substance at standard atomspheric pressure is called its normal melting point . Let us do the following activity to understand the process of melting of ice. Take a slab of ice. Take a metallic wire and fix two blocks, say kg each, at its ends.
Put the wire over the slab as shown in Fig. . . You will observe that the wire passes through the ice slab.
This happens due to the fact that just below the wire, ice melts at lower temperature due to increase in pressure. When the wire has passed, water above the wire freezes again. Thus, the wire passes through the slab and the slab does not split. This phenomenon of refreezing is called regelation .
Skating is possible on snow due to the formation of water under the skates. Water is formed due to the increase of pressure and it acts as a lubricant. Fig. .
After the whole of ice gets converted into water and as we continue further heating, we shall see that temperature begins to rise (Fig. . ). The temperature keeps on rising till it reaches nearly ° C when it again becomes steady.
The heat supplied is now being utilised to change water from liquid state to vapour or gaseous state. The change of state from liquid to vapour (or gas) is called vaporisation . It is observed that the temperature remains constant until the entire amount of the liquid is converted into vapour. That is, both the liquid and vapour states of the substance coexist in thermal equilibrium, during the change of state from liquid to vapour.
The temperature at which the liquid and the vapour states of the substance coexist is called its boiling point . Let us do the following activity to understand the process of boiling of water. Take a round-bottom flask, more than half filled with water. Keep it over a burner and fix a Triple Point The temperature of a substance remains constant during its change of state (phase change).
A graph between the temperature T and the Pressure P of the substance