JEE Main & Advanced Physics Thermometry, Calorimetry & Thermal Expansion Phase Change and Latent Heat

(1) Phase : We use the term phase 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.

(i) For any given pressure a phase change takes place at a definite temperature, usually accompanied by absorption or emission of heat and a change of volume and density.

(ii) In phase change ice at \[{{0}^{o}}C\] melts into water at \[{{0}^{o}}C\]. Water at \[{{100}^{o}}C\] boils to form steam at \[{{100}^{o}}C\].

(iii) In solids, the forces between the molecules are large and the molecules are almost fixed in their positions inside the solid. In a liquid, the forces between the molecules are weaker and the molecules may move freely inside the volume of the liquid. However, they are not able to come out of the surface. In vapours or gases, the intermolecular forces are almost negligible and the molecules may move freely anywhere in the container. When a solid melts, its molecules move apart against the strong molecular attraction. This needs energy which must be supplied from outside. Thus, the internal energy of a given body is larger in liquid phase than in solid phase. Similarly, the internal energy of a given body in vapour phase is larger than that in liquid phase.

(iv) In case of change of state if the molecules come closer, energy is released and if the molecules move apart, energy is absorbed.

(2) Latent heat : The amount of heat required to change the state of the mass m of the substance is written as : Q = mL, where L is the latent heat. Latent heat is also called as Heat of Transformation. It's unit is cal/gm or J/kg and Dimension: \[[{{L}^{2}}{{T}^{-2}}]\]

(i) Latent heat of fusion : The latent heat of fusion is the heat energy required to change 1 kg of the material in its solid state at its melting point to 1 kg of the material in its liquid state. It is also the amount of heat energy released when at melting point 1 kg of liquid changes to 1 kg of solid. For water at its normal freezing temperature or melting point \[({{0}^{o}}C),\] the latent heat of fusion (or latent heat of ice) is \[{{L}_{F}}={{L}_{\text{ice}}}\approx 80\,cal/gm\approx 60\,kJ/mol\approx 336\,kilo\,joule/kg\]

(ii) Latent heat of vaporisation : The latent heat of vaporisation is the heat energy required to change 1 kg of the material in its liquid state at its boiling point to 1 kg of the material in its gaseous state. It is also the amount of heat energy released when 1 kg of vapour changes into 1 kg of liquid. For water at its normal boiling point or condensation temperature \[({{100}^{o}}C),\] the latent heat of vaporisation (latent heat of steam) is \[{{L}_{V}}={{L}_{\text{steam}}}\approx 540\,cal/gm\approx 40.8\,kJ/mol\approx 2260\,kilo\,joule/kg\]

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