Change of State

Matter can exist in three states of matter viz, solid, liquid and gas under different conditions of temperature and pressure. The process of change from one state to another state at constant temperature is called change of state.
When water is heated, its temperature increases slowly till it begins to boil. Till boiling is complete,the temperature remains constant. The heat supplied is used to change water into steam without changing its temperature. The change of state from liquid to gas is called vaporization.

Boiling point is the constant temperature at which a liquid changes into its vapour.

Condensation is the change of state from gas to liquid. Solidification is the change of state from liquid to solid. This change is also called freezing. Sublimation is the change of state from solid to vapour without becoming liquid.

Melting Point

The change of state of matter from solid to liquid is called melting of fusion. Melting point is defined as the constant temperature at which the solid is converted into liquid. It is characteristic of the substance. It also depends on pressure. The melting point of a substance at standard atmospheric pressure is called its normal melting point. Skating is possible on snow due to the formation of water below the skates. Water is formed due to the increase of pressure and it acts as a lubricant.

Latent Heat

When we are heating a subject, certain amount of heat is transferred between the object and the surroundings when it undergoes a change of state. The amount of heat per unit mass transferred during change of state of the substance is called latent heat of the substance for the process.

The heat required during a change of state depends upon the heat of transformation and the mass of the substance undergoing a change of state.

If mass m of a substance undergoes a change from one state to the other, then the quantitiy of heat required is given by,
Q = mL

Where L is known as latent heat and is a characteristic of the substance. The SI unit of latent heat is J Kg-1. The value of L also depends on the pressure. Its value is usually quoted at standard atmospheric pressure. The latent heat for a solid to liquid state change is called the latent heat of fusion (Lf), and that for a liquid to gas state change is called the latent heat of vaporisation (Lv). These are often referred to as the heat of fusion and the heat of vaporisation.