Change Of State

Card # 37‐Change of state

Matter normally exists in three states solid, liquid and gas. A transition from one of these states to another is called a change of state. Certain amount of heat energy is transferred between a substance and its surroundings when it undergoes a change of state. “At constant temperature the amount of heat required to change the state of unit mass of substance is called its latent heat”. It is observed that during melting and vaporization of a substance, its temperature does not increase. Thus the heat given is utilized in changing its state. This heat is called latent heat. If the mass of the substance is m and the heat required to change its state is L then Q = m L

L=

Q = Latent Heat m

Latent heat is a characteristic of the substance. It also depends on pressure. Latent heat of fusion = L f = for solid to liquid change Latent heat of vaporization = L

L

f

v

= for liquid to gas change.

For water = 334 J/kg or 80 k cal/Kg

It means that to change 1 kg of ice to 0 80 k cal heat

ο

C into liquid water at 0 ο C we must supply

L

v

For water = 540 k cal/kg or 2260 KJ/kg

It means that to convert 1 kg of ice at 1 0 0 must be added to it.

ο

C into steam at 1 0 0 ο C , 540 k cal of heat