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Energy is not, and cannot, be created or destroyed. It can, however, be changed from one form to another (e.g electricity being used to make a lamp light up - electrical energy to heat energy). Energy changes occur in chemical reactions and in some physical processes, such as a solid (ice) dissolving into a liquid (water).

Simple explanation of Exothermic Reactions

Exothermic reactions are chemical reactions in which heat energy is given out. This is a result of bonds being created, which gives out/releases energy. If it helps, you can think of exit or expelled for 'exo', and 'thermal' for heat. (Exothermic describes any reaction/process that releases any form of energy, but only heat energy is studied at GCSE/IGCSE level).

Simple explanation of Endothermic Reactions

Not all reactions give out heat. Endothermic reactions, the opposite of exothermic, is a chemical reaction in which heat energy is absorbed/taken in. Endothermic reactions are a result of bonds being broken, which requires energy. If it helps, you can think of 'en' as 'in', or the 'entrance' for heat energy. (Endothermic describes any reaction/process that takes in any form of energy, but only heat energy is studied at GCSE/IGCSE level).

Note: Change in temperature

1. It's not always correct to decide if a reaction is endothermic or exothermic through it being hot or cold. It's best to always think of the direction heat moves.

• If heat goes in, it is endothermic (e.g. for ice to become liquid, heat must move in from the surroundings into the ice). 
• If heat goes out, it is exothermic (e.g. for liquid to turn to ice, heat must move out to cool it down).
  

2. Another point to note: If the surrounding temperature of a chemical reaction increases (the change in temperature is positive), this means that heat has been given out. Therefore, this reaction is exothermic.

Vice versa, if the temperature decreases this means that it is an endothermic reaction (the reaction has taken in the heat). This does not mean that the reaction has decreases in temperature, it has taken in heat energy which reduces the surrounding temperature.

• Endothermic: Change in temperature is positive.
  
• Exothermic: Change in temperature is negative.

Characteristics of Exothermic Reactions

• Heat is given out and transferred from the chemical to the surrounding environment.
  
• The temperature of the mixture rises, the container will therefore feel hot.
  

In an exothermic reaction, the temperature will rise until the highest temperature is reached. When the reaction is complete, the temperature will fall until it reaches room temperature.

Examples of Exothermic Reactions

• The combustion of fuels.
• The rusting of iron.
• The corrosion of metals.
• The neutralisation reaction between acid and alkali.
• Respiration.

Characteristics of Endothermic Reactions

• Heat energy is absorbed and transferred from the surrounding environment to the reactants.
• The temperature of the mixture falls, the container will therefore feel cold.
  

The temperature of the reaction mixture falls until the lowest temperature is reached. When the reaction is completed, the temperature of the mixture will rise until it reaches room temperature. 

Examples of Endothermic Reactions

• Photosynthesis.
• The action of light on silver bromide in photographic film.
• Thermal decomposition.

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