Properties of Gas

In our surrounding, there are lots of things like furniture, ceiling fan, tube light, refrigerator, TV, mobile, water, oil, juice etc. We can touch, feel and see these substances. But we can neither see nor touch Air without which the life is meaningless. Have you ever thought about Air? Why can not touch it? Why we can not see? How the Air is formed?

Air is a mixture of gasses. It contains oxygen gas, carbon dioxide gas, water vapour, nitrogen gas, nitrogen dioxide gas, other different gasses, dust particle and microbes etc. Still, we can not touch the air. Due to this unique behaviour of gas, many scientists researched and gave a conclusion. Some of that has been proved and some are valid only for the ideal case. Let us discuss the nature and properties of the gas.

Properties of Gas :

1 ) Shape and volume: The distance between the molecules of a gas is much much larger than that of liquid and solid. The intermolecular force of interaction between the molecules is negligible. For this reason, the gas has neither shape nor specific volume. It has volume as same as the volume of the container. Its volume is variable that means it can change with respect to some other factors like pressure, temperature and available space.

2 ) Motion of the gas molecules and gas pressure:  The gas molecules always move randomly in all possible direction. During the motion, these molecules collide each other, as well as they, collide with the wall of the container. During this collision, it exerts pressure on the wall of the container. This pressure is called as Gas pressure.

3 ) Expansibility and compressibility: Gas has two unique and very important properties expansibility and compressibility. If the external pressure is increased on the surface of the gas then the molecules will tend to close to each other. Due to this, the volume will decrease and this property is termed as the compressibility. If we decrease the pressure on the surface of the gas, there is more space than the previous, the molecules tend to occupy this space and they move far to each other. In this way, the volume of the gas increases. This property is termed as the expansibility. But there are some common factors which should be kept constant throughout the experiment and these factors are temperature, mass ( number of molecules ) and nature of the gas. These two properties have been explained through Boye’s law.

4 ) Diffusibilty: The different gasses can be diffused to each other. There are two different gasses in two different containers. These gasses are mixed in a new container. Then the molecules of the gasses are diffused.

A very good example of the diffusibility of the gasses is the atmosphere. In the atmosphere, there are different gasses and their molecules are mixed but they can not affect the nature of each other.

5 ) Temperature and volume : If we increase the temperature of the gas, then the kinetic energy of the molecules is also increased. The rate of the motion of the molecules also increases and the molecules tend to occupy more space. Due to this, the volume of the gas increase. But the pressure, mass ( number of the molecules ) and nature of the gas should be kept constant. This is explained through Charl’s law.

Kinetic theory of the Gas :

1 ) The gas molecule is of spherical shape and has negligible mass and volume with respect to the mass and volume of the container.

2 ) Gas molecules are always in random motion ( Brownian Motion ) in all possible direction.

3 ) Molecular interaction is negligible.

4 )  The gas consists of a large number of molecules.

5 ) The gas molecules of the gas collide with each other as well as with the wall of the container during the rapid, constant and random motion. This collision has elastic in nature.

Note :

This theory is applicable only for the ideal case. In non-ideal case, 

1 ) The mass of the molecule of the gas and its volume can not be neglected. It has an effective mass and volume. Even a single molecule contributes volume in calculating the volume of the gas kept in a container. 

2 ) The intermolecular force of interaction can not be neglected. In calculating the pressure of the gas, it contributes an effective value.