Kinetic theory of matter attempts to explain macroscopic properties of matter by considering their molecular composition and motion.
(a) In a particular experiment, smoke particles in a transparent box are observed using a microscope. A small point of light is seen to move around as shown.
(i) What does this motion demonstrate about the air molecules?
(ii) State and explain the change in the motion of the smoke particles when:
--- (1) the size of the smoke particle increases
--- (2) the temperature of the surrounding increases
(b) During the process of boiling, heat is supplied to the liquid but there is no increase in temperature. Explain this process in terms of the molecular behaviour.
(c) Explain how the process of evaporation leads to a cooling of the liquid.
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Answer:
(a) (i) It shows that air molecules are in continuous random motion.
(ii) 1.
The path of the particles are shorter.
The speed of particle is slower as it requires more molecules colliding on the particle to result in a change in motion.
There is less agitation as the number of collisions between particles and air molecules decreases
2. Higher temperature increase the kinetic energy of the air molecules. Hence the smoke particle will move faster and in a more random manner.
(b) Heat is used to increase the potential energy/ increase the distance between the molecules/weaken intermolecular forces. However, during boiling, the average kinetic energy of the molecules remained constant hence temperature remains the same.
(c) Molecules on the surface with higher kinetic energy will be able to break free from the surrounding water molecules. Resultant internal energy/average kinetic energy of molecules remaining in water decreases, and hence temperature decreases.
