Change Of State - Practice Questions & MCQ

What is Phase?

We know that there are three states of matter. So, the term phase is used to describe a specific state of matter, such as solid, liquid or gas. A transition from one phase to another is called a phase change. So we need to supply or extract heat from any substance to change its phase or state. For any given pressure a phase change takes place at a definite temperature. So the temperature will not change during phase change.

Water is very common substance known to us. So at 0⁰C temperature ice and liquid water can change its phase and at 100⁰C the liquid water and steam can change its phase to each other at the atmospheric pressure.

Latent heat : Latent heat is also called hidden heat. In this there is no change in the temperature of the body and because of that it is said to be hidden or latent as we are not feeling any change in temperature of the body. The amount of heat required to change the state of the mass m of the substance is written as:
$Q=m L_{\text {, where }} L$ is the latent heat. It's unit is $\mathrm{cal} / \mathrm{gm}$ or, $J / \mathrm{kg}$ and Dimension: $\left[L^2 T^{-2}\right]$

Basically the latent heat is classified in two types -
(i) Latent heat of fusion : The latent heat of fusion is the heat energy required to change one kilogram of the material in its solid state at its melting point to one kilogram of the material in its liquid state. The latent heat of fusion for water (or latent heat of ice) is -

$$
L_F=L_{\text {ice }} \approx 80 \mathrm{cal} / \mathrm{gm} \approx 60 \mathrm{~kJ} / \mathrm{mol} \approx 336 \text { kilo }- \text { joule } / \mathrm{kg}
$$

(ii) Latent heat of vaporisation : The latent heat of vaporisation is the heat energy required to change one kilogram of the material in its liquid state at its boiling point to one kilogram of the material in its gaseous state. The latent heat of vaporisation of water (latent heat of steam) is $L_V=L_{\text {steam }} \approx 540 \mathrm{cal} / \mathrm{gm} \approx 40.8 \mathrm{~kJ} / \mathrm{mol} \approx 2260$ kilojoule $/ \mathrm{kg}$

Latent heat of vaporisation is more than the latent heat of fusion. This is because when a substance gets converted from liquid to vapour, so the increase in volume is large. Hence more amount of heat is required. But when a solid gets converted to a liquid, then there is negligible increase in volume. Hence very less amount of heat is required.