Latent Heat

Latent heat is the heat energy absorbed or released during a phase change of a substance without a change in temperature. It occurs during transitions such as:

• Solid to liquid (melting) and liquid to solid (freezing): Heat of Fusion
• Liquid to gas (vaporization) and gas to liquid (condensation): Heat of Vaporization

Latent heat is related to the enthalpy of a substance and is responsible for overcoming the attractive forces between molecules during a phase transition.

• When a solid melts into a liquid, it absorbs energy to allow molecules to move freely.
• When a gas condenses into a liquid, it releases energy, bringing molecules closer together.
• Despite absorbing or releasing heat, the temperature remains constant during the phase change.

History of Latent Heat: The concept was introduced by Joseph Black (1750–1762) while studying distillation. James Prescott Joule later described latent heat as a form of potential energy, dependent on molecular structure and bonding.

1. Latent Heat of Fusion (Melting and Freezing)

The heat energy required to convert a solid into a liquid at a constant temperature. Example: Ice melts into water at 0°C, absorbing heat without a temperature rise.

2. Latent Heat of Vaporization (Boiling and Condensation)

The heat energy required to convert a liquid into gas at a constant temperature. Example: Water boils at 100°C, absorbing heat until all of it turns into steam.

1. Aim: To study the latent heat of fusion and vapourisation by observing phase changes in water.

2. Requirements: ice cubes, beaker, thermometer, stand, burner, and stirring rod.

3. Procedure

• Place ice cubes in a beaker and insert a thermometer into the ice.
• Start heating the beaker on a burner and record the temperature every minute.
• Observe the ice melting into water and stir to maintain uniform heating.
• Continue heating the water until it reaches 100°C and begins to boil.
• Observe the temperature remaining constant until all water converts into steam.
• Plot a temperature vs. time graph to analyse phase changes.

Latent heat

Temperature vs. Time Graph

Temperature vs. Time Graph

• Line AB: Ice melts at 0°C; temperature remains constant.
• Line BC: Water heats from 0°C to 100°C; temperature rises.
• Line CD: Water boils at 100°C; temperature remains constant.

This shows that latent heat is used to change the state, not to increase temperature.

4. Conclusion

• Melting Point: Ice melts at 0°C, absorbing heat without a temperature change.
• Boiling Point: Water boils at 100°C, absorbing heat until it fully vaporises.
• Latent Heat: Energy is absorbed during both phase changes without a rise in temperature.

The heat energy absorbed at constant temperature during the conversion of a liquid to gas is called the latent heat of vaporization. The specific latent heat of vaporisation is the heat required per unit mass of a liquid to change into gas. This energy is used to break intermolecular bonds rather than increasing temperature.

Effect of Atmospheric Pressure:

• The boiling point and latent heat of a substance depend on atmospheric pressure.
• Higher pressure increases the boiling point, while lower pressure decreases it.
• This principle is used in pressure cookers (higher pressure, higher boiling point, faster cooking) and mountain boiling (lower pressure, lower boiling point, slower cooking).

Melting and Boiling Points of Different Substances:

The heat energy absorbed (or liberated) in change of phase is not externally manifested by any rise or fall in temperature, it is called the latent heat.

Specific latent heat of a phase is the quantity of heat energy absorbed (or liberated) by the unit mass of the substance for the change in its phase at a constant temperature.

S.I. unit of specific latent heat is J kg⁻¹, and 1 cal g⁻¹ = 4.2 × 10³ J kg⁻¹.

L = \[\frac{\text{heat absorbed (or liberated) for the change of phase}}{\text{mass}}\]

or

L = \[\frac {Q}{m}\]

The heat energy absorbed at constant temperature during transformation of solid into liquid is called the latent heat of fusion.

The amount of heat energy absorbed at constant temperature by unit mass of a solid to convert into liquid phase is called the specific latent heat of fusion.

The amount of heat energy absorbed at constant temperature by unit mass of a liquid to convert into gaseous phase is called the specific latent heat of vapourization.

The constant temperature, at which the ice converts into water is called the melting point of ice.

The constant temperature at which the liquid transforms into gaseous state is called the boiling point of the liquid.