Matter can exist in different states solid,liquid and vapour. These are known as the phases of matter. Any substance can be taken to any state by providing suitable conditions. It is the inter molecular binding in the given condition which decides as in what state the matter is . Certain substance exist as solids (metals) at ordinary temperatures and pressures, certain substances exist as liquids (water) and yet other substances (O2 and N2 ) exist in the vapour (or gaseous ) state. Matter can be changed from one state to the other by suitably manipulating the temperature and pressure conditions.
When water is given heat at a certain temperature it starts getting vapourised. This takes place at constant temperature known as the boiling point. It is nearly 1000 C at normal pressure. But water can also be vaporised at a lower temperature by lowering pressure. Ice can be converted into water at a fixed temperature known as melting point which is 00 C at ordinary pressure. If the pressure is increased ice can melt at a lower temperature than 00 C. However, at a given pressure the temperature of the substance remains constant until the change of state is complete. Similarly steam can be condensed into water and water can be freezed into ice at the respective temperatures.
The changes in state are shown in the below figure at normal pressure.
The region AB of the curve represents the rise of temperature from -20 0 C to 00 C when we heat the beaker containing some pieces of ice. The regions 'BC' and "DE' of eh graph are horizontal and represent the changes of state. 'BC' indicates the change of state from solid to liquid at constant temperature 00 C and DE represents change of state from liquid to vapour state at constant temperature 1000 C.
Certain substance can also be directly converted into vapour and the process is known as sublimation (Ex. Camphor)