Acids, Bases and the pH Scale: KS3 Chemistry Guide

An acid is a substance that produces hydrogen ions (H⁺) when dissolved in water. A base is a substance that neutralises an acid. An alkali is a base that dissolves in water and produces hydroxide ions (OH⁻). The pH scale runs from 0 to 14 and measures how acidic or alkaline a solution is. This is core KS3 chemistry content.

What is the pH scale?

The pH scale measures the concentration of hydrogen ions in a solution:

• pH 0–6 = acidic (the lower the number, the stronger the acid)
• pH 7 = neutral (pure water)
• pH 8–14 = alkaline (the higher the number, the stronger the alkali)

The scale is logarithmic: each step of 1 represents a tenfold change in hydrogen ion concentration. This means a solution at pH 2 is ten times more acidic than one at pH 3, and 100 times more acidic than one at pH 4.

Everyday substances and their approximate pH values

How do you test pH?

Three common methods used in KS3 practicals are:

1. Universal indicator — a mixture of dyes that changes colour across the pH range: red/orange for strong acids, yellow/green for neutral/weak, blue/purple for alkalis. It gives a rough estimate of pH.
2. Litmus — a simpler indicator: red in acid, blue in alkali, purple in neutral solutions. Litmus cannot show the strength of the acid or alkali — only which side of 7 a solution is on.
3. pH probe (datalogger) — gives a precise numerical reading and can track continuous changes (e.g., during a titration or neutralisation reaction). The most accurate method.

What is the difference between a base and an alkali?

• A base is any substance that can neutralise an acid. Bases include metal oxides (e.g., copper(II) oxide, CuO) and metal hydroxides (e.g., sodium hydroxide, NaOH). Not all bases dissolve in water.
• An alkali is a base that dissolves in water to produce OH⁻ ions. All alkalis are bases, but not all bases are alkalis.

For example, calcium carbonate (CaCO₃, found in limestone and chalk) is a base — it neutralises acids — but it does not dissolve in water, so it is not an alkali.

What is neutralisation?

Neutralisation is the reaction between an acid and a base to form a salt and water. The general word equation is:

acid + base → salt + water

The H⁺ ions from the acid combine with the OH⁻ ions from the alkali to form water:

H⁺(aq) + OH⁻(aq) → H₂O(l)

A worked example: making copper sulphate

A classic Year 8 practical involves reacting copper(II) oxide (a base) with sulphuric acid:

Equation:

copper(II) oxide + sulphuric acid → copper(II) sulphate + water CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)

Method:

1. Warm dilute sulphuric acid in a beaker.
2. Add an excess of black copper(II) oxide powder and stir.
3. Filter off the unreacted copper(II) oxide (the filtrate — the liquid that passes through — contains copper sulphate solution).
4. Evaporate the solution slowly to crystallise blue copper(II) sulphate crystals.

The excess base is added to ensure all the acid is neutralised — any remaining acid would make the product impure. This experiment demonstrates neutralisation and introduces salt formation.

Neutralisation reactions in everyday life

• Indigestion tablets contain bases such as calcium carbonate or magnesium hydroxide, which neutralise excess stomach acid.
• Treating acidic soil — farmers add calcium hydroxide (slaked lime) to fields where the soil pH is too low for crops, raising the pH towards neutral.
• Bee stings and wasp stings — bee stings are acidic (around pH 5); applying a weak alkali such as baking soda soothes the sting. Wasp stings are slightly alkaline; a weak acid such as vinegar can help.
• Treating acid rain damage — limestone (calcium carbonate) is used to neutralise acid rain in lakes, raising the pH to protect aquatic life.

Indicators and their colour changes

According to the Department for Education's Science Programmes of Study for Key Stage 3, pupils should be taught about the pH scale, the use of indicators to identify acids and alkalis, and the products of neutralisation.

BBC Bitesize KS3 Chemistry covers the pH scale, the universal indicator colour code, and the acid + alkali → salt + water equation as key content for Year 7 and Year 8.

Frequently asked questions

What is the pH of a neutral solution?

A neutral solution has a pH of exactly 7. Pure water is the standard example. A neutral solution contains equal concentrations of H⁺ and OH⁻ ions.

What is the difference between a strong acid and a weak acid?

A strong acid (e.g., hydrochloric acid, sulphuric acid) fully dissociates into ions when dissolved in water, producing a high concentration of H⁺ ions. A weak acid (e.g., citric acid, ethanoic acid in vinegar) only partially dissociates, so fewer H⁺ ions are produced. Both strong and weak acids can be dilute or concentrated — strength and concentration are different ideas.

How do you name the salt formed in a neutralisation reaction?

The name of the salt comes from the acid and the base used. The first part of the salt name comes from the metal in the base (or the metal that replaced hydrogen). The second part comes from the acid: hydrochloric acid → chloride; sulphuric acid → sulphate; nitric acid → nitrate. For example, hydrochloric acid + sodium hydroxide → sodium chloride + water.

Why is the pH scale important in biology?

Many biological processes only work within a narrow pH range. Enzymes in the human body have an optimal pH — amylase works best near pH 7, while pepsin in the stomach needs pH 2. Significant deviation from the optimal pH can denature the enzyme (change its shape permanently) and stop it working.

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