Acids, Bases and the pH Scale

Acids, bases and alkalis

• An acid is a substance that releases hydrogen ions (H⁺) when dissolved in water.
• A base neutralises an acid. A base that dissolves in water is called an alkali, which releases hydroxide ions (OH⁻).

The pH scale

The pH scale runs from 0 to 14 and measures how acidic or alkaline a solution is:

• pH 0–6 = acidic (lower = more acidic).
• pH 7 = neutral (e.g. pure water).
• pH 8–14 = alkaline (higher = more alkaline).

pH can be measured with a pH probe/meter (accurate number) or universal indicator, which changes colour: red (strong acid) → green (neutral) → purple (strong alkali).

Strong vs weak acids

• A strong acid (e.g. hydrochloric, sulfuric, nitric) fully ionises in water — all molecules release H⁺.
• A weak acid (e.g. ethanoic, citric, carbonic) only partially ionises — few molecules release H⁺.
• Concentration (how much acid per volume) is different from strength (how fully it ionises). As pH decreases by 1, the H⁺ concentration increases ×10.

Neutralisation

When an acid reacts with a base/alkali, they neutralise to form a salt and water:

acid + base → salt + water
acid + alkali → salt + water

The H⁺ from the acid reacts with OH⁻ from the alkali: H⁺ + OH⁻ → H₂O.

Reactions of acids (learn these)

Reaction	Products
acid + metal	salt + hydrogen
acid + metal oxide/hydroxide (base)	salt + water
acid + metal carbonate	salt + water + carbon dioxide

The salt produced depends on the acid: hydrochloric → chloride, sulfuric → sulfate, nitric → nitrate.

Making a soluble salt

React an acid with an excess of an insoluble base, filter off the leftover base, then crystallise the salt solution by evaporating some water and leaving it to form crystals.

Worked example

Name the products of: hydrochloric acid + calcium carbonate.

• acid + carbonate → salt + water + carbon dioxide → calcium chloride + water + carbon dioxide. (The salt is a chloride because the acid is hydrochloric.) ✓

Common mistakes

• Confusing strength (degree of ionisation) with concentration (amount per volume).
• Forgetting the CO₂ when an acid reacts with a carbonate.
• Getting the salt name wrong — match it to the acid (chloride/sulfate/nitrate).

Exam tips

• Learn the three acid reactions and their products off by heart.
• Remember H⁺ + OH⁻ → H₂O is the essence of neutralisation.
• State clearly: strong acid = fully ionised; weak acid = partially ionised.

Key facts to remember

• Acids release H⁺; alkalis release OH⁻; pH scale 0–14 (7 = neutral).
• Strong acids fully ionise, weak acids partially; strength ≠ concentration.
• acid + base → salt + water; acid + metal → salt + hydrogen; acid + carbonate → salt + water + CO₂.