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Latent heat is energy that a substance absorbs or releases during a change in its physical state (phase) that occurs without changing its temperature. The heat of fusion is the latent heat associated with melting a solid or freezing a liquid; the heat of vaporisation is the latent heat associated with vaporising a liquid or solid or condensing a vapour. Latent heat is the amount of heat (in joules or calories) per mole or unit mass of a substance undergoing a state change.
When a pot of water is kept boiling, the temperature remains at 100 degrees Celsius (212 degrees Fahrenheit) until the last drop evaporates, since all of the heat added to the liquid is absorbed as latent heat of vaporisation and carried away by the exiting vapour molecules. Similarly, as the ice melts, it maintains a temperature of 0 °C (32 °F), and the liquid water created by the latent heat of fusion maintains a temperature of 0 °C.
Water has a heat of fusion of roughly 334 joules (79.7 calories) per gramme at 0 degrees Celsius, and a heat of vaporisation of approximately 2,230 joules (533 calories) per gramme at 100 degrees Celsius. Because of the high heat of vaporisation, steam carries a large amount of thermal energy that is released when it condenses, making water an excellent working fluid for heat engines.
Latent heat is the work required to overcome the forces that hold atoms or molecules together in a material.
A crystalline solid’s regular structure is maintained by forces of attraction between its individual atoms, which oscillate slightly about their average positions in the crystal lattice.
As the temperature rises, these motions become more violent until, at the melting point, the attractive forces are no longer sufficient to keep the crystal lattice stable.
However, additional heat (the latent heat of fusion) must be added (at constant temperature) to achieve the transition to the even more disordered liquid state, in which individual particles are no longer held in fixed lattice positions but are free to move about in the liquid.
Specific latent heat is defined as the amount of heat energy (heat, Q) consumed or discharged when a body goes through a steady-state temperature process. The formula for specific latent heat is as follows:
L = Q/m
Where:
The specific latent heat is denoted by the letter L.
Q denotes the amount of heat retained or discharged. The mass of a substance is denoted by m.
Dimensional Formula of Latent Heat
The latent heat dimensional formula is given by,
[M0 L2 T-2]
Where M stands for mass.
T = Time L = Length
Latent heat (L) = Heat × [Mass]-1 . . . . . . (1)
The mass dimensional formula = [M1 L0 T0] . . . . (2)
the dimensions of heat = dimensions of energy = dimensions of work
work = force × displacement . . . . (3)
Displacement = [M0 L1 T0] . . . (5)
Force = m a = [M1 L1 T-2] . . . (4)
When we substitute equations (4) and (5) into equation (3), we get
Work = [M1 L1 T-2] × [L1]
the work or heat dimensions = [M1 L2 T-2] . . . . (6)
When we substitute equations (2) and (6) into equation (1), we get
Latent heat (L) = Heat × [Mass]-1
Or, L = [M1 L2 T-2] × [M1 L0 T0]-1 = [M0 L2 T-2].
As a result, latent heat is represented dimensionally as [M0 L2 T-2].
FAQs
What exactly is latent heat?
Latent heat is energy that a substance absorbs or releases during a change in its physical state (phase) that occurs without changing its temperature.
What is the nature of latent heat?
Latent heat of fusion (melting) and latent heat of vaporisation are two common types of latent heat (boiling). These terms describe the direction of energy flow when transitioning from one phase to the next: solid to liquid and liquid to gas.
