What you'll learn
This revision guide covers everything you need to know about strong and weak acids for AQA GCSE Chemistry. You'll learn how to distinguish between acid strength and concentration, understand ionisation in aqueous solutions, and apply this knowledge to pH calculations and reactions. These concepts appear regularly in both Paper 1 and Paper 2, often worth 4-6 marks in extended response questions.
Key terms and definitions
Strong acid — an acid that completely ionises (dissociates) in aqueous solution, releasing all its hydrogen ions into the solution.
Weak acid — an acid that only partially ionises in aqueous solution, establishing an equilibrium between undissociated molecules and ions.
Ionisation — the process by which an acid releases hydrogen ions (H⁺) when dissolved in water to form an aqueous solution.
pH — a measure of the hydrogen ion concentration in a solution, calculated as pH = -log₁₀[H⁺], where a lower pH indicates higher acidity.
Concentration — the amount of acid dissolved in a given volume of solution, typically measured in mol/dm³ or g/dm³.
Dissociation — the breaking apart of an acid molecule into ions when dissolved in water (used interchangeably with ionisation at GCSE level).
Aqueous solution — a solution where water is the solvent, essential for acid behaviour as ionisation requires water molecules.
Reversible reaction — a chemical reaction that can proceed in both forward and reverse directions, indicated by the ⇌ symbol, characteristic of weak acid ionisation.
Core concepts
Strong acids vs weak acids
The fundamental difference between strong and weak acids lies in their degree of ionisation in water, not their concentration.
Strong acids completely ionise in aqueous solution. Every molecule of a strong acid releases its hydrogen ion(s):
- Hydrochloric acid: HCl → H⁺ + Cl⁻
- Sulfuric acid: H₂SO₄ → 2H⁺ + SO₄²⁻
- Nitric acid: HNO₃ → H⁺ + NO₃⁻
The arrow (→) shows complete, irreversible ionisation. In a solution of hydrochloric acid, virtually no HCl molecules remain—only H⁺ and Cl⁻ ions exist.
Weak acids only partially ionise in aqueous solution. Most molecules remain intact, with only a small percentage releasing hydrogen ions:
- Ethanoic acid: CH₃COOH ⇌ H⁺ + CH₃COO⁻
- Citric acid (found in citrus fruits)
- Carbonic acid: H₂CO₃ ⇌ H⁺ + HCO₃⁻
The reversible arrow (⇌) indicates equilibrium. In a solution of ethanoic acid, most molecules remain as CH₃COOH, with only approximately 1% ionised at typical concentrations.
Concentration vs strength
Students frequently confuse these two distinct properties:
Acid strength refers to the proportion of acid molecules that ionise:
- Strong acids: 100% ionisation
- Weak acids: typically <5% ionisation
Acid concentration refers to the number of moles of acid per unit volume:
- Concentrated: high number of moles per dm³
- Dilute: low number of moles per dm³
You can have:
- Concentrated strong acid (e.g., 10 mol/dm³ HCl)
- Dilute strong acid (e.g., 0.01 mol/dm³ HCl)
- Concentrated weak acid (e.g., 10 mol/dm³ CH₃COOH)
- Dilute weak acid (e.g., 0.01 mol/dm³ CH₃COOH)
A dilute strong acid can have a lower pH than a concentrated weak acid because pH depends on hydrogen ion concentration, which depends on both the amount of acid present and how much of it ionises.
pH and hydrogen ion concentration
The pH scale measures acidity from 0 to 14, with acids having pH values below 7. The relationship between pH and hydrogen ion concentration is logarithmic.
For strong acids, you can calculate hydrogen ion concentration directly from the acid concentration because ionisation is complete:
- 1 mol/dm³ HCl produces [H⁺] = 1 mol/dm³
- 0.1 mol/dm³ HCl produces [H⁺] = 0.1 mol/dm³
- 0.01 mol/dm³ HCl produces [H⁺] = 0.01 mol/dm³
Key pattern for strong monoprotic acids: When you dilute a strong acid by a factor of 10, the hydrogen ion concentration decreases by a factor of 10, and the pH increases by 1 unit.
Examples:
- [H⁺] = 1 mol/dm³ → pH = 0
- [H⁺] = 0.1 mol/dm³ → pH = 1
- [H⁺] = 0.01 mol/dm³ → pH = 2
- [H⁺] = 0.001 mol/dm³ → pH = 3
For weak acids, the hydrogen ion concentration is much lower than the acid concentration because only partial ionisation occurs. A 1 mol/dm³ solution of ethanoic acid typically has a pH around 2.4 (not 0), indicating [H⁺] ≈ 0.004 mol/dm³.
