Strong and Weak Acids [H]

This lesson covers strong and weak acids, as specified in the AQA GCSE Chemistry specification (4.4.2 — Higher Tier only). You need to understand the difference between strong and weak acids in terms of ionisation, relate acid strength to pH and hydrogen ion concentration, and distinguish between acid strength and acid concentration. This is a Higher Tier topic, marked [H] on the specification, and will only appear on Higher Tier exam papers.

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Ionisation of Acids

When an acid dissolves in water, its molecules ionise (split apart) to release hydrogen ions (H+). The degree to which this happens determines whether the acid is strong or weak.

Strong Acids

A strong acid is one that is completely ionised (fully dissociated) in aqueous solution. Every molecule of the acid splits up to release H+ ions.

Examples of strong acids:

Strong Acid	Formula	Ionisation in water
Hydrochloric acid	HCl	HCl → H+ + Cl-
Sulfuric acid	H2SO4	H2SO4 → 2H+ + SO4 2-
Nitric acid	HNO3	HNO3 → H+ + NO3-

The arrow (→) indicates that the ionisation is complete and irreversible — all the acid molecules break apart.

Weak Acids

A weak acid is one that is only partially ionised (partially dissociated) in aqueous solution. Only a small fraction of the acid molecules release H+ ions. The rest remain as intact molecules.

Examples of weak acids:

Weak Acid	Formula	Ionisation in water
Ethanoic acid (in vinegar)	CH3COOH	CH3COOH ⇌ CH3COO- + H+
Citric acid	C6H8O7	Partial ionisation
Carbonic acid	H2CO3	H2CO3 ⇌ 2H+ + CO3 2-

The reversible arrow (⇌) indicates that the ionisation is partial and reversible — an equilibrium is established between the undissociated acid molecules and the ions.

Exam Tip: The key distinction is the arrow. Strong acids use a forward arrow (→) showing complete ionisation. Weak acids use a reversible arrow (⇌) showing partial ionisation and equilibrium. If asked to write an ionisation equation, choosing the correct arrow is essential for full marks.

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Comparing Strong and Weak Acids

graph TD
    A["Acid dissolved<br/>in water"] --> B{"Degree of<br/>ionisation?"}
    B -->|"100% ionised<br/>(complete)"| C["STRONG ACID"]
    B -->|"Partially ionised<br/>(equilibrium)"| D["WEAK ACID"]
    C --> E["High concentration<br/>of H+ ions"]
    D --> F["Low concentration<br/>of H+ ions"]
    E --> G["Lower pH<br/>(e.g. pH 1)"]
    F --> H["Higher pH<br/>(e.g. pH 3-5)"]
    E --> I["Reacts faster<br/>with metals/carbonates"]
    F --> J["Reacts slower<br/>with metals/carbonates"]

    style C fill:#e74c3c,color:#fff
    style D fill:#f39c12,color:#fff
    style G fill:#c0392b,color:#fff
    style H fill:#e67e22,color:#fff

Property	Strong Acid (e.g. HCl)	Weak Acid (e.g. CH3COOH)
Ionisation	Complete (100%)	Partial (small %)
Arrow in equation	→ (forward only)	⇌ (reversible)
H+ ion concentration	High (for a given concentration)	Low (for the same concentration)
pH (at same concentration)	Lower (e.g. 1)	Higher (e.g. 3)
Rate of reaction (with Mg)	Faster (vigorous fizzing)	Slower (gentle fizzing)
Electrical conductivity	Higher (more ions)	Lower (fewer ions)
Volume of gas produced (with excess Mg)	Same as weak acid at same concentration	Same as strong acid at same concentration

Important Distinction: Same Volume of Gas

Although a strong acid reacts faster than a weak acid of the same concentration, the total volume of gas produced is the same (assuming excess metal is used). This is because both solutions contain the same total number of acid molecules — the weak acid just releases its H+ ions more slowly as the equilibrium shifts.

Exam Tip: Students often confuse acid STRENGTH with acid CONCENTRATION. They are completely different things. Strength refers to how completely the acid ionises (strong = fully ionised, weak = partially ionised). Concentration refers to how many moles of acid are dissolved per unit volume. A concentrated weak acid and a dilute strong acid are both perfectly valid concepts.

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Strength vs Concentration

Term	Definition	Determined by
Acid strength	How completely the acid ionises in water	The type of acid (its chemical identity)
Acid concentration	The amount of acid (in moles) dissolved per unit volume of solution (mol/dm3)	How much acid is dissolved in how much water

Examples to Illustrate

• Concentrated ethanoic acid — lots of ethanoic acid dissolved in a small volume, but still a weak acid (only partially ionised).
• Dilute hydrochloric acid — a small amount of HCl dissolved in a large volume, but still a strong acid (fully ionised).
• A concentrated weak acid may have a similar pH to a dilute strong acid, but for different reasons.

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pH and Hydrogen Ion Concentration [H]

The pH of a solution is related to the concentration of hydrogen ions:

• As the concentration of H+ ions increases, the pH decreases.
• As the concentration of H+ ions decreases, the pH increases.
• Each unit change in pH represents a tenfold change in H+ ion concentration.

pH	Relative H+ concentration
1	10,000 times more H+ than pH 5
2	1,000 times more H+ than pH 5
3	100 times more H+ than pH 5
4	10 times more H+ than pH 5
5	Baseline (1x)

Comparing Strong and Weak Acids at the Same Concentration

If you have 0.1 mol/dm3 hydrochloric acid (strong) and 0.1 mol/dm3 ethanoic acid (weak):

• The HCl has pH 1 (fully ionised, high H+ concentration)
• The ethanoic acid has pH approximately 3 (partially ionised, lower H+ concentration)
• The HCl has approximately 100 times more H+ ions than the ethanoic acid

Exam Tip: If a question gives you two acids at the same concentration and asks you to compare their pH values, the strong acid will always have the lower pH (more H+ ions). If asked to explain, state that the strong acid is fully ionised so produces more H+ ions, while the weak acid is only partially ionised so produces fewer H+ ions.

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Effect of Dilution on pH

When an acid is diluted (water is added):

• The concentration of H+ ions decreases
• The pH increases (moves towards 7)
• For every tenfold dilution, the pH increases by approximately 1 unit (for strong acids)