Gas

Substance with no fixed shape, size or volume is said to be in gaseous state. Attractive forces among gaseous particles are very less or weak. They occupy the whole space available to them. Molecules of gas move freely. When the fast moving particles hit the walls of the container, they exert a pressure which is known as gas pressure.

Characteristics
(a) Shape and volume : A gas neither has a specific shape nor a specific volume. It acquires the shape and occupies the volume of the container.

(b) Density: The density of a gas is lower than that of a solid or liquid, because its particles are very far from each other. Therefore, mass per unit volume i.e. density is very less.

(c) Compressibility: A gas is highly compressible. This is due to large interparticle spaces which can be reduced by applying pressure. For example, LPG (used in home), O₂ (used in hospitals) & CNG (used in vehicles and homes), can be compressed and transported easily.

(d) Fluidity: A gas shows the property of fluidity because gaseous particles can flow from a region of high concentration to lower concentration.

(e) Diffusion : Due to very less interparticle forces of attraction and large interparticle spaces, gases readily intermix with each other without any external aid. This property of readily intermixing of particles is known as diffusion. With increase in temperature, rate of diffusion increases.

(f) Melting and boiling point : At normal atmospheric pressure, the melting and boiling points of a gas

are below room temperature.

(g) Effect of heating and cooling of a gas: A gas generally expands and contracts respectively on heating and cooling. Actually, on heating, the gaseous particles gain energy and move apart from one another (due to very less interparticle forces of attraction) and on cooling they contract.

(h) Liquefaction of a gas: The conversion of a gas into liquid is called 'Liquefaction'. Liquefaction can be carried out either by decreasing temperature or by increasing pressure.

Both methods (either decreasing temperature or increasing pressure) tend the gaseous particles to come closer by reducing their kinetic energy. During liquefaction, the interparticle forces of attraction increases while interparticle space decreases.

(i) Pressure of a gas : The gaseous molecules exert force on the walls of the container due to continuous movement and collisions with one-another and with the walls of the container.

The pressure of gas is the force applied by a gas per unit area on the walls of container. The pressure

of air at any location on the Earth is called atmospheric pressure. It is caused by the weight of the column of air above it.