What are the characteristic properties of acids according to the Cambridge IGCSE Coordinated Science syllabus?

In the Cambridge IGCSE Coordinated Science syllabus, acids are characterized by their ability to donate protons (H+ ions) in aqueous solutions. They typically have a sour taste, turn blue litmus paper red, and react with metals to produce hydrogen gas. Acids also react with bases to form salts and water in a neutralization reaction.

How do bases differ from acids in terms of their characteristic properties?

Bases differ from acids in that they accept protons (H+ ions) in aqueous solutions. They usually have a bitter taste and a slippery feel. Bases turn red litmus paper blue and can neutralize acids to form salts and water. Common examples include metal oxides and hydroxides, such as sodium hydroxide and calcium oxide.

Why do acids and bases change the color of litmus paper?

Acids and bases change the color of litmus paper due to their effect on the pH level of a solution. Acids increase the concentration of hydrogen ions (H+), turning blue litmus paper red, while bases increase the concentration of hydroxide ions (OH-), turning red litmus paper blue. This color change is a result of the chemical reaction between the litmus dye and the ions in the solution.

What is the role of the pH scale in distinguishing between acids and bases?

The pH scale is a numerical scale used to specify the acidity or basicity of an aqueous solution. It ranges from 0 to 14, with 7 being neutral. Acids have a pH less than 7, indicating a higher concentration of hydrogen ions, while bases have a pH greater than 7, indicating a higher concentration of hydroxide ions. The pH scale is crucial for identifying the strength and concentration of acids and bases in the Cambridge IGCSE Coordinated Science curriculum.

Can you explain the concept of neutralization in the context of acids and bases?

Neutralization is a chemical reaction in which an acid and a base react to form a salt and water. This process involves the combination of hydrogen ions (H+) from the acid and hydroxide ions (OH-) from the base to produce water (H2O). The resulting solution tends to have a pH closer to 7, indicating a neutral state. Neutralization is a key concept in the study of acids, bases, and salts in the Cambridge IGCSE Coordinated Science syllabus.

Why is "Confusing 'strong' with 'concentrated' for acids" a common mistake in The Characteristic Properties of Acids and Bases?

Why it happens: Students use the everyday meaning of 'strong' to mean 'a lot of acid'. How to avoid it: Strong/weak = degree of ionisation (chemistry concept). Concentrated/dilute = amount of substance per volume. HCl is always strong; ethanoic acid is always weak regardless of concentration. Source: 0654 Examiner Report 2022

Why is "Adding the indicator after some acid has already been added" a common mistake in The Characteristic Properties of Acids and Bases?

Why it happens: Students forget to add the indicator before starting the titration. How to avoid it: Add the indicator to the conical flask before adding any acid from the burette. The indicator must be present from the start to detect the endpoint.

Why is "Averaging all titration results including the rough titre" a common mistake in The Characteristic Properties of Acids and Bases?

Why it happens: Students include the first (rough) titre in their average, which may be far from the true value. How to avoid it: The rough titre identifies the approximate endpoint. Only average the concordant (accurate) titres — those within 0.10 cm³ of each other.

Acids, Bases and SaltsCambridge IGCSE Coordinated Sciences 0654

The Characteristic Properties of Acids and Bases

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Acids and Bases — Cambridge IGCSE Coordinated Science 0654

Acids donate H⁺ ions; bases accept them. Cambridge tests acid reactions (with metals, carbonates, oxides, bases), pH, indicators, strong vs weak acids, and the Brønsted-Lowry definition.

What you’ll learn

Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).

C8.1 — Define acid and base in terms of H⁺ ions (Arrhenius) and proton transfer (Brønsted-Lowry).
C8.2 — Describe properties and reactions of acids.
C8.3 — Distinguish strong acids from weak acids.
C8.4 — Use pH and indicators to classify solutions.

Acids, Bases, and Alkalis — Definitions

Acids donate H⁺; bases accept H⁺. Alkalis are water-soluble bases that produce OH⁻ in solution.

