CBSE Class 10 Answered

When a chemical reaction happens, energy is transferred to or from the surroundings and often there is a temperature change. For example, when a bonfire burns it transfers heat energy to the surroundings. Objects near a bonfire become warmer and the temperature rise can be measured with a thermometer.

Exothermic reactions
These are reactions that transfer energy to the surroundings. The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to get hotter. The temperature increase can be detected using a thermometer. Some examples of exothermic reactions are:

• burning
• neutralisation reactions between acids and alkalis, and
• the reaction between water and calcium oxide

Endothermic reactions
These are reactions that take in energy from the surroundings. The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to get colder. The temperature decrease can be detected using a thermometer. Some examples of endothermic reactions are:

• electrolysis
• the reaction between ethanoic acid and sodium carbonate, and
• the thermal decomposition of calcium carbonate in a blast furnace