(a) The particles of a gas have high kinetic energy and negligible forces of attraction amongst them. Due to this the particles of a gas are constantly moving with high speeds in all the directions and the gas completely fills the vessel in which it is kept.
(b) Because of high kinetic energy and negligible forces of attraction, the particles of a gas move with high speeds in all directions. When the fast moving gas particles hit the walls of its container from inside, they exert a pressure (called gas pressure). Thus, the pressure exerted by a gas is due to the constant collisions of the fast moving gas particles against the inner walls of the container.
(c ) A wooden table is a rigid object having a definite shape and a definite volume. Since a wooden table has these basic characteristics of solid state (rigidity, definite shape and definite volume), it should be called a solid.
(d) Air is a gas (or a mixture of gases) whose particles are very far apart and there are very weak forces of attraction between them. The extremely weak forces between particles of air can be overcome easily due to which 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 there are very strong forces of attraction between the particles of wood. It needs a huge outside force to overcome the strong interparticle attractions of a block of wood and break it apart by moving hand which only a karate expert can apply.