Arrhenius definition:

Acid: produces H⁺ ions when dissolved in water
Base: produces OH⁻ ions when dissolved in water (alkali) OR accepts H⁺ ions

Brønsted-Lowry definition (more general):

Acid: proton (H⁺) donor
Base: proton (H⁺) acceptor

Alkali: a base that is SOLUBLE in water and produces OH⁻ ions in solution. (All alkalis are bases, but not all bases are alkalis — insoluble bases like CuO cannot form alkalis.)

Common acids:

Hydrochloric acid (HCl) — monobasic, strong
Sulfuric acid (H₂SO₄) — dibasic, strong
Nitric acid (HNO₃) — monobasic, strong
Ethanoic acid (CH₃COOH) — monobasic, weak
Carbonic acid (H₂CO₃) — dibasic, weak

Common alkalis: NaOH, KOH, Ca(OH)₂, NH₃(aq)

Strong vs weak acids:

	Strong acid	Weak acid
Ionisation	Complete (fully dissociated)	Partial (equilibrium)
pH (same concentration)	Lower (more H⁺)	Higher (fewer H⁺)
Examples	HCl, H₂SO₄, HNO₃	CH₃COOH, H₂CO₃, citric acid
Reactivity	More reactive	Less reactive

Acid: H⁺ donor (Brønsted-Lowry). Alkali: soluble base, produces OH⁻.
Strong acid: fully ionised. Weak acid: partially ionised. Same concentration → strong is more acidic.
HCl, H₂SO₄, HNO₃ = strong. CH₃COOH = weak.

Reactions of Acids

Acids react with metals, metal oxides, metal carbonates, and alkalis — always giving a salt as a product.

1. Acid + Metal → Salt + Hydrogen

Zn + H₂SO₄ → ZnSO₄ + H₂↑ Mg + 2HCl → MgCl₂ + H₂↑

Test for H₂: burns with a squeaky pop with a lit splint

2. Acid + Metal Oxide → Salt + Water

CuO + H₂SO₄ → CuSO₄ + H₂O ZnO + 2HCl → ZnCl₂ + H₂O

Metal oxide is a base

3. Acid + Metal Carbonate → Salt + Water + Carbon Dioxide

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑ Na₂CO₃ + H₂SO₄ → Na₂SO₄ + H₂O + CO₂↑

Test for CO₂: turns limewater milky

4. Acid + Alkali → Salt + Water (Neutralisation)

NaOH + HCl → NaCl + H₂O 2KOH + H₂SO₄ → K₂SO₄ + 2H₂O

Ionic equation for all neutralisations: H⁺(aq) + OH⁻(aq) → H₂O(l)

Naming salts:

HCl → chloride salts (NaCl, CaCl₂)
H₂SO₄ → sulfate salts (Na₂SO₄, CuSO₄)
HNO₃ → nitrate salts (KNO₃, Ca(NO₃)₂)
H₃PO₄ → phosphate salts

Acid + metal → salt + H₂. Acid + metal oxide → salt + H₂O.
Acid + carbonate → salt + H₂O + CO₂. Acid + alkali → salt + H₂O (neutralisation).
Salt name: metal from base/carbonate + anion from acid.

pH and Indicators

pH measures H⁺ concentration. Indicators change colour at different pH values — used to identify acids, bases, and end-points in titrations.

pH scale: 0–14

pH 0–6: acidic (lower = stronger acid, more H⁺)
pH 7: neutral
pH 8–14: alkaline (higher = stronger alkali, more OH⁻)

Each unit of pH = 10× change in H⁺ concentration:

pH 3 has 10× more H⁺ than pH 4 and 100× more than pH 5

Indicators:

Indicator	Colour in acid	Colour in neutral	Colour in alkali
Litmus	Red	Purple	Blue
Universal indicator	Red → orange → yellow	Green	Blue → violet
Phenolphthalein	Colourless	Colourless	Pink
Methyl orange	Red	Orange	Yellow

In titrations:

Phenolphthalein: good for strong acid–strong base and strong acid–weak base titrations (sharp colour change at endpoint)
Methyl orange: good for strong acid–strong base titrations
Litmus: poor for titrations (gradual colour change, difficult to detect endpoint precisely)

Universal indicator colours map directly onto pH: red at strong acid, green at neutral, deep blue at strong alkali.

pH < 7: acid. pH 7: neutral. pH > 7: alkali.
Each pH unit = 10× change in H⁺ concentration.
Phenolphthalein: colourless in acid, pink in alkali (used in titrations).

