Give reasons:

(a) A gas fills completely the vessel in which it is kept.

(b) A gas exerts pressure on the walls of the container.

(c) A wooden table should be called a solid.

(d) We can easily move our hand in air but to do the same through a solid block of wood, we need a karate expert.

(a) A gas is able to fill the container because its particles are able to vibrate randomly in all the directions. The particles of gas have very high kinetic energy and very negligible forces of attraction. As a result, gases are able to fill the container fully.

(b) The particles of a gas have very high kinetic energy. They have negligible forces of attraction. They move randomly with high speeds in all directions. Due to this random movement, the particles hit each other and also the walls of the container. As a result, a gas exerts pressure on the walls of the container. The pressure exerted by the gas is because of this force exerted by gas particles per unit area on the walls of the container.

(c) A wooden table has a definite shape and a definite volume. The particles in a wooden table are tightly packed and have no intermolecular spaces and hence, it cannot be compressed. Since, a wooden table has the basic characteristics of a solid i.e. rigidity, definite shape and definite volume, it should be called a solid.

(d) Air is a mixture of gases. The particles in air are very far away from each other and have very weak forces of attraction between them. These weak forces of attraction can be easily overcome and a very small external force can separate them. As a result, we can easily move our hand in air. On the other hand, the particles of a solid block of wood are very closely packed and have very strong forces of attraction between them.

A large amount of external force is required break apart a solid block of wood which only a karate expert can apply.