Exothermic and endothermic changes

Some changes heat up their surroundings and others cool them down. Measuring the temperature tells you which.

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Exothermic and endothermic

An exothermic change transfers energy to the surroundings, so the temperature of the surroundings rises.
Examples: combustion, respiration, neutralisation, and the reaction in a hand warmer.
An endothermic change takes energy from the surroundings, so the temperature falls.
Examples: thermal decomposition, photosynthesis, and dissolving some salts, such as in an instant cold pack.

Changes of state

Melting and boiling take in energy from the surroundings, so they are endothermic.
Freezing and condensing give out energy, so they are exothermic.
While a pure substance is melting or boiling, its temperature stays constant: the energy goes into separating the particles, not into raising the temperature. This shows as a flat part on a heating curve.
Time Temperature solid melting(solid + liquid) liquid boiling(liquid + gas) gas flat flat
Heating curve of a pure substance: the temperature stays constant while it melts and while it boils.

Measuring energy changes

Mix the reactants in an insulated cup, such as a polystyrene cup with a lid, so less heat is lost.
Measure the starting temperature, mix, and record the highest (or lowest) temperature.
Temperature change = final temperature − starting temperature
A positive change (a rise) means exothermic. A negative change (a fall) means endothermic.
Worked example

Citric acid and sodium hydrogencarbonate are mixed. The temperature falls from 20 °C to 14 °C. Is the reaction exothermic or endothermic?

  1. Change = 14 − 20 = −6 °C
  2. The temperature fell, so energy was taken from the surroundings.

Answer: Endothermic: the temperature fell by 6 °C

Key idea

Exothermic: energy out, temperature rises (combustion, respiration, neutralisation). Endothermic: energy in, temperature falls (thermal decomposition, photosynthesis). Melting and boiling are endothermic; freezing and condensing are exothermic.

Key words
Exothermic
A change that gives out energy to the surroundings, so the temperature rises.
Endothermic
A change that takes in energy from the surroundings, so the temperature falls.
Combustion
Burning: a reaction in which a fuel reacts with oxygen and releases energy.
Neutralisation
The reaction between an acid and an alkali (or base) that makes a salt and water.
Thermal decomposition
A reaction in which heat breaks one substance down into two or more new substances.
Photosynthesis
How plants make glucose using light: carbon dioxide + water → glucose + oxygen.

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  1. Which of these is an endothermic reaction?

    1. Photosynthesis
    2. Neutralisation
    3. Burning methane
    4. Respiration
    Show the answer

    Answer: Photosynthesis

    It takes in energy from light.

  2. In a neutralisation, the temperature rises from 17 °C to 29 °C. By how many degrees does it rise?

    Show the answer

    Answer: 12 °C

    29 − 17 = 12 °C

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.