Energy changes are everywhere

Whenever matter changes state or a chemical reaction happens, energy is either taken in from the surroundings or given out to them. We usually notice this as a change in the temperature of the surroundings.

• Ice melting or water boiling — energy is taken in.
• Steam condensing or water freezing — energy is given out.
• Burning fuel, or an iron hand-warmer warming up — energy is given out.

Golden rule: energy is never made or destroyed — it only moves between the chemicals and their surroundings. This is called conservation of energy.

Energy and changes of state

To go from a more ordered state to a more spread-out one (solid → liquid → gas), particles need more energy, so energy is taken in. Going the other way (gas → liquid → solid) gives energy out.

Watch out: in a change of state no new substance is made — it is still water, just in a different form. Energy still moves though!

Take in or give out?

Exothermic: gives out heat

An exothermic reaction gives out energy to the surroundings, usually as heat. Because energy leaves the reaction and warms everything around it, the temperature rises.

• Burning fuels (combustion) — e.g. a candle, a Bunsen burner, a bonfire.
• Neutralisation — an acid reacting with an alkali.
• Iron hand-warmers — iron powder reacting with oxygen in the air.
• Respiration in your cells.

Remember: exo = exit — heat exits the reaction, so it feels hot and the thermometer reading goes up.

Endothermic: takes in energy

An endothermic reaction takes in energy from the surroundings. Some take it from the mixture around them, so the temperature falls. Others need energy supplied by heating or by light.

• Citric acid with sodium hydrogencarbonate (bicarbonate of soda) — the mixture goes cold.
• Thermal decomposition — a substance breaks down when heated. It takes in energy from the flame.
• Photosynthesis — plants take in energy from light.

Remember: endo = into — energy goes into the reaction. A heated tube is hot because of the flame, not the reaction.

The thermometer test

For a reaction in a beaker or cup, the easiest way to tell the two apart is to measure the temperature before and after with a thermometer:

Misconception buster: "exothermic" does not mean "gets cold". It means heat exits the reaction, so the surroundings get warmer.

Read the thermometer

Exothermic or endothermic?

Tap a reaction, then tap the box it belongs in.

Energetics in everyday life

These energy changes are all around you. Here are ones you should be able to name:

Watch out: ice melting takes in energy, but it is not a reaction. No new substance is made — it is a change of state.

Match the example to its type

Tap an example on the left, then its matching type on the right.

Needs a flame to start?

A candle needs a match to light it. Does that make burning endothermic? No.

Once lit, the candle keeps burning by itself. The energy comes from the wax and oxygen reacting, not from the match. So burning is exothermic.

Misconception buster: how a reaction starts does not decide its type. Ask: does it give out or take in energy overall?

Started by a flame

Where does the "lost" energy go?

In an exothermic reaction it can look like energy is created — but it isn't. Energy is transferred from the reacting chemicals to the surroundings.

In an endothermic reaction it can look like energy disappears — but it doesn't. It is transferred from the surroundings into the reacting chemicals.

Misconception buster: energy is never made or destroyed. It only moves between the chemicals and their surroundings — this is conservation of energy.

Your turn — explain

Spot the odd one out

Where does the energy go?

The key ideas to know

You've covered the key ideas in KS3 Energetics. Press Finish to see your score.