× J kg – K – ) ( ° C ) s Al = . kJ kg – K – . CHANGE OF STATE Matter normally exists in three states: solid, liquid and gas. A transition from one of these states to another is called a change of state.
Two common changes of states are solid to liquid and liquid to gas (and, vice versa). These changes can occur when the exchange of heat takes place between the substance and its surroundings. To study the change of state on heating or cooling, let us perform the following activity. Take some cubes of ice in a beaker.
Note the temperature of ice. Start heating it slowly on a constant heat source. Note the temperature after every minute. Continuously stir the mixture of water and ice.
Draw a graph between temperature and time (Fig. . ). You will observe no change in the temperature as long as there is ice in the beaker.
In the above process, the temperature of the system does not change even though heat is being continuously supplied. The heat supplied is being utilised in changing the state from solid (ice) to liquid (water). Fig. .
A plot of temperature versus time showing the changes in the state of ice on heating (not to scale). The change of state from solid to liquid is called melting or fusion and from liquid to solid is called freezing . It is observed that the temperature remains constant until the entire amount of the solid substance melts. That is, both the solid and the liquid states of the substance coexist in thermal equilibrium during the change of states from solid to liquid.
The temperature at which the solid and the liquid states of the substance is in thermal equilibrium with each other is called its melting point . It is characteristic of the substance. It also depends on pressure. The melting point of a