Heat is the form of energy transferred between two (or more) systems or a system and its
surroundings by virtue of temperature difference. The SI unit of heat energy transferred is expressed in joule (J) while SI unit of temperature is Kelvin (K), and degree Celsius (°C) is a commonly used unit of temperature.
The change in temperature of a substance, when a given quantity of heat is absorbed or rejected
by it, is characterised by a quantity called the heat capacity of that substance.
Substance has a unique value for the amount of heat absorbed or given off to change the
temperature of unit mass of it by one unit. This quantity is referred to as the specific heat capacity of the substance.
The change of state from solid to liquid is called melting or fusion and from liquid to solid is called
freezing.
The temperature at which the solid and the liquid states of the substance is in thermal equilibrium with each other is called its melting point.
The change of state from liquid to vapour (or gas) is called vaporisation. It is observed that the temperature remains constant until the entire amount of the liquid is converted into vapour. That is, both the liquid and vapour states of the substance coexist in thermal equilibrium, during the change of state from liquid to vapour.
The temperature at which the liquid and the vapour states of the substance coexist is called its boiling point.
The change from solid state to vapour state without passing through the liquid state is called sublimation.
Thus, if mass m of a substance undergoes a change from one state to the other, then the quantity of heat required is given by Q = m L or L = Q/m, where L is known as latent heat and is a
characteristic of the substance. The value of L also depends on the pressure. Its value is usually quoted at standard atmospheric pressure.
The latent heat for a solid-liquid state change is called the latent heat of fusion, and that for a liquid-gas state change is called the latent heat of vaporisation.