Latent heat is the amount of heat energy that a substance absorbs or releases during a change of state (phase change) without any change in its temperature. When a substance changes from one state to another, such as solid to liquid or liquid to gas, heat is required to break the forces between its particles. This heat does not increase the temperature, so it is called latent heat, meaning “hidden heat.”
The graph given below shows the change in various forms of matter and the heat related to each process.
A → B: Heating of Solid
- The substance is in solid state
- Temperature increases
- No change of state
B → C: Melting (Latent Heat of Fusion)
- Temperature remains constant
- Solid changes into liquid
- Heat used to overcome intermolecular forces
C → D: Heating of Liquid
- The substance is in liquid state
- Temperature increases
D → E: Boiling (Latent Heat of Vaporisation)
- Temperature remains constant
- Liquid changes into gas
- Heat used to separate particles completely
E → F: Heating of Gas
- The substance is in gaseous state
- Temperature increases
Formula for Latent Heat
Q = ML
- L = Latent Heat
- Q = Amount of Heat Released or Absorbed
- M = Mass of Substance
Unit of Latent Heat
- Its unit is J/kg.
- The dimension formula for latent heat is [ M0L2 T-2].
Examples of Latent Heat
- During boiling, the temperature of water remains constant until all the water changes into steam, even though heat is continuously supplied.
- All ice melts at 0°C even when heat is continuously absorbed.
- Vertical movement of air (winds) occurs due to the absorption and release of latent heat by land and water.
Type of Latent Heat
Depending on the type of change of state involved, latent heat is classified into different types
1. Latent Heat of Fusion
It is the amount of heat required to transform 1 kg of solids into liquid by melting and yet maintaining the temperature constant, and it is called latent heat of fusion.
Example:
- When 0°C ice is melted to 0°C water, heat is taken in, yet the temperature does not change.
- For Ice: Latent heat of fusion of ice = 3.34 x 10⁵ J/kg.
Q = m × L_f
- Q = heat energy
- m = mass
- Lf = latent heat of fusion
2. Latent Heat of Vaporization
Latent heat of vaporization is the amount of heat required to convert 1 kg of a liquid into vapour at its boiling point without any change in temperature.
Example:
- When 100°C of water turns into 100°C of steam, the temperature will be the same, but the heat will be absorbed.
- For Water: Latent heat of vaporization of water = 2.26 x 10⁶ J/kg.
Q = m × L_v
- Q = heat energy
- m = mass
- Lv = latent heat of vaporisation.
3. Latent Heat of Sublimation
- Latent heat of sublimation is the amount of energy in the form of heat that is needed to convert 1 kg of a solid substance to vapor at the same temperature (and not by means of passing through a liquid state).
- Some materials directly change to the gaseous state upon heating in the solid state.
- This is referred to as sublimation.
- When the sublimation process occurs, the temperature of the substance does not change although heat is added continuously.
- The heat energy supplied is utilized in overcoming the strong intermolecular forces existing in the solid to allow the particles to move freely like gas particles.
- This is the heat energy that is absorbed when the process of sublimation is taking place, and this heat energy is referred to as the latent heat of sublimation.
L=\frac{Q}{m}
- Q = heat energy supplied (in joules)
- m = mass of the substance (in kg)
Specific Latent Heat
Specific latent heat is defined as the amount of heat energy required to change the state of 1 kg of a substance at a constant temperature, without any change in temperature.
It is similar to latent heat, but in specific latent heat the mass of the substance is fixed as 1 kg, which makes comparison between substances easy.
L = \frac{Q}{m}
- L = specific latent heat
- Q = heat absorbed or released
- m = mass of a substance
Sensible Heat
Sensible heat is the heat energy that causes a change in temperature of a substance without changing its state.
When sensible heat is supplied to a body:
- The temperature of the body increases or decreases
- No phase change (solid, liquid, gas) occurs
Examples of Sensible Heat
- Heating water in a kettle before boiling.
- Cooling of hot tea when kept in open air.
- Heating of air during daytime by the Sun.
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Solved Examples
Example 1: Find the latent heat of a 20 kg substance if the amount of heat required for the phase change is 600 kcal.
Solution: Given,
Q = 600 Kcal
M = 20 KgFormula for latent heat is given as:
L = Q ⁄ M
= 600 / 20
= 30 Kcal/KgHence, the latent heat required for phase change is 30 Kcal / Kg.
Example 2: Find the mass of ice required to lower the temperature of 500 g of water at 20°C to water at 0°C. Given, the specific latent heat of ice is 336 J/g, and the specific heat capacity of water is 4.2 J/g°C.
Solution: Heat absorbed by Ice = Heat released by water
mIceL = mwaterc∆θ
Given,
mwater = 500 g = 0.5 kg
c = 4.2 J/ g°C
L = 336 J /g
∆θ = 20°C - 0°C = 20°CmIce×336 = 500×4.2×20
mIce = (500×4.2×20) / 336
= 125 g
The required amount of ice is 125 gram