How it’s examined

Paper 4: 'Write the equation for the reaction between zinc carbonate and dilute hydrochloric acid' (2 marks — balanced equation with state symbols). 'Explain why hydrochloric acid is a stronger acid than ethanoic acid of the same concentration' (2 marks — HCl fully ionises, more H⁺, lower pH). 'State the pH of a 0.1 mol/dm³ NaOH solution' (1 mark — above 7, approximately 13). 'Name a suitable indicator for this titration and state its colour at the endpoint' (2 marks). Acid reactions are tested in almost every paper.

Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (C8); 0654 Examiner Reports 2022-2024. Last reviewed 2026-05-14.

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Step-by-step worked examples — The Characteristic Properties of Acids and Bases

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

1Reaction of zinc with hydrochloric acid
Core
Question
State what is observed when excess zinc is added to dilute hydrochloric acid, and write the balanced equation.
Step-by-step solution
1. Step 1
  Bubbles of hydrogen gas are produced (the gas makes a squeaky pop with a burning splint).
2. Step 2
  Zinc dissolves/disappears from the surface as the reaction proceeds.
3. Step 3
  The solution warms up (exothermic reaction).
  $\text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2$
Answer
Observations: bubbles of gas (H₂), zinc dissolves, solution warms. Equation: $\text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2$
2Titration calculation — concentration of NaOH
Extended
Question
In a titration, $25.0\,\text{cm}^3$ of NaOH solution is exactly neutralised by $20.0\,\text{cm}^3$ of $0.100\,\text{mol/dm}^3$ HCl. Calculate the concentration of the NaOH solution.
Step-by-step solution
1. Step 1
  Calculate moles of HCl: n = cV (V in dm³).
  $n(\text{HCl}) = 0.100 \times \frac{20.0}{1000} = 0.100 \times 0.0200 = 2.00 \times 10^{-3}\,\text{mol}$
2. Step 2
  Mole ratio NaOH : HCl = 1 : 1 from the equation NaOH + HCl → NaCl + H₂O.
  $n(\text{NaOH}) = 2.00 \times 10^{-3}\,\text{mol}$
3. Step 3
  Calculate concentration of NaOH.
  $c(\text{NaOH}) = \frac{n}{V} = \frac{2.00 \times 10^{-3}}{0.0250} = 0.0800\,\text{mol/dm}^3$
Answer
$c(\text{NaOH}) = 0.0800\,\text{mol/dm}^3$
Examiner tip
Show the mole ratio step explicitly — it earns a mark even if you get the final answer wrong.
3Strong vs weak acid
Extended
Question
Explain the difference between a strong acid and a weak acid and give one example of each.
Step-by-step solution
1. Step 1
  A strong acid completely dissociates into ions in aqueous solution. Every molecule ionises.
  $\text{HCl} \rightarrow \text{H}^+(\text{aq}) + \text{Cl}^-(\text{aq})$
2. Step 2
  A weak acid only partially dissociates; an equilibrium exists between ionised and un-ionised forms.
  $\text{CH}_3\text{COOH} \rightleftharpoons \text{CH}_3\text{COO}^- + \text{H}^+$
3. Step 3
  At the same concentration, a strong acid has a lower pH (more H⁺) than a weak acid.
Answer
Strong acid (e.g. HCl): fully dissociates → more H⁺ → lower pH. Weak acid (e.g. ethanoic): partially dissociates → equilibrium → higher pH at same concentration.
Examiner tip
Strong/weak refers to degree of ionisation, NOT concentration. A dilute strong acid and a concentrated weak acid can have similar pH values.
4Titration procedure
Extended
Question
Describe how to carry out a titration to find the exact volume of $0.100\,\text{mol/dm}^3$ HCl needed to neutralise $25\,\text{cm}^3$ of NaOH solution.
Step-by-step solution
1. Step 1
  Rinse the burette with distilled water then with the HCl solution. Fill the burette with HCl and record the initial reading.
2. Step 2
  Use a pipette to transfer exactly 25 cm³ of NaOH solution into a clean conical flask. Add 2–3 drops of a suitable indicator (e.g. phenolphthalein or methyl orange).
3. Step 3
  Add HCl from the burette slowly, swirling the flask continuously. Near the endpoint, add HCl drop by drop.
4. Step 4
  Stop when the indicator permanently changes colour (endpoint). Record the final burette reading. Titre = final − initial reading.
5. Step 5
  Repeat the titration until concordant results (within 0.10 cm³) are obtained. Average the concordant titres.
Answer
Pipette 25 cm³ NaOH into flask + indicator; add HCl from burette slowly; stop at colour change; record titre; repeat for concordant results.

