Have you ever thought about what happens when any matter changes its state? Changing of phase undergoes a heat transfer, but the temperature of the substances remains constant. So, the heat necessary for phase changes of water from solid to liquid or gas, or liquid to solid or gas, without any temperature alteration is known as latent heat of water.
Definition of Latent Heat
The energy radiated or absorbed by a thermodynamic system or a body during its change of state and without any change in its temperature is termed as latent heat (also called latent energy). Moreover, latent heat is generally represented in calories or joules per unit mass or mole of the body experiencing phase change.
For instance, consider melting ice blocks. Ice melts as it absorbs heat and undergoes a rise in its temperature. Furthermore, during the melting process, ice absorbs latent energy which helps in the change of its state from solid to liquid. However, the temperature of ice does not change when it intakes latent heat.
Furthermore, two primary forms of latent heat are as follows:
Latent Heat of Fusion
It refers to the energy related to freezing of liquid and melting of solid.
Latent Heat of Vaporization
Whereas, latent heat of vaporization refers to the energy related with changing of solid or liquid to gas and condensation of vapour.
Take an example of a bucket of water which is boiling. Moreover, when water in the bucket is boiling, its temperature stays at 100 degree Celsius or 212 degrees Fahrenheit till it completely evaporates. This happens because water absorbs the heat applied as latent heat of vaporization. Further, it is taken by the evaporating molecules.
Additionally, in this same way, the temperature of ice when it melts is 0 degree Celsius or 32 degrees Fahrenheit. Also, the water which is formed with the effect of latent heat of fusion is 0 degree.
Specific Latent Heat
Specific latent heat refers to the quantity of energy in heat form needed for the complete change of phase for one unit of mass of a particular matter. This is an intensive property. The specific latent heat expression is:
L = Q / m
Material or substance features are intensive properties which do not depend on the extent or shape of any matter. So, accordingly, to evaluate specific latent heat of water value for fusion and vaporization use the following expression:
Q = m x L
Where, Q is quantity of absorbing or radiating heat or during change of state of any matter (joules or calories)
m is mass of matter (kilograms)
L is specific latent heat for a particular matter (kJ kg-1); Lv for vaporization and Lf for fusion
Note: The latent heat of water at 0 degree Celsius for fusion is nearest to 334 joules per gram or 79.7 calories per gram. On the other hand, the latent heat of water at 100°C for vaporization is approximately 2230 joules per gram or 533 calories per gram.
Do It Yourself
(i) The Energy Absorbed or Released During a Change of State is Known as:
(a) thermal heat
(b) kinetic energy
(d) latent heat
(ii) Water Molecules have the Greatest Kinetic Energy in
(a) Ice at 0°C
(b) Water at 373 K
(c) Water at 98°C
(d) Steam at 150°C.
Latent heat of water is a vital concept in Physics, and you must have a thorough knowledge of its related concepts for good grades in your examinations. Now you can even download our Vedantu app for convenient access to detailed study materials and interactive online sessions.
1. What is the Latent Heat of Ice?
A. The latent heat of ice in calories per gram is 80. In the SI unit, it is 3.36X10^5 J/Kg.
2. What is the Latent Heat Formula?
Ans. The formula for latent heat is Q = m X L, where Q is sensible heat, m is mass of the body and L is latent heat.
3. Can a Thermometer Measure Latent Heat?
Ans. Usually, a thermometer reads temperature and a calorimeter measures calorific value or heat energy. Furthermore, the heat necessary for change of phase is latent heat. Hence, temperature change during supply of heat cannot be measured by a thermometer.