pH Scale, Neutralisation and Salt Production — AQA GCSE Chemistry
The pH scale measures acidity and alkalinity, and neutralisation reactions produce salts.
The pH scale
The pH scale runs from 0 to 14 and measures how acidic or alkaline a solution is:
- pH less than 7 — acidic.
- pH 7 — neutral.
- pH greater than 7 — alkaline.
pH can be measured with universal indicator (which changes colour) or, more accurately, with a pH probe/meter.
Acids and alkalis in terms of ions
- Acids produce hydrogen ions (H⁺) in aqueous solution.
- Alkalis (soluble bases) produce hydroxide ions (OH⁻) in solution.
Neutralisation
When an acid reacts with a base or alkali, they neutralise each other to form a salt and water: $$H^+ + OH^- \rightarrow H_2O$$
Making soluble salts
A soluble salt can be made by reacting an acid with an insoluble base (or a metal/carbonate):
- Add the base to warm acid until no more reacts (excess base).
- Filter off the excess solid.
- Crystallise the salt solution and dry the crystals.
Exam tips
- Learn the pH scale: <7 acidic, 7 neutral, >7 alkaline.
- Acids give H⁺; alkalis give OH⁻.
- Neutralisation: H⁺ + OH⁻ → H₂O.
- Know the steps to make a pure, dry salt (add excess base, filter, crystallise).