Key Formulae — The Characteristic Properties of Acids and Bases

The formulae you need to memorise for the characteristic properties of acids and bases on the Cambridge IGCSE 0654 paper, with every variable defined in plain English and a note on when to use it.

Ionic equation for neutralisation
$\text{H}^+(\text{aq}) + \text{OH}^-(\text{aq}) \rightarrow \text{H}_2\text{O}(l)$
$\text{H}^+$
hydrogen ion from acid
$\text{OH}^-$
hydroxide ion from base/alkali
When to use
Writing the net ionic equation for any acid–alkali neutralisation reaction.

Key Definitions and Keywords — The Characteristic Properties of Acids and Bases

Definitions to memorise and the exact keywords mark schemes credit for the characteristic properties of acids and bases answers — sharpened from recent examiner reports for the 2026 0654 sitting.

Acid
Examiner keyword
A substance that produces hydrogen ions (H⁺) in aqueous solution; has a pH less than 7; turns blue litmus red.
Base
Examiner keyword
A substance that reacts with an acid to form a salt and water only (neutralisation); usually a metal oxide or metal hydroxide.
Alkali
Examiner keyword
A soluble base that produces hydroxide ions (OH⁻) in aqueous solution; has a pH greater than 7; turns red litmus blue.
Neutralisation
Examiner keyword
The reaction between an acid and a base (or alkali) to form a salt and water; the net ionic equation is $\text{H}^+ + \text{OH}^- \rightarrow \text{H}_2\text{O}$ .
Strong acid
Examiner keyword
An acid that completely dissociates into ions in aqueous solution (e.g. HCl, H₂SO₄, HNO₃). At the same concentration, it has a lower pH than a weak acid.
Weak acid
Examiner keyword
An acid that only partially dissociates in aqueous solution; an equilibrium exists between ionised and un-ionised forms (e.g. ethanoic acid, citric acid).

Common Mistakes and Misconceptions — The Characteristic Properties of Acids and Bases

The traps other students keep falling into on the characteristic properties of acids and bases questions — taken from recent Cambridge IGCSE 0654 examiner reports and mark schemes — and how to avoid them.

✕Confusing 'strong' with 'concentrated' for acids
0654 Examiner Report 2022
Why it happens
Students use the everyday meaning of 'strong' to mean 'a lot of acid'.
How to avoid it
Strong/weak = degree of ionisation (chemistry concept). Concentrated/dilute = amount of substance per volume. HCl is always strong; ethanoic acid is always weak regardless of concentration.
✕Adding the indicator after some acid has already been added
Why it happens
Students forget to add the indicator before starting the titration.
How to avoid it
Add the indicator to the conical flask before adding any acid from the burette. The indicator must be present from the start to detect the endpoint.
✕Averaging all titration results including the rough titre
Why it happens
Students include the first (rough) titre in their average, which may be far from the true value.
How to avoid it
The rough titre identifies the approximate endpoint. Only average the concordant (accurate) titres — those within 0.10 cm³ of each other.

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