Key Stage 3 science · Chemistry – Chemical reactions

KS3 chemical reactions and equations: a guide for parents

A chemical reaction is a change that makes new substances. In Years 7 to 9 children learn that every reaction is a rearrangement of atoms, which are never created or destroyed, and they learn to show this using formulae and balanced equations. They study combustion, thermal decomposition, oxidation and displacement, and find out what a catalyst does.

Chemical reactions and equations, explained

Changes of colour, fizzing, a rise in temperature, a flash of light or a new solid forming are all clues that a reaction may have happened, but the real test is whether a new substance has been made, with different properties from the starting materials. Pupils learn to explain reactions with atoms. In the burning of methane gas, for example, the atoms in methane and in oxygen separate from their old partners and join up in new ways to form carbon dioxide and water. The atoms are the same atoms as before, so the total mass is unchanged.

This idea, called conservation of mass, often surprises children when they weigh a reaction. In an open flask the mass can seem to drop because a gas has escaped, and when magnesium burns in air its mass goes up because it joins with oxygen from the air. Repeating the experiment in a sealed container shows the mass staying the same. Learning to account for “missing” or “extra” mass is one of the central reasoning skills in this topic.

Elements are written with symbols such as C, O and Mg. Compounds are written as formulae, which show which atoms are joined and how many of each: CO₂ has one carbon atom and two oxygen atoms, and H₂O has two hydrogen atoms and one oxygen atom. Pupils start with word equations, such as magnesium + oxygen → magnesium oxide, then move on to symbol equations. A balanced equation has the same number of each kind of atom on both sides. You balance it by changing the big numbers in front of formulae, never the small numbers inside them, because changing a small number would make a different substance.

Four types of reaction are named in the programme of study. Combustion is burning in oxygen, which releases energy; burning a fuel containing carbon and hydrogen produces carbon dioxide and water. Oxidation is any reaction where a substance gains oxygen, such as magnesium turning into magnesium oxide, or iron slowly rusting. Thermal decomposition is when heating breaks one compound into simpler substances, such as green copper carbonate becoming black copper oxide and carbon dioxide. Displacement is when a more reactive element takes the place of a less reactive one in a compound, as when magnesium is added to copper sulfate solution and pink-brown copper appears.

A catalyst is a substance that speeds up a reaction without being used up, so it can be recovered unchanged at the end. Only a small amount is needed. Pupils often see manganese dioxide speed up the breakdown of hydrogen peroxide into water and oxygen. Enzymes in the body are natural catalysts, and the exhaust system of a car contains catalysts that help turn harmful gases into less harmful ones. At this stage children need to know what catalysts do, not how they work.

Words your child will hear

Reactant: A substance that is used up in a reaction. Reactants are on the left of an equation.
Product: A new substance made in a reaction. Products are on the right of an equation.
Formula: A way of showing which elements are in a compound and how many atoms of each, such as CO₂.
Balanced equation: An equation with the same number of each kind of atom on both sides.
Thermal decomposition: A reaction where heat breaks one compound down into simpler substances.
Displacement: A reaction where a more reactive element takes the place of a less reactive one in a compound.
Catalyst: A substance that speeds up a reaction and is not used up.

What the National Curriculum says

The programme of study for Key Stage 3 says:

Pupils should be taught about chemical reactions as the rearrangement of atoms
Pupils should be taught about representing chemical reactions using formulae and using equations
Pupils should be taught about combustion, thermal decomposition, oxidation and displacement reactions
Pupils should be taught about what catalysts do

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

A worked example

Write a balanced symbol equation for methane burning in oxygen to make carbon dioxide and water.

Write the word equation, then replace the words with formulae. Methane is CH₄, oxygen is O₂, carbon dioxide is CO₂ and water is H₂O.
```
methane + oxygen
→ carbon dioxide + water
CH₄ + O₂ → CO₂ + H₂O
```
Count the atoms of each kind on each side. The equation is not balanced yet, because the hydrogen and oxygen counts differ.
```
Left:  C 1, H 4, O 2
Right: C 1, H 2, O 3
```
Balance the hydrogen first. There are 4 on the left and 2 on the right, so put a 2 in front of H₂O. That also changes the oxygen on the right to 2 + 2 = 4.
```
CH₄ + O₂ → CO₂ + 2H₂O
```
Now the oxygen. There are 4 on the right but only 2 on the left, so put a 2 in front of O₂.
```
CH₄ + 2O₂ → CO₂ + 2H₂O
```
Check every element again. If the counts match, stop. Never alter the small numbers inside a formula.
```
Left:  C 1, H 4, O 4
Right: C 1, H 4, O 4
```

CH₄ + 2O₂ → CO₂ + 2H₂O

Common mistakes and how to help

Believing things disappear when they burn

A candle shrinks and wood turns into a small pile of ash, so the mass seems to vanish. The gases that carry most of it away, carbon dioxide and water vapour, cannot be seen.

What helps: Talk about where the atoms went. Teachers often repeat a reaction in a closed container to show the mass staying the same. At home, the weighing activity below shows mass seeming to drop as a gas escapes, which is a good prompt for asking where it went.

Balancing an equation by changing the small numbers

If there is one oxygen too few, writing H₂O₂ instead of H₂O looks like the quickest fix.

What helps: Explain that H₂O is water and H₂O₂ is a different substance. Only the large numbers in front of a formula, which say how many of each substance there are, can be changed.

Thinking a catalyst is a reactant that gets used up

Children see something added and the reaction speed up, and assume it is part of the recipe.

What helps: Ask whether the catalyst appears in the equation as a reactant or product. It can be filtered out and weighed at the end, and the mass is the same as at the start.

Treating every change as a chemical reaction

Dissolving, melting and mixing can all change how a substance looks or seems to make it disappear.

What helps: Ask whether a new substance has been made. Salt dissolving can be reversed by evaporating the water, which suggests no new substance, while burning a match cannot be reversed.

How to help at home

Rearrange the bricks. Use building bricks or sweets in different colours as atoms. Build the molecules for methane and oxygen, take them apart and rebuild them as carbon dioxide and water. Count the bricks before and after to see that none are lost.
Weigh a reaction. Put a cup containing a few spoons of vinegar and a saucer holding a teaspoon of bicarbonate of soda on kitchen scales together and note the reading. Tip the powder into the vinegar, let the fizzing finish and weigh again. The reading drops slightly. Ask your child where the “missing” mass went and how they could show that no atoms were destroyed. An adult should supervise and keep the mixture away from eyes.
Cut apple experiment. Leave slices of apple in the open air, coated in lemon juice, coated in water and wrapped tightly in cling film. Your child can say which one browns first and use the word oxidation to explain why.
Start a rust trial. Put a plain steel nail (not a shiny galvanised one) in each of three jars: one dry, one in tap water, one in salty water. Check them each day for a couple of weeks and record the changes. Rusting is the oxidation of iron, and your child can explain why water, and salt in the water, speed it up. Schools usually extend this to show that air is needed too.
Spot reactions in the kitchen. Ask your child to name the signs of a reaction in cooking: bread rising, toast browning, an egg setting, a cut avocado going brown. Which are reversible, and which have made new substances?

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