Key Stage 3 science · Chemistry – Energetics
KS3 exothermic and endothermic reactions: a guide for parents
Changes of state and chemical reactions both involve energy moving between a substance and its surroundings. In Years 7 to 9 children learn, without calculations, which changes take energy in and which give it out, and how to describe reactions as exothermic or endothermic using temperature evidence.
Energy changes in reactions and changes of state, explained
Pupils start from the particle model. In a solid the particles vibrate in fixed positions, in a liquid they slide past each other, and in a gas they are far apart and moving freely. To melt a solid, energy must be supplied to loosen the particles from their fixed positions. To boil a liquid, more energy is needed to pull the particles completely apart. The reverse changes, freezing and condensing, release energy to the surroundings as the particles settle back together. So melting, evaporating and boiling take energy in, while freezing and condensing give energy out.
One result that surprises many children is that the temperature stays constant during a change of state. If ice is heated steadily the temperature climbs to 0 °C and then stays there until all the ice has melted, before rising again. It then pauses at 100 °C while the water boils. The energy is being used to separate the particles instead of making them move faster, so the thermometer reading does not change. Everyday examples include sweat cooling your skin as it evaporates, and the way steam condensing on skin releases extra energy, which is why steam can scald worse than boiling water at the same temperature.
A reaction is exothermic when it transfers energy to the surroundings, so the surroundings get warmer. Burning fuels, neutralisation reactions, rusting (very slowly) and the reactions in some hand warmers are examples. A reaction is endothermic when it takes in energy from the surroundings, so the surroundings get cooler. Thermal decomposition is a typical example, because the reaction keeps going only while heating continues. The reaction between citric acid and sodium hydrogencarbonate (bicarbonate of soda) in water is another, and it is part of why sherbet feels cool on the tongue. Instant cold packs work on a similar idea, although most use a solid that takes in energy as it dissolves in water. The word “surroundings” matters: pupils describe the temperature of the water or the air around the reaction, not the chemicals alone.
Energy is never created or destroyed in these changes. In an exothermic reaction, energy stored in the chemicals is transferred to the surroundings as thermal energy, and in an endothermic reaction it goes the other way. Pupils investigate this with a thermometer and an insulated cup, measuring the starting and final temperatures. Teachers use these practicals to talk about fair testing: the same amounts of each substance, the same starting temperature, and repeats to check that the results are repeatable.
Words your child will hear
- Exothermic
- A change that gives out energy to the surroundings, so the surroundings warm up.
- Endothermic
- A change that takes in energy from the surroundings, so the surroundings cool down.
- Surroundings
- Everything around the reaction or substance, such as the water in a cup, the air or a thermometer.
- Energy transfer
- Energy moving from one place or store to another. It is not made or lost.
- Melting
- Changing from solid to liquid, which takes in energy.
- Condensing
- Changing from gas to liquid, which gives out energy.
- Change of state
- A physical change between solid, liquid and gas. The particles stay the same but are arranged differently.
What the National Curriculum says
The programme of study for Key Stage 3 says:
- Pupils should be taught about energy changes on changes of state (qualitative)
- Pupils should be taught about exothermic and endothermic chemical reactions (qualitative)
Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.
A worked example: is it exothermic or endothermic?
A pupil mixes citric acid solution with sodium hydrogencarbonate solution in a cup. The temperature falls from 21 °C to 12 °C. What type of reaction is this, and how do we know?
- Work out the temperature change.
12 °C − 21 °C = −9 °C The temperature fell by 9 °C.
- The thermometer is measuring the surroundings, the liquid in the cup. The surroundings have cooled down, so thermal energy has moved from them into the reaction.
- A reaction that takes in energy from its surroundings is endothermic. If the temperature had risen, energy would have been given out and the reaction would be exothermic.
- To make the result more reliable, the pupil should repeat the experiment, use the same amounts and starting temperatures each time, stir to spread the temperature evenly, and use an insulated cup to cut heat gains from the room.
The reaction is endothermic: it takes in energy from the surroundings, so the temperature falls by 9 °C.
Common mistakes and how to help
Thinking endothermic reactions “give out cold”
Touching a cold pack feels as though coldness is flowing into your hand, so cold seems to be a thing that is released.
What helps: Explain that coldness is not a substance. Energy moves out of your hand and out of the surroundings into the reaction, which is why they get cooler.
Believing the temperature always rises while something is being heated
Heating usually does raise the temperature, so children expect a straight upward line on a graph.
What helps: Use a graph of heating ice. It has flat sections at 0 °C and 100 °C where the energy goes into changing state. Ask what the particles are doing during each flat part.
Saying that exothermic reactions create energy
Burning seems to produce heat and light from nothing.
What helps: Say “transfer” instead of “make”. The energy was stored in the chemicals and has been moved to the surroundings, and the total amount stays the same.
Believing a reaction that needs heat to get started must be endothermic
If a flame or spark is needed, it seems that the reaction must be taking in energy.
What helps: Compare with a match. A small spark starts the burning, but the fuel gives out far more energy than the spark supplied, so the overall reaction is exothermic. Judge the reaction by the final temperature change.
How to help at home
- Ice cubes and warm hands. Put one ice cube in a bowl of lukewarm water and another on a plate. Time how long each takes to melt and ask where the energy came from. Hold a cube in your hand briefly and describe how your fingers feel and why.
- Evaporation cooling. Dab a little water on the back of your child’s hand and blow gently across it. It feels cooler because the water takes energy from the skin as it evaporates. Try the same with a dry hand for comparison.
- Watch condensation. Take a cold glass of water out of the fridge and watch drops form on the outside. Ask what changed state, and which way the energy moved.
- Compare a hand warmer or cold pack. If you have a reusable hand warmer or an ice pack, ask your child to decide whether the change inside it is exothermic or endothermic and explain how the temperature tells them.
- Kitchen temperature test. With an adult supervising and a kitchen thermometer, stir a teaspoon of citric acid and a teaspoon of bicarbonate of soda into a cup of cool water. Record the temperature before and after. Keep the powders away from eyes, and do not drink the mixture.
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