General Science - Physics: Change of State (Notes)

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.

-  The change of state from solid to liquid is called melting and from liquid to solid is called fusion. It is observed that the temperature remains constant until the entire amount of the solid substance melts. That is, both the solid and 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 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 substance at standard atomspheric pressure is called its normal melting point.

-  The change of state from liquid to vapour (or gas) is called vaporisation.

It is observed that the temperature remains constant until the entire amount of the liquid is converted into vapour. That is, both the liquid and vapour states of the substance coexist in thermal equilibrium, during the change of state from liquid to vapour.

-  The temperature at which the liquid and the vapour states of the substance coexist is called its boiling point. At high altitudes, atmospheric pressure is lower, reducing the boiling point of water as compared to that at sea level.

On the other hand, boiling point is increased inside a pressure cooker by increasing the pressure. Hence cooking is faster.

-  The boiling point of a substance at standard atmospheric pressure is called its normal boiling point.

-  However, all substances do not pass through the three states: solid-liquidgas.

There are certain substances which normally pass from the solid to the vapour state directly and vice versa.

The change from solid state to vapour state without passing through the liquid state is called sublimation, and the substance is said to sublime. Dry ice (solid CO2) sublimes, so also iodine.

During the sublimation process both the solid and vapour states of a substance coexist in thermal equilibrium.

-  Certain amount of heat energy is transferred between a substance and its 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 amount of heat energy supplied to a solid at its melting point, such that it changes into liquid state without any rise in temperature is called latent heat of fusion and that for a liquid-gas state change is called the latent heat of vaporisation.

-  Newton’s Law of Cooling says that the rate of cooling of a body is proportional to the excess temperature of the body over the surroundings.