Question

One litre of cool air weighs heavier than one litre of hot air because of the:
(A). Increased number of collisions between the molecules
(B). Increased number of molecules at low temperature
(C). Greater energy of molecules at high temperature
(D). Lower energy of molecules at high temperature

Answer 1

Hint: The temperature of a substance tells us about the amount of interatomic vibrations between molecules or atoms of a material. High temperature implies high interatomic motion than at a lower temperature.

Complete step by step solution:
The significance of temperature of a substance is that it gives the measure of kinetic energy of the molecules/atoms in substance.
At a low temperature, the interatomic motion and collisions are less and an interatomic attractive force dominates and solid state is formed. When temperature is raised, the vibrations increase and molecules get free and start to move around freely.
There is an increase in the number of collisions between the molecules which leads to decrease in the density of the substance and increase in the volume. The denser air is heavier than the rarer air due to which cool air weighs heavier than hot air.

Additional information:
In a room containing a mixture of hot and cold air, the hot air being less dense rises while cold air sinks which takes place through the convection of air. Convection is the process to transfer the heat from one place to another by motion of the particles. It occurs in a medium which has ability to flow , for example, fluids like liquid and gas. It is not possible in solids.

Note: The ventilators or air conditioners are installed at the top near the roof in a room because the hot air gathers at the top of the room. Installing them near the floor will not help in cooling the room. Similar is the case with room heaters which must be installed near the floor rather than top.