## Changing of solid to liquid state (melting)

When a solid is heated it starts melting at a certain fixed temperature (melting point). At this stage even when the heating is continued, the temperature does not change until the whole of solid is converted into liquid. The state when solid and liquid phases of a substance coexist is called solid-liquid equilibrium. Solid-liquid equilibrium is described as,

• If no heat is exchanged with the surroundings, then the temperature and the mass of the two phases (solid and liquid) remain constant.

For example, if we place ice and water at 273 K (0°C) under normal atmospheric pressure in a perfectly insulated thermos flask, since the flask is insulated, there will be no exchange of heat between its contents and the surroundings. We notice that

• The temperature of both the phases remains constant, i.e., temperature of the system does not change.
• Mass of each phase (ice and water) does not change with time.

Rate of transfer of molecules from ice to water = Rate of transfer of molecules from water to ice