Latent Heat

The word latent comes from Latin latere, meaning "to lie hidden." When a substance changes its physical state — solid to liquid, or liquid to gas — it absorbs or releases a certain quantity of heat energy. This energy transfer occurs without any change in temperature. Because a thermometer shows no change, the heat appears to be "hidden."

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.

OR

Latent heat is the quantity of heat energy required to change the state of unit mass of a substance from one phase to another, at constant temperature and constant pressure.

The quantity of heat required to convert unit mass of a substance from its solid state to the liquid state, at its melting point, without any change in its temperature, is called its latent heat of fusion (L_f).

OR

The heat energy absorbed at constant temperature during the 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 quantity of heat required to convert unit mass of a substance from its liquid state to vapour state, at its boiling point without any change in its temperature is called its latent heat of vapourization (L_v).

Q = m × L

where,

Q = Heat energy absorbed or released during phase change
m = Mass of the substance undergoing phase change
L = Specific Latent Heat (characteristic of the substance & process)

SI Units = J kg⁻¹

Procedure

• Place pieces of ice in a glass beaker. Insert a thermometer bulb into the ice and record the initial temperature.
• Mount the beaker on a stand and apply heat uniformly using a spirit burner.
• Record the temperature using the thermometer at regular 1-minute intervals.
• Stir the ice-water mixture continuously as melting begins to maintain thermal equilibrium.
• Continue heating even after all ice has melted — do not stop until all water converts to steam.
• Plot a graph of temperature (y-axis) versus time (x-axis) to visualise the phase transitions.

Fig.: Experimental setup: ice in a beaker, thermometer, and burner on a stand

Observations Table:

The Heating Curve

Fig. 3 — Temperature vs. Time graph for heating ice → water → steam (at 1 atm). The flat segments A→B and C→D are where latent heat is released.

All values at one standard atmospheric pressure (1 atm). Values vary at different pressures.

Problem: When 0.1 kg of ice at 0°C is mixed with 0.32 kg of water at 35°C in a container, the resulting temperature of the mixture is 7.8°C. Calculate the latent heat of fusion of ice.

Given: s_water = 4186 J kg⁻¹ K⁻¹

Given Data:

Mass of ice (m_ice) = 0.1 kg
Mass of water (m_w) = 0.32 kg
Temp. of ice (T_ice) = 0°C
Temp. of water (T_w) = 35°C
Final temp. (T_F) = 7.8°C
Specific heats = 4186 J kg⁻¹K⁻¹

• Formula: Q = mL. Specific latent heat L has SI unit J kg⁻¹.
• Temperature stays constant during any phase change. Heat energy goes into breaking or forming intermolecular bonds, not into raising kinetic energy.
• Latent Heat of Fusion (water): L_f = 3.33 × 10⁵ J kg⁻¹ = 80 cal/g. Heat needed to melt 1 kg of ice at 0°C.
• Latent Heat of Vaporisation (water): L_v = 22.6 × 10⁵ J kg⁻¹ = 540 cal/g. Heat is needed to convert 1 kg of water to steam at 100°C.
• L_v ≫ L_f because vaporisation requires complete molecular separation and work against atmospheric pressure during expansion.
• All latent heat values depend on atmospheric pressure. Standard values quoted at 1 atm. Increasing pressure raises the boiling point (pressure cooker effect).