Reactions of acids
Both strong and weak acids undergo the same types of reactions, but weak acids react more slowly because they have fewer hydrogen ions available in solution at any given moment.
Reaction with metals:
- Acid + Metal → Salt + Hydrogen
- 2HCl + Mg → MgCl₂ + H₂
- 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂
Magnesium reacts vigorously with dilute hydrochloric acid (strong) but reacts more slowly with ethanoic acid of the same concentration (weak), producing fewer bubbles per second initially.
Reaction with metal oxides:
- Acid + Metal oxide → Salt + Water
- H₂SO₄ + CuO → CuSO₄ + H₂O
- 2CH₃COOH + CuO → (CH₃COO)₂Cu + H₂O
Reaction with metal carbonates:
- Acid + Metal carbonate → Salt + Water + Carbon dioxide
- 2HNO₃ + CaCO₃ → Ca(NO₃)₂ + H₂O + CO₂
- 2CH₃COOH + CaCO₃ → (CH₃COO)₂Ca + H₂O + CO₂
The slower reaction rate of weak acids occurs because as H⁺ ions are used up, the equilibrium shifts to produce more, but this takes time. Strong acids have all their hydrogen ions immediately available.
Electrical conductivity
Aqueous acid solutions conduct electricity because they contain mobile ions. The conductivity provides evidence for ionisation.
Strong acid solutions conduct electricity well because:
- Complete ionisation produces a high concentration of ions
- More charge carriers means better conductivity
- 1 mol/dm³ HCl contains 1 mol/dm³ H⁺ ions and 1 mol/dm³ Cl⁻ ions
Weak acid solutions conduct electricity poorly because:
- Partial ionisation produces a low concentration of ions
- Fewer charge carriers means lower conductivity
- 1 mol/dm³ CH₃COOH contains only ~0.004 mol/dm³ H⁺ ions and ~0.004 mol/dm³ CH₃COO⁻ ions
Testing equal concentrations of hydrochloric acid and ethanoic acid with a simple circuit (battery, bulb, electrodes) shows a brighter bulb with the strong acid, demonstrating higher ion concentration.
Equilibrium in weak acids
Weak acid ionisation establishes a dynamic equilibrium:
CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq)
At equilibrium:
- The forward reaction (ionisation) and backward reaction (recombination) occur at equal rates
- The concentrations of all species remain constant
- Most acid molecules remain undissociated
- The position of equilibrium lies far to the left
When weak acids react with metals or carbonates, H⁺ ions are removed from solution. Le Chatelier's principle states that the equilibrium shifts right to replace them, allowing the reaction to continue until all the metal/carbonate is consumed. This explains why weak acids eventually produce the same amount of product as strong acids (when in excess) despite reacting more slowly.
Worked examples
Example 1: Comparing acid strength and concentration
Question: A student has four solutions:
- Solution A: 2 mol/dm³ hydrochloric acid
- Solution B: 0.1 mol/dm³ hydrochloric acid
- Solution C: 2 mol/dm³ ethanoic acid
- Solution D: 0.1 mol/dm³ ethanoic acid
a) Which solution contains the highest concentration of hydrogen ions? [1 mark] b) Which two solutions would react with magnesium at the same rate? [1 mark] c) Explain why Solution B has a lower pH than Solution D. [3 marks]
Answer:
a) Solution A [1 mark]
b) Solutions B and D [1 mark] (Both have the same hydrogen ion concentration despite different total acid concentrations—B is dilute strong, D is concentrated weak, but D only ~1% ionised)
Actually, this is incorrect. Solution B would have higher [H⁺] and react faster.
Correction for b): No two solutions react at exactly the same rate. Solution B reacts faster than D because HCl is fully ionised. [Accept: "None" with correct explanation]
c) Solution B contains hydrochloric acid, which is a strong acid that completely ionises [1 mark]. Solution D contains ethanoic acid, which is a weak acid that only partially ionises [1 mark]. Therefore Solution B has a higher concentration of hydrogen ions, giving it a lower pH [1 mark].
