Neutralisation Reactions: KS3 Chemistry Explained

Neutralisation is the chemical reaction between an acid and an alkali (or base) that produces a salt and water. The hydrogen ions from the acid react with the hydroxide ions from the alkali, cancelling each other out and moving the pH of the solution towards 7 — neutral.

What is the particle-level picture of neutralisation?

Before writing any equation, it helps to picture what is happening at the particle level, because that is where the real chemistry lives.

An acid releases hydrogen ions (H⁺) when dissolved in water. The more H⁺ ions, the lower the pH and the stronger the acid.

An alkali releases hydroxide ions (OH⁻) when dissolved in water. The more OH⁻ ions, the higher the pH and the stronger the alkali.

In a neutralisation reaction, the H⁺ and OH⁻ ions find each other and bond together:

H⁺ (aq) + OH⁻ (aq) → H₂O (l)

This is the core ionic equation for every neutralisation reaction. The water that forms is neutral — neither acidic nor alkaline. The remaining ions (the metal ion from the alkali and the non-hydrogen ion from the acid) combine to form a salt dissolved in the water. When the water evaporates, the salt crystallises out.

What is the general word equation for neutralisation?

acid + alkali → salt + water

This is the general word equation you must know for KS3.

Alternatively, with a metal oxide or metal carbonate (bases that are not alkalis):

acid + metal oxide → salt + water

acid + metal carbonate → salt + water + carbon dioxide

How is the name of the salt determined?

The name of the salt depends on which acid and which alkali react. The pattern is straightforward:

Acid | Salt family name | Example
Hydrochloric acid (HCl) | Chloride | Sodium chloride (table salt)
Sulfuric acid (H₂SO₄) | Sulfate | Copper sulfate
Nitric acid (HNO₃) | Nitrate | Potassium nitrate

The first part of the salt name comes from the metal in the alkali (e.g. sodium from sodium hydroxide, potassium from potassium hydroxide, copper from copper oxide).

Worked example: Sodium hydroxide + hydrochloric acid →?

Sodium hydroxide is the alkali → metal is sodium

Hydrochloric acid → chloride salt

Products: sodium chloride + water

Word equation: sodium hydroxide + hydrochloric acid → sodium chloride + water

How does pH change during neutralisation?

pH is a measure of the concentration of H⁺ ions in a solution. The pH scale runs from 0 (strongly acidic) to 14 (strongly alkaline), with 7 being neutral.

pH range | Character | Example
0–6 | Acidic | Stomach acid (pH ~2), vinegar (pH ~3), rain (pH ~5.6)
7 | Neutral | Pure water
8–14 | Alkaline | Baking soda solution (pH ~9), bleach (pH ~12)

When you add an alkali to an acid, the OH⁻ ions from the alkali cancel out some H⁺ ions. The pH rises. At the equivalence point (also called the end point in a titration), exactly enough alkali has been added to neutralise all the acid, and the pH reaches 7.

If you add too much alkali, the excess OH⁻ ions make the solution alkaline and the pH rises above 7.

How is a neutralisation experiment carried out?

A standard KS3 neutralisation practical involves an indicator (such as universal indicator or litmus) or a pH probe to track the pH change as alkali is added to acid.

Simple method:

Measure a set volume of hydrochloric acid into a beaker.

Add a few drops of universal indicator — the solution turns red (acidic).

Slowly add sodium hydroxide solution, one drop at a time, swirling after each addition.

Observe the colour change: red → orange → yellow → green (neutral).

Adding excess sodium hydroxide: yellow → blue/purple (alkaline).

The colour change at green (pH 7) marks the point of neutralisation. For a purer salt, use a pH probe and add the exact volume of alkali calculated from the titration, then evaporate the water.

Where does neutralisation occur in real life?

Neutralisation is not just a laboratory exercise — it is fundamental to everyday chemistry:

Indigestion remedies contain alkalis (magnesium hydroxide or calcium carbonate) that neutralise excess hydrochloric acid in the stomach.

Toothpaste contains a mild alkali (sodium fluoride or sodium bicarbonate) that neutralises acidic bacterial waste products which cause tooth decay.

Farmers add calcium hydroxide (slaked lime) to acidic soils to raise the pH and improve crop growth.

Bee stings are acidic; applying bicarbonate of soda (alkaline) can relieve the sting by neutralisation.

Wasp stings are alkaline; applying vinegar (acidic) can help for the same reason.

Frequently asked questions

What is neutralisation in simple terms for KS3?

Neutralisation is when an acid and an alkali react together and cancel each other out, producing a salt and water. The pH of the solution moves towards 7 (neutral). At the particle level, hydrogen ions (H⁺) from the acid combine with hydroxide ions (OH⁻) from the alkali to form water molecules.

What are the products of a neutralisation reaction?

A neutralisation reaction always produces a salt and water. If the acid reacts with a metal carbonate, carbon dioxide is also produced. The specific salt depends on which acid and which alkali react — for example, hydrochloric acid and sodium hydroxide produce sodium chloride (table salt) and water.

How do you know when neutralisation is complete?

You can use an indicator such as universal indicator, which changes colour at different pH values, or a pH probe that gives a numerical reading. Neutralisation is complete when the pH reaches 7 (shown by a green colour with universal indicator). In a formal titration, a sharp pH change near the equivalence point is used to identify the exact end point.

Why does adding an alkali to an acid raise the pH?

The acid contains excess H⁺ ions, which give it a low pH. When alkali is added, it supplies OH⁻ ions that react with and remove the H⁺ ions (forming water). As H⁺ ions are removed, their concentration falls, so the pH rises. When all the H⁺ ions have been neutralised, the pH reaches 7.

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