PHASE CHANGES AND LATENT HEAT

Suppose that we slowly heat a cube of ice whose temperature is below 0°C at atmospheric pressure, what changes do we observe in the ice? Initially we find that its temperature increases according to equation Q = mc(T₂ – T₁). Once 0°C is reached, the additional heat does not increase the temperature of the ice. Instead, the ice melts and temperature remains at 0°C. The temperature of the water then starts to rise and eventually reaches 100°C, whereupon the water vaporizes into steam at this same temperature.

During phase transitions (solid to liquid or liquid to gas) the added heat causes a change in the positions of the molecules relative to one another, without affecting the temperature.

The heat necessary to change a unit mass of a substance from one phase to another is called the latent heat (L). Thus, the amount of heat required for melting and vaporizing a substance of mass m are given by,

Q = mL … (1)

For a solid-liquid transition, the latent heat is known as the latent heat of fusion (L_f) and for the liquid-gas transition, it is known as the latent heat of vaporization (L_v).

PRINCIPLE OF CALORIMETRY

When two bodies (one being solid and other liquid or both being liquid) at different temperatures are mixed, heat will be transferred from body at higher temperature to body at lower temperature till both acquire same temperature. The body at higher temperature releases heat while body at lower temperature absorbs it, so that:

Heat lost = Heat gained,

i.e. principle of calorimetry represents the law of conservation of heat energy.

While, using this principle always keep in mind that:

(a) Temperature of mixture (T) is always $⩾$ lower temperature (T_L) and $⩽$ higher temperature (T_H), i.e.,

T_L $⩽$ T $⩽$ T_H

i.e. the temperature of mixture can never be lesser than lower temperatures (as a body cannot be cooled below the temperature of cooling body) and greater than higher temperature (as a body cannot be heated above the temperature of heating body when there is no chemical reaction).

(b) When temperature of a body changes, the body releases heat if its temperature falls and absorbs heat when its temperature rises. The heat released or absorbed by a body of mass m is given by:

Q = mc $\Delta$T

Where c is specific heat of the body and $\Delta$T change in its temperature in C° or K.

(c) When state of a body changes, change of state takes place at constant temperature [m.pt. or b.pt.] and heat released or absorbed is given by

Q = mL

Where L is latent heat. Heat is absorbed if solid converts into liquid (at m.pt.) or liquid converts into vapours (at b.pt.) and is released if liquid converts into solid or vapours converts into liquid.