Latent Heat

Let Us Learn About Latent Heat

When a substance changes phase, that is it goes from either a solid to a liquid or liquid to gas, the energy, it requires energy to do so. The potential energy stored in the interatomics forces between molecules needs to be overcome by the kinetic energy the motion of the particles before the substance can change phase.

Latent heat is the energy absorbed or released when a substance changes its physical state. Latent heat is absorbed upon evaporation, and released upon condensation to liquid (as in clouds). Latent heat is also absorbed when water melts, and released when it freezes.

Latent heat is the name given to energy which is either lost or gained by a substance when it changes state, for example from gas to liquid. It is measured as an amount of energy, joules, rather than as a temperature.

If we measure the temperature of the substance which is initially solid as we heat it we produce a graph like

Temperature change with time. Phase changes are indicated by flat regions where heat energy used to overcome attractive forces between molecules

Starting a point A, the substance is in its solid phase, heating it brings the temperature up to its melting point but the material is still a solid at point B. As it is heated further, the energy from the heat source goes into breaking the bonds holding the atoms in place. This takes place from B to C. At point C all of the solid phase has been transformed into the liquid phase. Once again, as energy is added the energy goes into the kinetic energy of the particles raising the temperature, (C to D). At point D the temperature has reached its boiling point but it is still in the liquid phase. From points D to E thermal energy is overcoming the bonds and the particles have enough kinetic energy to escape from the liquid. The substance is entering the gas phase. Beyond E, further heating under pressure can raise the temperature still further is how a pressure cooker works.

Latent Heat of Fusion and Vaporization

The energy required to change the phase of a substance is known as a latent heat. The word latent means hidden. When the phase change is from solid to liquid we must use the latent heat of fusion, and when the phase change is from liquid to a gas, we must use the latent heat of vaporization.

The energy require is Q= m L, where m is the mass of the substance and L is the specific latent heat of fusion or vaporisation which measures the heat energy to change 1 kg of a solid into a liquid.