Acids, Bases and pH

Acids release $\mathrm{H^+}$ ions in water (proton donors); pH below 7. Bases neutralise acids (proton acceptors); soluble bases are alkalis, releasing $\mathrm{OH^-}$ , pH above 7. Acids react with metals (→ salt + hydrogen), bases (→ salt + water) and carbonates (→ salt + water + carbon dioxide). Strong acids ionise completely; weak acids only partly.

Cambridge 5070 Paper 1Cambridge 5070 Paper 2Cambridge 0620 Papers 2, 4Cambridge 0620 Papers 1, 3Edexcel 4CH1 Paper 1CEdexcel 4CH1 Paper 2C≈ 40 minRevision 1 · updated 2026-10-30

Section 1 of 14

Learning objectives

Describe the reactions of acids with metals, bases and carbonates.
Use indicators and the pH scale.
Define acids and bases in terms of proton transfer.
Distinguish strong and weak acids.

Pause and think

Why does indigestion medicine fizz when it meets stomach acid?

Section 2 of 14

Core idea

Start hereSour and slippery

Lemon juice tastes sour; soap feels slippery. Long before anyone knew about ions, people sorted substances by these clues. Today we know both come down to one tiny particle: the hydrogen ion.

The big idea

Acids donate $\mathrm{H^+}$ ions; bases accept them. When an acid meets a base, $\mathrm{H^+}$ and $\mathrm{OH^-}$ combine into water and a salt is left behind.

1. Reactions of acids

What acids react with

Drawing diagram…

Every route makes a salt.

Worked exampleWriting the equation

Sodium carbonate reacts with sulfuric acid.

Goal: Balanced equation.
Strategy: Carbonate + acid → salt + water + $\mathrm{CO_2}$ .

$\mathrm{Na_2CO_3+H_2SO_4\rightarrow Na_2SO_4+H_2O+CO_2}$
Sodium sulfate is the salt.
$\text{Na }2,\ \text{S }1,\ \text{C }1,\ \text{O }7,\ \text{H }2$
Check each element on both sides.

Answer

\mathrm{Na_2CO_3+H_2SO_4\rightarrow Na_2SO_4+H_2O+CO_2}

✓ CheckAlready balanced ✓.

2. pH and strength

The pH scale runs from 0 to 14. Each unit lower means ten times more $\mathrm{H^+}$ ions. A strong acid ionises completely, $\mathrm{HCl\rightarrow H^++Cl^-}$ ; a weak acid only partly, $\mathrm{CH_3COOH\rightleftharpoons CH_3COO^-+H^+}$ .

Common misconceptionStrong ≠ concentrated Strength is about how fully the acid ionises. Concentration is how much acid is dissolved per dm³. You can have a dilute strong acid or a concentrated weak one.

Key takeawayIndicators Litmus red/blue; methyl orange red/yellow; thymolphthalein colourless/blue (acid/alkali).

Pause and think

A weak acid and a strong acid of the same concentration react with magnesium. Which fizzes faster?

Summary

Acid = $\mathrm{H^+}$ donor; base = acceptor.
Three acid reactions all make salts.
Strong = fully ionised.

Alkali: A base that dissolves in water.
Neutralisation: Acid and base react to form a salt and water.
Weak acid: An acid only partly ionised in water.

HistoryArrhenius. Svante Arrhenius proposed in 1884 that acids produce hydrogen ions in water. His examiners almost failed his PhD; in 1903 the same idea won him a Nobel Prize.

Fun factRed cabbage indicator. Boiled red cabbage water is an indicator: red in vinegar, purple in water, green-yellow in soap solution.

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