What is the latent heat of vaporisation?

The latent heat of vaporisation is the energy required to change a substance from liquid to gas without changing its temperature.

When a liquid turns into a gas, it needs energy to break the bonds between its molecules. This energy is called the latent heat of vaporisation. Even though the temperature of the substance doesn't change during this process, a significant amount of energy is still absorbed. This is why, for example, boiling water stays at 100°C until it has completely turned into steam.

The concept of latent heat is crucial in understanding phase changes. When you heat a liquid, its temperature rises until it reaches its boiling point. At this point, any additional energy you provide goes into changing the state of the liquid to gas, rather than increasing its temperature. This energy is used to overcome the intermolecular forces that hold the liquid molecules together.

Different substances have different latent heats of vaporisation. For instance, water has a relatively high latent heat of vaporisation, which is why sweating is an effective way for humans to cool down. The energy required to evaporate the sweat from our skin takes away a lot of heat, helping to lower our body temperature.

In GCSE Physics, understanding the latent heat of vaporisation helps you to analyse various phenomena, such as why steam burns are more severe than boiling water burns. Steam contains more energy due to the latent heat absorbed during vaporisation, which it releases when it condenses on your skin.

To calculate the energy required for a phase change, you can use the formula \( Q = mL \), where \( Q \) is the heat energy, \( m \) is the mass of the substance, and \( L \) is the specific latent heat of vaporisation. This formula helps you quantify the energy involved in changing a liquid to a gas, which is essential for solving related problems in your exams.