Latent heat of any substance is defined as the heat required to change the phase of that material. The latent heat is absorbed at a constant temperature. This means that the heat absorbed is solely used for changing the phase of the substance. All the energy in the heat is absorbed by the constituent molecules or atoms to break the bond between them or to move apart.
The change in the net attractive force between the molecules is responsible for the material's phase change. Phase change can be from solid to liquid or from liquid to gas. In both cases, phase changes from one with stronger inter-particle force to one with weaker inter-particle force.
The concept of latent heat is of great interest in the field of thermodynamics. Any thermodynamical process involves changes in one of the state variables of pressure, temperature or volume. Latent heat of a substance describes the changes in two of these variables in particular: pressure and volume.
So for all thermodynamical processes happening at a constant temperature, the concept of latent heat is a central one. Heat is a form of energy, and the thermodynamic state of any system is contingent on the total energy content of the system.
Latent heat is thought of as the hidden energy that any substance absorbs. Because this always happens at the point of phase change, its effects are not immediately noticeable since the temperature is constant. That is why this heat is termed ‘latent,’ meaning hidden. Latent heat depends on the substance's pressure and temperature, which is undergoing a phase change.
The concept of latent heat was given to the world by Joseph Black. He was a British chemist working on the concepts of heat in the eighteenth century. Joseph was studying calorimetry when he encountered processes where the temperature was constant, yet the system was absorbing heat. To describe this phenomenon, he came up with the term' latent heat'. However, he contested this term with 'sensible heat', which is the heat that brings about a change in the body's temperature.
Later, James Prescott Joule made the assumption that the latent heat goes into increasing the potential energy between the particles of the system, and this heat energy increases the average kinetic and vibrational energy of the particles. Increased vibrational and translational (in the case of liquids and gases) motion leads to a change in the temperature of a body and can be measured by a thermometer.
A separate term, called the specific latent heat to calculate the heat required to change the phase of a given amount of substance. Specific latent heat is an intensive property of the system and is measured by dividing the heat required to change the phase of a given amount of substance by the mass of the substance. This gives us the heat required per unit mass to change the phase of any system.
Therefore, to calculate the latent heat, we only have to multiply its specific latent heat with the mass of the substance, and we will get the total latent heat required to change the phase. The formula for the same can be given by:
Q = mL
Here, Q is the latent heat
L is the specific latent heat
m is the mass of the given substance
A very famous case is that of water. Water exists in three phases in nature: solid, also called ice, liquid, also called water, gas, also called steam. The specific latent heat of fusion of water is 334 kilojoules per kilogram of water. This means that to convert one kilogram of water from solid to liquid, 334 kilojoules of heat is required. Normally, the atmospheric temperature, which is responsible for the translational and vibrational motion of the air particles, is enough to melt ice.
However, this process occurs very slowly if left totally to the natural processes because the heat is transferred to the surface of the ice. The water molecules on the surface absorb the heat from the surroundings and break loose. This exposes the next layer of molecules, and that layer then starts absorbing the latent heat. This process continues like this for layers that follow and the thawing of a frozen surface takes place.
Similarly, the latent heat of vapourisation of water is approximately 2264 kilojoules per kilogram of water. We notice that this is significantly greater than the latent heat of fusion of water. This is because, in the gaseous state, the particles exist with much higher energy than in the liquid state.