question_answer

    Give reasons for the following observations: (a)A gas completely fills the vessel in which it is kept. (b) A gas exerts pressure on the walls of the container.               (c) Naphthalene balls disappear with time without leaving any 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.                               [NCERT] (e) Our palm feel cold when we put some acetone or petrol or perfume at it.        [NCERT]

Answer:

                 (a) The molecules of a gas possess high kinetic energy and very weak intermolecular forces. The molecules move in all directions. As a result the vessel is completely filled with gas. (b) The molecules of a gas possess high kinetic energy and random motion the molecules keep hitting the walls of the container. As a result, the pressure is exerted by the gas.                                       (c) Naphthalene is a volatile solid; therefore, it changes into vapour completely and no solid is left. Sublimation of naphthalene occurs. (d) Solids have very strong intermolecular attraction among their particle while intermolecular attraction is very poor in case of gases. Due to the reason, we are able to easily move our hands in air but cannot do the same through a solid block of wood. (e) Acetone, petrol and perfumes are highly volatile compounds. When we place them on our palm, they evaporate at a high rate and absorb heat from our palm; as a result, our palm feel cold.