Applies to: AQA GCSE Chemistry and Combined Science (Required Practical 4). Energy changes in reactions are on every GCSE and IGCSE specification, so the ideas here are useful whichever board you're studying.

Most people expect a fizzing reaction to get hot. Pop a thermometer into vinegar, tip in some bicarbonate of soda, and watch the number drop instead. It's a lovely surprise, and it's exactly the kind of thing that helps exothermic and endothermic finally click.

What it tests

How to measure the temperature change in a reaction, and how to decide whether the reaction is exothermic (gives out energy, so the temperature rises) or endothermic (takes in energy, so the temperature falls).

What you need

A polystyrene cup or a disposable coffee cup with a lid (make a small hole in the lid for the thermometer), a kitchen, aquarium or digital food thermometer, 400 ml of white vinegar, bicarbonate of soda and a half teaspoon measure, a measuring jug, a teaspoon for stirring, and a mug to stand the cup in. For the extra test you'll also want 2 teaspoons of Epsom salt or washing soda.

How to do it

Stand the cup inside a mug so it can't tip over and pour in 100 ml of vinegar. Put the thermometer in, wait a minute and record the starting temperature. Add one level half teaspoon of bicarbonate of soda, put the lid on with the thermometer through the hole, stir gently and record the lowest temperature reached. Rinse the cup and repeat with 2, 3 and 4 half teaspoons. The temperature change is the final temperature minus the starting temperature.

For a bonus test, dissolve 2 teaspoons of Epsom salt or washing soda in 50 ml of water and see which way the temperature goes.

Safety note

Don't stir with a glass thermometer, as it can break. The vinegar and bicarbonate mixture fizzes up, so stand the cup in a bowl to catch any overflow. Washing soda irritates eyes and skin, so wear gloves and glasses and have an adult with you. Don't drink any of the mixtures, and wash your hands at the end.

What you should find

Vinegar and bicarbonate of soda is endothermic, so the temperature falls. More bicarbonate gives a bigger drop, until all the acid has been used up. After that, extra bicarbonate has nothing left to react with, so the temperature change stops getting bigger. In the school version, acid and alkali neutralisation is exothermic, so the temperature rises to a peak and then falls as extra alkali cools the mixture.

Why it matters for the exam

You'll be expected to explain why a polystyrene cup with a lid is used (it's an insulator, so less energy is lost to the surroundings and the reading is more accurate), spot anomalous results, calculate means, and explain the shape of a temperature graph. One exam tip worth practising: drawing two straight lines of best fit on the graph and reading off where they meet. There's also a 6-mark planning question in the PDF, which is exactly the kind of question that catches students out.

On motivation: a result that goes the opposite way to what you expected is gold for memory. If your teen remembers "the fizzy one got colder", they've already got the key idea.

Free download: the Practicals at Home PDF

I've put everything for this practical into one free PDF: the home method, a results table, sample results if you don't have the kit, the school lab method the exam questions are based on, exam-style questions and a full mark scheme so you can check your own answers.

Download the Temperature changes in reactions PDF

Watch and listen: Key Ideas

To go with this practical I've made a short Key Ideas video and a podcast on exothermic and endothermic reactions, including the reaction profile diagrams that come up in exams. Both are short enough to fit in a bus journey or a dog walk.

• Watch the Key Ideas video: Exothermic and endothermic reactions

• Listen to the Key Ideas podcast: Exothermic and endothermic reactions

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