Physics · Thermal Properties Of Matter · NEET
During a change of state the added heat does not raise the temperature. Instead it goes into breaking the bonds that hold the molecules together (or into pushing molecules apart against pressure). This hidden energy is the latent heat. So the thermometer stays at 0 C while ice melts and at 100 C while water boils, even though heat is still going in. Temperature rises again only after the whole state change is finished.
Latent heat of fusion (Lf) is the heat per kg to change a solid into a liquid at the melting point (ice to water at 0 C). Latent heat of vaporisation (Lv) is the heat per kg to change a liquid into a gas at the boiling point (water to steam at 100 C). Both use Q = mL, but for water Lf = 3.33 x 10^5 J/kg while Lv = 22.6 x 10^5 J/kg, so Lv is much larger.
To melt a solid you only need to loosen the molecules so they can slide past each other; they stay close. To boil a liquid you must pull the molecules completely apart and also do work pushing back the surrounding air as the gas expands to a much bigger volume. This needs far more energy, so Lv is much greater than Lf for the same substance.
It is released. Latent heat is absorbed when a substance goes to a higher-energy state (melting, boiling) and released when it goes to a lower-energy state (freezing, condensing). When 1 kg of water freezes it gives out 3.33 x 10^5 J of heat to the surroundings, the same amount that was needed to melt it.
The specific latent heat L (per kg) does not depend on mass; it is a fixed property of the substance at a given pressure. But the total heat Q = mL does depend on mass. More mass needs more total heat, yet each kilogram still needs the same amount L.
A piece of ice falls from a height h so that it melts completely. Only one-quarter of the heat produced is absorbed by the ice, and all the gravitational energy is converted into heat. The value of h is (latent heat of ice = 3.4 x 10^5 J/kg, g = 10 N/kg):
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The SI unit of specific latent heat is joule per kilogram (J/kg), because it is heat per unit mass. Total heat Q = mL is in joules (J).
For water, latent heat of fusion Lf = 3.33 x 10^5 J/kg (to melt ice at 0 C) and latent heat of vaporisation Lv = 22.6 x 10^5 J/kg (to boil water at 100 C).
Both are at 100 C, but steam carries extra latent heat of vaporisation (22.6 x 10^5 J/kg). When steam touches skin and condenses it releases this large hidden heat, so it burns much more than the same mass of boiling water.
Sensible heat changes the temperature and uses Q = mcT. Latent heat changes the state at constant temperature and uses Q = mL. You can sense one as a temperature change and not the other.
Yes. Latent heat values are quoted at standard atmospheric pressure. If pressure changes, the boiling and melting points shift and the latent heat value changes too.
