Neutralisation

Neutralisation (neutralization in the US) is the reaction between an acid and a base that makes a salt and water. When the base is an alkali, the hydrogen ions from the acid react with the hydroxide ions from the alkali: H⁺ + OH⁻ → H₂O. The pH moves towards 7, and the reaction gives out heat, so it is exothermic.

Acids, bases and alkalis

• Acid: a substance that releases hydrogen ions, H⁺, in water. Examples: hydrochloric acid, sulfuric acid, nitric acid.
• Base: a substance that neutralises an acid, such as a metal oxide or metal hydroxide.
• Alkali: a base that dissolves in water, releasing hydroxide ions, OH⁻. Examples: sodium hydroxide, potassium hydroxide.

Alkalis are the soluble members of the base family. Copper oxide, for example, neutralises acids but doesn't dissolve in water, so it is a base and not an alkali. Metal carbonates neutralise acids too, giving off carbon dioxide as well.

Neutralisation equations

AQA's GCSE Chemistry specification lists these: acids are neutralised by alkalis such as soluble metal hydroxides, and by metal carbonates, to make salts. It also asks you to predict the products from the reactants.

Naming the salt

The first part of the salt's name comes from the metal in the base, and the second part from the acid:

To write the formula, balance the charges of the ions: Ca²⁺ and NO₃⁻ need two nitrates for each calcium, so calcium nitrate is Ca(NO₃)₂.

The ionic equation for neutralisation

In any reaction between an acid and an alkali, the ions that actually react are the same. The hydrogen ions from the acid join with the hydroxide ions from the alkali to make water:

The metal ions and the ions from the acid, such as Na⁺ and Cl⁻, are spectator ions: they are in the solution before and after, and they make up the salt. See ionic equations for how to write these.

pH and neutralisation

The pH scale runs from 0 to 14, and you can measure it with universal indicator or a pH probe. Add an alkali to an acid a little at a time and the pH climbs slowly at first, then jumps quickly past 7 when the acid is nearly used up. That sharp jump is why an indicator changes colour so suddenly at the end of a titration. Watch it happen in the titration lab below.

Try it

Titration Lab

Run a virtual titration and plot the pH curve as you go

Open the simulatorGo to its page

Neutralisation is exothermic

AQA lists neutralisation with combustion as an exothermic reaction: it transfers energy to the surroundings, so the temperature of the mixture rises. One of AQA's required practicals investigates the variables that affect temperature changes in reacting solutions, and neutralisation is one of its examples. See reaction profiles for how to draw one.

Making a soluble salt

AQA's required practical 1 is preparing a pure, dry sample of a soluble salt from an insoluble oxide or carbonate. For copper sulfate from copper oxide and sulfuric acid:

1. Warm some dilute sulfuric acid in a beaker.
2. Add copper oxide a spatula at a time, stirring, until no more dissolves. The leftover black solid shows the acid is all used up.
3. Filter to remove the excess copper oxide.
4. Heat the blue filtrate in an evaporating basin to evaporate some of the water.
5. Leave it to cool so crystals form, then pat them dry with filter paper.

Using an excess of the solid base makes sure all the acid reacts, and filtering removes what is left, so the salt isn't contaminated with acid or base.

Titration calculations

A titration finds exactly how much acid neutralises a measured volume of alkali, using an indicator to show the end point. Titrations are in AQA's separate GCSE Chemistry only, and the calculations are Higher tier.

Example: 22.50 cm³ of 0.150 mol/dm³ sulfuric acid neutralises 25.0 cm³ of sodium hydroxide solution. Find the concentration of the sodium hydroxide. The equation is H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O.

1. Moles of acid = 0.150 × 22.50 ÷ 1000 = 0.003375 mol.
2. The equation shows 2 NaOH for each H₂SO₄, so moles of NaOH = 2 × 0.003375 = 0.00675 mol.
3. Concentration = moles ÷ volume in dm³ = 0.00675 ÷ 0.0250 = 0.270 mol/dm³.
4. In g/dm³, multiply by the relative formula mass of NaOH, 40: 0.270 × 40 = 10.8 g/dm³.

The titration lab lets you run the experiment and plot the pH curve, and the molarity calculator checks the concentration step.

Neutralisation in everyday life

• Indigestion tablets contain bases, such as calcium carbonate or magnesium hydroxide, that neutralise excess stomach acid.
• Farmers spread lime (calcium hydroxide or calcium carbonate) on fields to neutralise acidic soil.

Common mistakes

• Forgetting carbon dioxide when the base is a carbonate.
• Naming the salt from the wrong part. The metal comes from the base and the ending from the acid.
• Saying all bases are alkalis. Only soluble bases are alkalis.
• Not balancing the formula of the salt. Sulfate is SO₄²⁻, so sodium sulfate is Na₂SO₄.
• Using the 1 : 1 ratio for sulfuric acid. It reacts with sodium hydroxide in a 1 : 2 ratio.

To practise acids and salts under time pressure, try today's GCSE Chemistry Higher or GCSE Combined Science Higher short timed paper. Each paper mixes topics.

Read next

Chemistry · Reactions and electrolysisIonic equations: how to write them, with examplesWhat an ionic equation is, how to write one in four steps by removing spectator ions, and examples for neutralisation, precipitation and displacement.