This is the quantity of energy required to change the state of a substance without any temperature change. It is also the heat absorbed or released during the change of states such as boiling, melting. Latent heat refers to as hidden heat which implies invisible heat. We have two types of latent heat, namely;

• Specific latent heat of fusion( L_f)
• Specific latent heat of vaporization(L_v)

1. Specific latent heat of fusion( L_f)

This is the quantity of energy required to change 1kg of a substance from solid to liquid without change in temperature. It can be calculated from the formula below;

From Q = mL_f;           Therefore,      Lf= Qm

Where L_f = Specific latent heat of fusion,                

m = mass

Q = Heat energy

The S.I unit of specific latent heat of fusion is J/kgK

Example: What energy is required to change 2kg of ice from 0°C to 20°C, given that  Lf=33400J/kgK.

Solution

Q=mLf+ mc(θ2-θ1)

But m=2kg,Lf=33400J/kgK, C=2100J/kgK,  θ1=0°C  ,   θ2=20°C.

Q = (2*33400) + (2*2100*20)

Q = 66800 + 84000 = 150800J

Q = 150800J

2. Specific Latent Heat of Vaporization (L_V)

This is the quantity of energy required to change the state of 1kg of a substance from liquid to vapor without change in temperature. It can be calculated from the formula below;

From Q = mL_V                     Therefore LV= Qm

Where LV is the specific latent heat of vaporization

The S.I unit of specific latent heat of vaporization is J/kgK