Strong and Weak Acids and pH
What Is an Acid?
An acid is a substance that produces hydrogen ions (H⁺) when dissolved in water. It is these H⁺ ions that give acids their characteristic properties.
More precisely, H⁺ ions in water exist as hydronium ions (H₃O⁺), but at GCSE we simply write H⁺(aq).
The pH Scale
The pH scale runs from 0 to 14 and measures how acidic or alkaline a solution is:
| pH | Nature |
|---|---|
| 0–3 | Strong acid |
| 4–6 | Weak acid |
| 7 | Neutral |
| 8–10 | Weak alkali |
| 11–14 | Strong alkali |
- pH is measured using universal indicator (gives a range of colours) or a pH meter (more precise)
- Each whole number change on the pH scale represents a ×10 change in H⁺ ion concentration
So pH 3 has 10 times more H⁺ ions than pH 4, and 100 times more than pH 5.
Strong Acids vs Weak Acids
Strong Acids
A strong acid is one that completely ionises (dissociates) in water. Every molecule of the acid breaks apart to release H⁺ ions.
Examples:
- Hydrochloric acid: HCl(aq) → H⁺(aq) + Cl⁻(aq)
- Sulfuric acid: H₂SO₄(aq) → 2H⁺(aq) + SO₄²⁻(aq)
- Nitric acid: HNO₃(aq) → H⁺(aq) + NO₃⁻(aq)
The single arrow (→) shows the reaction goes to completion — all molecules ionise.
Weak Acids
A weak acid only partially ionises in water. Only a small fraction of the molecules dissociate to release H⁺ ions at any given time. The rest remain as whole molecules.
Examples:
- Ethanoic acid (in vinegar): CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq)
- Citric acid (in citrus fruits)
- Carbonic acid (CO₂ dissolved in water)
The double arrow (⇌) shows the reaction is reversible — an equilibrium is set up where most molecules remain un-ionised.
Important Distinction
Strong/weak is NOT the same as concentrated/dilute.
- Strong/weak describes how completely the acid ionises
- Concentrated/dilute describes how much acid is dissolved in water
A dilute strong acid (e.g. 0.01 mol/dm³ HCl) is fully ionised but has few H⁺ ions per dm³ because there is little acid present.
A concentrated weak acid (e.g. 10 mol/dm³ ethanoic acid) has many molecules but only a fraction ionise.
At the same concentration, a strong acid:
- Has a lower pH (more acidic)
- Has a higher concentration of H⁺ ions
- Reacts faster (more H⁺ ions available to react)
- But produces the same total volume of gas or product (same number of moles of acid overall)
Reactions of Acids
With Metals
acid + metal → salt + hydrogen
2HCl(aq) + Mg(s) → MgCl₂(aq) + H₂(g)
With Metal Oxides (Bases)
acid + metal oxide → salt + water
2HCl(aq) + CuO(s) → CuCl₂(aq) + H₂O(l)
With Metal Hydroxides (Alkalis)
acid + metal hydroxide → salt + water
HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
This is a neutralisation reaction. The ionic equation is:
H⁺(aq) + OH⁻(aq) → H₂O(l)
With Metal Carbonates
acid + metal carbonate → salt + water + carbon dioxide
2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g)
Naming Salts
The salt formed depends on the metal/base and the acid used:
| Acid | Salt type |
|---|---|
| Hydrochloric acid (HCl) | Chloride |
| Sulfuric acid (H₂SO₄) | Sulfate |
| Nitric acid (HNO₃) | Nitrate |
The first part of the salt name comes from the metal or base:
- NaOH + HCl → sodium chloride
- MgO + H₂SO₄ → magnesium sulfate
- CaCO₃ + HNO₃ → calcium nitrate
Required Practical: Making a Soluble Salt
To make a pure, dry sample of a soluble salt (e.g. copper sulfate from copper oxide and sulfuric acid):
1. Warm the acid gently in a beaker
2. Add the insoluble base (e.g. CuO) a spatula at a time, stirring, until in excess (some remains undissolved)
3. Filter to remove excess solid
4. Pour the filtrate into an evaporating basin
5. Heat gently to evaporate some water, then leave to crystallise
6. Pat dry with filter paper
Exam Tips
- Strong/weak refers to ionisation; concentrated/dilute refers to amount — never confuse them
- A weak acid at the same concentration as a strong acid has a higher pH, reacts more slowly, but gives the same total product
- Learn the ionic equation for neutralisation: H⁺ + OH⁻ → H₂O
- Know which acid produces which type of salt