\text{COOH} + \text{Na}_{2}\text{CO}_{3} \to 2\text{CH}_{3}\text{COONa} + \text{H}_{2}\text{O} + \text{CO}_{2}.$

CO₂ effervesces — turns limewater cloudy.

4. With ammonia. $\text{CH}_{3}\text{COOH} + \text{NH}_{3} \to \text{CH}_{3}\text{COONH}_{4}$

(Ammonium ethanoate.)

Salt naming. Salt = anion from acid (here: ethanoate, CH₃COO⁻) + cation from base.

Worked qualitative. Why does adding vinegar to baking soda fizz? Vinegar (ethanoic acid) reacts with sodium hydrogencarbonate / sodium carbonate; CO₂ gas is released, fizzing.

Same four reactions as inorganic acids.
- metal → metal salt + H₂.
- base → salt + water.
- carbonate → salt + water + CO₂.
- ammonia → ammonium salt.
Anion: -oate (e.g. ethanoate from ethanoic acid).

Esterification — carboxylic acid + alcohol → ester

Acid + alcohol $\rightleftharpoons$ ester + water. Concentrated H₂SO₄ catalyst. Esters smell sweet.

Esterification. $\text{carboxylic acid} + \text{alcohol} \rightleftharpoons \text{ester} + \text{water}.$

The reaction is REVERSIBLE; conc. sulfuric acid catalyses it AND removes water (helps shift equilibrium toward ester).

Worked. Ethanoic acid + ethanol → ethyl ethanoate + water: $\text{CH}_{3}\text{COOH} + \text{C}_{2}\text{H}_{5}\text{OH} \rightleftharpoons \text{CH}_{3}\text{COOC}_{2}\text{H}_{5} + \text{H}_{2}\text{O}.$

Esterification is reversible; concentrated sulfuric acid catalyses it and absorbs the water produced.

Naming an ester. Two parts:

From the alcohol → "alkyl" prefix (e.g. ethyl-).
From the acid → "-oate" suffix replacing -oic acid (e.g. -anoate from -anoic acid).

Examples:

Methanol + ethanoic acid → METHYL ethanoate.
Ethanol + ethanoic acid → ETHYL ethanoate.
Ethanol + methanoic acid → ETHYL methanoate.

Properties of esters.

Sweet, fruity smell — banana, pear, pineapple, etc.
Used in flavourings, perfumes, solvents.
Insoluble in water (the polar -OH and -COOH have combined into the non-polar ester linkage -COO-).

Worked qualitative. Why does pear flavouring smell of pears? Esters in real pears (3-methylbutyl ethanoate) match the artificial ester used in pear-flavoured products.

Cambridge tip. Always write esterification with reversible arrows ( $\rightleftharpoons$ ). The reaction goes in both directions; concentrated H₂SO₄ shifts the equilibrium by absorbing water.

Acid + alcohol → ester + water (reversible).
Conc. H₂SO₄ catalyses.
Naming: alkyl-oate.
Esters: sweet smell; flavourings, perfumes.

How it’s examined

Carboxylic acids appear every Paper 4 (5-7 marks): structure, naming, reactions, esterification. Examiner reports flag students writing the salt name as just 'sodium' instead of 'sodium ethanoate', and missing concentrated H₂SO₄ as the catalyst for esterification.

Sources: Cambridge IGCSE Chemistry 0620 syllabus 2026-2028 (13.7); 0620/22 May/Jun 2024 — Q20 (carboxylic acids, esterification); 0620 Examiner Reports 2022-2024. Last reviewed 2026-05-07.

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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.

1Reactions of ethanoic acid
Extended• Adapted from 0620/42 May/Jun 2024 Q15• acid reactions
Question
Write equations for ethanoic acid reacting with (a) NaOH, (b) Na₂CO₃, (c) Mg.
Step-by-step solution
1. Step 1
  (a) Neutralisation:
  $\text{CH}_{3}\text{COOH} + \text{NaOH} \to \text{CH}_{3}\text{COONa} + \text{H}_{2}\text{O}$
2. Step 2
  (b) Carbonate:
  $2\text{CH}_{3}\text{COOH} + \text{Na}_{2}\text{CO}_{3} \to 2\text{CH}_{3}\text{COONa} + \text{H}_{2}\text{O} + \text{CO}_{2}$
3. Step 3
  (c) Reactive metal:
  $2\text{CH}_{3}\text{COOH} + \text{Mg} \to (\text{CH}_{3}\text{COO})_{2}\text{Mg} + \text{H}_{2}$
Answer
Forms ethanoate salts in each case, plus water (with NaOH), water+CO₂ (with carbonate), or H₂ (with metal).
2Make ethanoic acid by oxidation
Extended• oxidation
Question
Describe two routes to make ethanoic acid from ethanol.
Step-by-step solution
1. Step 1
  (a) Heat with acidified potassium dichromate ( $\text{K}_{2}\text{Cr}_{2}\text{O}_{7}$ ) — orange to green.
2. Step 2
  (b) Bacterial action (acetobacter) on ethanol exposed to air → vinegar.
Answer
Acidified $\text{K}_{2}\text{Cr}_{2}\text{O}_{7}$ + heat, OR bacterial fermentation in air.
3Why ethanoic acid is a weak acid
Extended• weak acid
Question
Compare $1\,\text{mol/dm}^{3}$ ethanoic acid with $1\,\text{mol/dm}^{3}$ HCl in terms of reactions and pH.
Step-by-step solution
1. Step 1
  Ethanoic acid PARTIALLY ionises → fewer $\text{H}^{+}$ ions in solution.
2. Step 2
  HCl FULLY ionises at the same concentration → more $\text{H}^{+}$ .
3. Step 3
  Ethanoic acid: higher pH, slower reactions.
Answer
Ethanoic acid: weak (partial ionisation) → higher pH and slower reactions than equal-concentration 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 0620 sitting.

Carboxylic acid
Examiner keyword
Organic compound containing $-\text{COOH}$ . Weak acid in water.
Ethanoate salt
Examiner keyword
Salt of ethanoic acid. e.g. sodium ethanoate ( $\text{CH}_{3}\text{COONa}$ ).
Vinegar
Examiner keyword
Dilute aqueous ethanoic acid (typically 4-7%).

Common Mistakes and Misconceptions — Carboxylic Acids

The traps other students keep falling into on carboxylic acids questions — taken from recent Cambridge IGCSE 0620 examiner reports and mark schemes — and how to avoid them.

✕Treating ethanoic acid as a strong acid
0620/42 — recurring
Why it happens
All acids look the same.
How to avoid it
Carboxylic acids are WEAK — only partially ionised. Reactions slower than HCl.
✕Calling the salt 'sodium acetic'
Why it happens
Mixing common with IUPAC name.
How to avoid it
IUPAC: 'ethanoate' (from ethanoic). 'Acetate' is the older common name.
✕Forgetting CO₂ in carbonate reaction
Why it happens
Speed.
How to avoid it
Carboxylic acid + carbonate → salt + water + CO₂. Three products.

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Carboxylic Acids — frequently asked questions

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Carboxylic Acids

Short Study Notes in the form of Slides

Detailed Notes

What you’ll learn

How it’s examined

1Reactions of ethanoic acid

2Make ethanoic acid by oxidation

3Why ethanoic acid is a weak acid

Carboxylic acid

Ethanoate salt

Vinegar

✕Treating ethanoic acid as a strong acid

✕Calling the salt 'sodium acetic'

✕Forgetting CO₂ in carbonate reaction

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Assess your understanding

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Carboxylic Acids — frequently asked questions