An exothermic change releases energy to the surroundings, so the surroundings get warmer. An endothermic change absorbs energy from the surroundings, so they get cooler. This appears in almost every IGCSE Chemistry paper, often as a one-line interpretation of a temperature reading.
It is the first lesson in energy changes and bonds. Every later idea, from energy profiles to bond energies, builds on it.
How do you decide which one it is?
Start from the surroundings, not the chemicals. The thermometer measures the solution or the air around the reacting particles, so it shows where the energy went.
- Temperature of the surroundings rises: energy moved out of the reacting chemicals. The change is exothermic.
- Temperature of the surroundings falls: energy moved into the reacting chemicals. The change is endothermic.
The same idea gives the sign of ΔH, the energy change of the reaction, usually in kJ/mol. Exothermic means ΔH is negative because the chemicals end up with less energy. Endothermic means ΔH is positive because they end up with more.
Which everyday and school examples should you know?
Exothermic: burning fuels, respiration, and neutralisation of an acid by an alkali. Endothermic: thermal decomposition of calcium carbonate and photosynthesis (light energy is absorbed).
Dissolving some salts also cools the water, which is why a question may show a temperature drop with no obvious “reaction”. Any practical work on this should follow your teacher’s instructions and your school’s safety rules. This page treats the ideas only as reasoning.
Worked example
The data below are invented for practice. Two fictional reactions, X and Y, are each carried out in 50 cm³ of water.
| Reaction | Start temperature (°C) | Highest or lowest temperature (°C) |
|---|---|---|
| X | 21.0 | 28.5 |
| Y | 24.0 | 19.5 |
Step 1, find the change for X: 28.5 − 21.0 = 7.5. The temperature rose by 7.5 °C.
Step 2, interpret X: the surroundings got warmer, so energy was released. X is exothermic and its ΔH is negative.
Step 3, find the change for Y: 19.5 − 24.0 = −4.5. The temperature fell by 4.5 °C.
Step 4, interpret Y: the surroundings got cooler, so energy was absorbed. Y is endothermic and its ΔH is positive.
Check: a rise means energy out, a fall means energy in. Both interpretations match the thermometer.
The mistake to watch for
A very common slip is to describe the chemicals and the surroundings the wrong way round.
Mistaken answer: “The temperature went up, so the reaction absorbed heat, so it is endothermic.”
The student thought “the mixture has more heat, so it took heat in”. The warmth is energy that has arrived in the surroundings.
The correction is to ask “where did the energy go?” A temperature rise in the surroundings means the energy left the chemicals. Write the direction of transfer in your answer, for example “energy is released to the surroundings, so it is exothermic”.
Check yourself
1. A reaction mixture goes from 23.0 °C to 31.5 °C. Is it exothermic or endothermic, and what is the temperature change?
Show answer
31.5 − 23.0 = 8.5, a rise of 8.5 °C. The surroundings warmed, so energy was released. Exothermic, and ΔH is negative.
2. A reaction has ΔH = +92 kJ/mol. Describe the energy transfer and the expected temperature trend of the surroundings.
Show answer
A positive ΔH means the chemicals gain energy, so energy is absorbed from the surroundings. The surroundings cool down. The reaction is endothermic.
3. A student writes “exothermic means energy is taken in”. Correct the statement in one sentence.
Show answer
Exothermic means energy is released to the surroundings (and endothermic means energy is taken in).
Where this leads next
Once direction of energy feels automatic, move on to interpreting an energy profile, then test the whole module with the energy changes and bonds practice set. If you want to keep your formula and equation habits sharp alongside this, the mole and equation-ratio tutor supports the amount work that later lessons use.
Some students can recite the definitions but still swap them in unfamiliar questions. A teacher in online one-to-one Chemistry tuition can work out which picture you are using and replace it with a reliable one.