question_answer 7)

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.  

Answer:

(a) The particles of a gas are constantly moving in all the directions with different speeds. Therefore they do not have a fixed volume and hence completely fill the vessel in which they are kept. (b) The molecules of a gas are free to move randomly in all directions. During their motion, they collide with one another and also with the walls of the container. The constant bombardment of the molecules on the walls of the container exerts a steady force. The force acting per unit area on the walls of the container is called pressure. Thus, gases exert pressure. (c) A wooden table is called solid because it has a definite mass, volume and shape. (d) In air, there is lot of empty space between the molecules and the forces between the particles are almost negligible. Hence, we can move our hand in air. However we cannot move our hand in a solid block because there are strong forces of attractions between particles in a solid and there is no empty space between them.