What is the heat of vaporisation of water?

The heat of vaporisation of water is approximately 40.7 kJ/mol.

The heat of vaporisation, also known as enthalpy of vaporisation, is the amount of energy required to transform a given quantity of a substance into a gas at a constant pressure. For water, this value is approximately 40.7 kilojoules per mole (kJ/mol) at its boiling point, which is 100 degrees Celsius at standard atmospheric pressure.

This energy is used to overcome the forces of attraction between the water molecules, allowing them to move freely and become a gas. In other words, it's the energy needed to break the hydrogen bonds between water molecules. Hydrogen bonds are relatively strong intermolecular forces, which is why water has a higher heat of vaporisation compared to many other liquids.

The heat of vaporisation is a crucial concept in many biological processes. For instance, it plays a significant role in the regulation of body temperature in mammals. When we sweat, the body uses the heat from our skin to evaporate the sweat, which is mostly water. This process cools the body down, as heat is taken from the body to provide the heat of vaporisation for the water.

Moreover, the high heat of vaporisation of water also helps to stabilise the temperature of the Earth's environment. Large bodies of water, such as oceans and lakes, can absorb a lot of heat energy from the sun during the day without their temperature rising significantly. At night, this heat is slowly released, helping to moderate temperature changes.

In summary, the heat of vaporisation of water is the energy required to change water from a liquid to a gas at its boiling point, which is approximately 40.7 kJ/mol. This property of water plays a vital role in many biological and environmental processes.