Example 2: pH calculations
Question: A student dilutes 100 cm³ of hydrochloric acid with a pH of 1 by adding 900 cm³ of water.
a) What is the hydrogen ion concentration in the original acid? [1 mark] b) Calculate the hydrogen ion concentration after dilution. [2 marks] c) State the pH of the diluted acid. [1 mark]
Answer:
a) [H⁺] = 0.1 mol/dm³ [1 mark] (pH 1 corresponds to [H⁺] = 10⁻¹ = 0.1 mol/dm³)
b) Volume increases from 100 cm³ to 1000 cm³ = 10× dilution [1 mark] New [H⁺] = 0.1 ÷ 10 = 0.01 mol/dm³ [1 mark]
c) pH = 2 [1 mark] ([H⁺] = 0.01 = 10⁻², so pH = 2)
Example 3: Explaining observations
Question: A student adds excess magnesium ribbon to two beakers, each containing 50 cm³ of 1 mol/dm³ acid. Beaker 1 contains hydrochloric acid. Beaker 2 contains ethanoic acid.
a) Describe what the student would observe in each beaker. [2 marks] b) Explain the difference in the rate of reaction in terms of ionisation. [3 marks] c) The student measures the total volume of hydrogen gas produced in each beaker. Compare the volumes and explain your answer. [2 marks]
Answer:
a) Beaker 1: vigorous/fast bubbling, magnesium dissolves quickly [1 mark] Beaker 2: slower/less vigorous bubbling, magnesium dissolves more slowly [1 mark]
b) Hydrochloric acid is a strong acid that completely ionises [1 mark], providing a high concentration of hydrogen ions available to react immediately [1 mark]. Ethanoic acid is a weak acid that only partially ionises, so has a lower concentration of hydrogen ions available at any moment, making the initial reaction slower [1 mark].
c) The same volume of hydrogen is produced in both beakers [1 mark] because both acids are in excess, so all the magnesium reacts, and the stoichiometry is the same regardless of acid strength [1 mark].
Common mistakes and how to avoid them
Confusing strength with concentration. Remember: strength is about percentage ionisation (strong vs weak), concentration is about moles per volume (concentrated vs dilute). A dilute strong acid is still a strong acid.
Thinking weak acids produce less product. Weak acids produce the same amount of product as strong acids when in excess—they just take longer because the reaction rate is slower due to lower hydrogen ion concentration at any instant.
Incorrectly writing weak acid equations. Always use the reversible arrow (⇌) for weak acids, never a forward arrow (→). This is a common mark-losing error.
Applying pH calculations to weak acids. At GCSE, you cannot directly calculate weak acid pH from concentration because you don't know the degree of ionisation. Only apply pH = -log₁₀[H⁺] calculations to strong acids unless additional information is given.
Forgetting that ionisation requires water. Acids only show acidic properties in aqueous solution. Pure (anhydrous) hydrochloric acid gas or ethanoic acid liquid don't conduct electricity or have low pH—they must dissolve in water first.
Misunderstanding electrical conductivity tests. Lower conductivity in weak acids doesn't mean fewer ions per molecule—it means fewer molecules have ionised, so overall ion concentration is lower despite the same starting concentration.
Exam technique for "Chemical changes: strong and weak acids"
Command words matter. "Describe" requires observations (what you see/measure). "Explain" requires reasons using scientific principles (ionisation, hydrogen ion concentration). "Compare" needs statements about both items with explicit comparative language (higher/lower, faster/slower).
Extended response questions on this topic typically reward: defining strong/weak (2 marks), explaining ionisation/equilibrium (2-3 marks), linking to pH or reaction rate (2 marks). Use the reversible arrow symbol for weak acids to secure a mark.
Practical context questions often involve comparing reaction rates or electrical conductivity. Structure answers: state the observation → explain using ionisation → link to hydrogen ion concentration. This three-step approach secures maximum marks.
Calculation questions appear infrequently but focus on pH-concentration relationships for strong acids. Remember the factor of 10 rule: dilute 10×, pH increases by 1. Show working for method marks even if your final answer is wrong.
Quick revision summary
Strong acids (HCl, H₂SO₄, HNO₃) completely ionise in water; weak acids (CH₃COOH, carbonic acid) only partially ionise, establishing equilibrium. Acid strength (percentage ionisation) differs from concentration (moles per volume). Strong acids have higher hydrogen ion concentrations at equal concentrations, giving lower pH, faster reactions, and better electrical conductivity. Weak acids eventually produce the same amount of product when in excess. For strong acids, 10× dilution increases pH by 1 unit. Always use ⇌ for weak acid equations.