Latent Heat Of Vaporization And Fusion

Latent Heat

What is latent heat?

If you remember, while heating a cube of ice, there is a short span of time when there is no change in temperature in spite of continued heating. Till the ice cubes have melted entirely, the sample in the beaker will experience no rise in temperature.

Latent Heat

Temperature of water remains zero degree Celsius till all the ice have melted.

What causes this temporary stagnancy in temperature?

If we continue heating, then we reach a temperature where the water changes states from liquid to gas; 1000C. At this temperature, water starts escaping the surface in the form of water vapour. From the instant the water touches 1000C and starts boiling till the time the entire water sample is boiled off, the water’s temperature remains constant. Now we see a pattern, don’t we? This stagnancy in temperature seems to occur along with the change of state. Why so?

What happens during the change of state; either solid gets converted to liquid or liquid to gas.

To understand this concept, we will have to examine the molecular behavior. In case of ice, the molecules are packed together and are mostly immovable. They still do vibrate in their position, but they don’t flow from place to place. This represents the lowest energy state of water. Now when you heat ice cubes, you are supplying energy in the form of heat and this energy increases the vibration of the molecules in the ice.

On continued heating, the water molecules gain enough energy to break the hydrogen bonds of the H2O which gives ice its rigidity. So the thing is, water starts melting at 00C but to break these hydrogen bonds, it needs more heat. This heat is known as the Latent heat. When speaking of latent heat in terms of change of state from solid to liquid we refer to it as the Latent heat of Fusion.

If the amount of latent heat required to change the state of a substance of mass ‘m’ is ‘Q’ then the latent heat formula is;

\(L \) = \( \frac{Q}{m}\)

The latent heat of fusion of water is;

\(L_{f} \) = \(3.33\times 10^{5}Jkg^{-1}\)

After the ice has all melted, the temperature of the water begins to rise again till it reaches 1000C. If Ice is the lowest energy state of water, gas is the other extreme; gas molecules have the most energy and are constantly whizzing around. This energy to whizz around comes from the heat we supply to it. Remember that water exists as a liquid due to intermolecular forces of attraction which needs to be overcome in order to turn it into gas.

Once it reaches 1000C, the energy supplied is used by the water molecules to break its bonds with other molecules and run away. This energy doesn’t show up as a temperature change as in the case of melting but is used by the molecules to break the bonds and enter a higher energy state i.e. gaseous state. The heat required for a phase change from liquid to gas is referred to as Latent heat of Vaporization.

The latent heat of vaporization of water is;

\(L_{f}\) = \( 22.6\times 10^{5}Jkg^{-1}\)

Latent Heat of Vaporization is 7.5 times as large as the Latent Heat of fusion i.e. the energy required to melt something is considerably lower than the energy required to vaporize something. This is the reason why a burn caused by steam at 1000C will be a lot more severe than a burn caused by water at 1000C. The graph below shows the two state changes of water and their corresponding temperature changes.

Temperature vs Heat

Temp vs Heat graph (Observe the Latent Heat)

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Practise This Question

The weather report reads, “Temperature 20C:  Relative humidity 100%”.  What is the dew point?