Change of state

Change of state

Any matter around us can be in three forms: solid, liquid and gasese forms of matter are called states of matter. Depending upon the temperature, pressure and transfer of heat, matter is converted from one state to another. The conversion of matter from one state to another is called change of state in matter. The process of changing of a substance from one physical state to another at a de nite temperature is de ned as change of state.

For example, water molecules are in liquid state at normal temperature. When water is heated to 1000C, it becomes steam which is a gaseous state of matter. On reducing the temperature of the steam it becomes water again. If we reduce the temperature further to 00C, it becomes ice which is a solid state of water. Ice on heating, becomes water again. us, water changes its state when there is a change in temperature. ere are different such processes in the change of state in matter. The gure 1.9 shows various processes of change state.

Melting – Freezing

The process in which a solid is converted to liquid by absorbing heat is called melting or fusion. The temperature at which a solid changes its state to liquid is called melting point. The reverse of melting is freezing. The process in which a liquid is converted to solid by releasing heat is called freezing. The temperature at which a liquid changes its state to solid is called freezing point. In the case of water, melting and boiling occur at 0˚C.

Boiling-Condensation

The process in which a liquid is converted to vapor by absorbing heat is called boiling or vaporization. The temperature at which a liquid changes its state to gas is called boiling point. The process in which a vapor is converted to liquid by releasing heat is called condensation. The temperature at which a vapour changes its state to liquid is called condensation point. Boiling point as well as condensation point of water is 100˚C.

Sublimation

Some solids like dry ice, iodine, frozen carbon dioxide and naphthalene balls change directly from solid state to gaseous state without becoming liquid. The process in which a solid is converted to gaseous state is called sublimation.

Various stages of conversion of state of matter with heat with the corresponding change in temperature is shown in Figure 1.10