Change of state:

Matter can be changed from one state to another. For such a change to occur thermal energy is added to or removed from a substance.

Activity:

Take a beaker and place it over a stand. Put small pieces of ice in the beaker and suspend a thermometer in the beaker to measure the temperature of the ice.

Now place a burner under the beaker. The ice will start melting. The temperature of the mixture containing ice and water will not increase above 0°C until all the ice melts and we get water at 0°C. If this water at 0°C is further heated, its temperature will begin to increase above 0°C as shown by the graph in the figure. 8.9.

Part AB: On this portion of the curve, the temperature of ice increases from 30°C to 0°C.

Part BC: When the temperature of ice reaches 0°C, the ice water mixture remains at this temperature until all the ice melts.

Part CD: The temperature of the substance gradually increases from 0°C to 100°C. The amount of energy so added is used up in increasing the temperature of the water.

Part DE: At 100°C water begins to boil and changes into steam. The temperature remains 100°C until all the water changes into steam.