Key Stage 3 science · Chemistry – Chemical reactions

KS3 acids, alkalis and neutralisation: a guide for parents

Acids and alkalis are two families of chemicals with opposite behaviour, and when they meet they cancel each other out. In Years 7 to 9 children learn to measure acidity on the pH scale, use indicators, and predict the products when an acid reacts with a metal or with an alkali.

Acids, alkalis and neutralisation, explained

Lemons, vinegar and fizzy drinks contain acids, which taste sour, while soap, bleach and many oven or drain cleaners are alkaline. In Key Stage 3 children define the two families by what they do to each other. An acid is neutralised by an alkali, and an alkali is neutralised by an acid, meaning each cancels out the other’s properties. A base is any substance that can neutralise an acid, and an alkali is a base that dissolves in water. Not all acids are harmful: citric acid in oranges is safe to eat, but concentrated acids and alkalis are corrosive, which is why school experiments use dilute solutions and eye protection.

The pH scale gives acidity a number, usually from 0 to 14. A pH below 7 is acidic, 7 is neutral (pure water is the usual example) and above 7 is alkaline. The further a value is from 7, the more strongly acidic or alkaline the substance is. Pupils measure pH with indicators, which are dyes that change colour depending on acidity. Litmus is red in acid and blue in alkali. Universal indicator shows a range of colours: red, orange and yellow for acids, green for neutral, and blue and purple for alkalis. Juice from red cabbage works as a natural indicator too.

Acids react with many metals to give a salt and hydrogen gas. For example, magnesium and hydrochloric acid make magnesium chloride and hydrogen. In chemistry a salt means any compound formed when a metal replaces the hydrogen in an acid, not only the table salt on the dinner table. The name follows a pattern: hydrochloric acid gives chlorides, sulfuric acid gives sulfates and nitric acid gives nitrates. The fizzing is hydrogen, which pops when a lit splint is held at the mouth of the tube. Unreactive metals such as copper do not react with dilute acids.

When an acid reacts with an alkali the products are a salt and water. Hydrochloric acid and sodium hydroxide make sodium chloride and water. Pupils carry out neutralisation by adding one solution drop by drop to the other with an indicator present, watching for the point where the colour shows neutral. Neutralisation is useful in everyday life: indigestion remedies neutralise excess stomach acid, and gardeners add lime to acidic soil. Evaporating the water from the neutral mixture gives crystals of the salt.

Words your child will hear

Acid: A substance with a pH below 7 that is neutralised by alkalis, such as lemon juice or hydrochloric acid.
Alkali: A base that dissolves in water, with a pH above 7. Alkalis neutralise acids.
Neutralisation: The reaction between an acid and an alkali that makes a salt and water.
pH scale: A scale from 0 to 14 for measuring how acidic or alkaline a solution is, with 7 as neutral.
Indicator: A dye that changes colour to show whether a solution is acidic, neutral or alkaline.
Salt: A compound made when an acid reacts, such as magnesium chloride or sodium sulfate.
Corrosive: Able to damage skin, eyes and materials. The hazard symbol warns that care is needed.

What the National Curriculum says

The programme of study for Key Stage 3 says:

Pupils should be taught about defining acids and alkalis in terms of neutralisation reactions
Pupils should be taught about the pH scale for measuring acidity/alkalinity; and indicators
Pupils should be taught about reactions of acids with metals to produce a salt plus hydrogen
Pupils should be taught about reactions of acids with alkalis to produce a salt plus water

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

A worked example

Dilute hydrochloric acid is added slowly to sodium hydroxide solution containing universal indicator. What happens, and what is made?

Start with the alkali. Sodium hydroxide has a high pH, so universal indicator shows it as purple.
Add the acid a few drops at a time, stirring after each addition. Each drop reacts with some of the alkali, so the pH falls and the colour changes: purple, then blue.
Stop when the indicator turns green. At this point the acid has exactly used up the alkali and the solution is neutral, with a pH of about 7. Adding more would turn the mixture yellow, then orange or red, showing the acid is now in excess.
Write the word equation: an acid plus an alkali gives a salt and water.
```
hydrochloric acid + sodium hydroxide
→ sodium chloride + water
```
The salt is dissolved in the water. To get it back, heat the neutral solution gently so that the water evaporates, and crystals of sodium chloride are left.

The colour goes from purple through blue to green, showing neutralisation. The products are sodium chloride (a salt) and water.

Common mistakes and how to help

Thinking acids are dangerous and alkalis are safe

The word “acid” sounds frightening, and alkalis are often described as “gentle” or “mild” because soap and baking soda are alkaline.

What helps: Compare examples at both ends of the pH scale. Vinegar and lemon juice are acids found in food, while oven cleaner and drain cleaner are strong alkalis that can burn skin. Safety depends on how concentrated the substance is, so read hazard symbols and never mix cleaning products.

Saying that the bubbles from an acid and a metal are carbon dioxide

Fizzing in school science is often carbon dioxide, so children assume all gas bubbles are the same gas.

What helps: Remember the pattern: an acid with a metal gives a salt and hydrogen. The hydrogen gives a squeaky pop with a lit splint, and carbon dioxide turns limewater cloudy.

Forgetting that neutralisation makes a salt as well as water

The word “neutral” suggests that everything has been cancelled out and only water is left.

What helps: Point to the evidence from school: when a neutralised solution of hydrochloric acid and sodium hydroxide is evaporated, white crystals of sodium chloride are left behind, so something other than water must have been made. Practise naming the salt from the acid and the metal.

Believing that a higher pH number means a stronger acid

Most scales children meet count upwards from “weak” to “strong”, so 14 seems the most acidic.

What helps: Draw the scale as a line with 7 in the middle: acid on the left, alkali on the right. Strongly acidic substances are at the low numbers and strongly alkaline ones at the high numbers.

How to help at home

Tea and lemon. Make a cup of tea, let it cool, then squeeze in a few drops of lemon juice. The colour lightens because substances in the tea change colour with acidity, so the tea is acting as an indicator. Ask your child what the colour change tells them about the lemon juice, and why scientists use indicators instead of tasting things.
Rank kitchen liquids. Ask your child to predict the order of lemon juice, vinegar, milk, tap water, a little bicarbonate of soda dissolved in water and some soapy water, from most acidic to most alkaline. Then test them with red cabbage indicator: an adult chops some red cabbage and pours hot water over it, and once it has cooled the purple liquid is strained off. Add a little to each liquid in a clear glass. Pink or red shows an acid, purple is close to neutral, and blue or green shows an alkali. Stick to kitchen ingredients and leave cleaning products out.
Read the labels. Find the hazard symbols on cleaning products in the house. Which ones warn about corrosive substances? This is a good moment to repeat that different cleaners must never be mixed.
Garden detective. Look at hydrangeas, which tend to produce blue flowers in acidic soil and pink flowers in alkaline soil. The colour of the flowers works a little like a natural indicator, and your child can find out why gardeners test their soil.
Talk about indigestion tablets. Ask what an indigestion remedy is designed to do. Your child should be able to explain that it contains a substance that neutralises extra stomach acid, and why too much of it is not sensible.

Leads on to

Key Stage 4: Formulae, equations and gas tests
Key Stage 4: Acids, pH, electrolysis and redox
Key Stage 4: Rates of reaction and reversible reactions
Key Stage 4: Purity, separation and reacting masses
Key Stage 4: Life cycle assessment and recycling
Key Stage 4: Crude oil, cracking and metal extraction