Latent Heat

When the addition of heat to a body does not cause a rise in temperature, then it is called as latent heat. In this case, the heat energy is used to produce a change of state to reduce the molecular bonds. When the substance changes from a solid to liquid state, then the latent heat of fusion occurs. And the latent heat of vaporization occurs when the latent heat required for a change from liquid to vapour state.

Required latent heat to change from solid to liquid = L_f x m

Required latent heat to change from liquid to vapour = L_v x m

L_f = the latent heat of fusion

L_v = the latent heat of vaporization

m = mass of the substance

        When heat is supplied to a body, the temperature is expected to rise and this sensible act is known as sensible heat.

Q = C x m x ∆t (Joules)

Q = sensible heat

C = specific heat capacity of the substance

∆t = temperature change

Water

Water is the best example as it can experience all three states of matter. Water at 100^oC requires 2,257kJ of heat energy to become a steam and hence L_v is 2,257kJ.

        The density of water increases until it reaches 4^oC and becomes less dense as it expands. The same phenomenon causes the water pipes to burst in frosty conditions. The water forms crystal lattice structure during freezing causing less density. But this freezing point of water can be lowered by adding substances like ethanol, ethylene glycol, propylene glycol or other methanol based solutions that prevent the crystal lattice formation in car radiators. These substances are usually highly inflammable. The more the anti-freezing mixture is added, the lower the freezing temperature.

        The boiling process of water can be divided into three following phases.

Nucleate boiling: During this phase, bubbles are formed from specific points of the heated area and rise to the water surface.

Transition boiling: In this, tiny bubbles stop raising and the liquid starts to become unstable.

Film boiling: The water becomes turbulent in this stage when the heated surface is so hot that a thin vapour layer occurs between the heated surface and the actual water.

        The process whereby a substance changes from a solid to a gas, or from a gas to a solid without passing through the liquid stage is called sublimation.