Question

Choose the correct statement from the following:
A. Molecular collisions are unaffected by gravity
B. Real gases obey gas laws under any conditions
C. Gas laws have no significance at absolute zero
D. At $0^\circ {\rm{C}}$, the molecular motion ceases

Answer 1

Hint: We know that the gaseous state of a substance is described by four physical properties, that is, temperature, pressure, volume and mass. The co-relations in the characteristics of gases have been established as a result of certain generalizations worked out by different scientists. These are known as gas laws.

Complete step by step answer:
Let’s discuss the options one by one.
Option A says that there is no effect of gravity on molecular collisions. We know that gravitational force is everywhere on the Earth. The collision of gases exerts force that overtakes the gravitational force. Thus, gravity has no effect on collisions of molecules.
Option B says real gases obey gas laws under any conditions. The statement is not correct. An Ideal gas obeys gas laws under all conditions of temperature and pressure. Real gases obey gas laws when pressure is very low and temperature is very high.
Option C says, at absolute zero there is no significance of gas laws. The hypothetical temperature of $ - 273^\circ {\rm{C}}$at which volume of any gas becomes zero is termed as absolute zero. So, there is no significance of gas laws at absolute zero.
Option D says at $0^\circ {\rm{C}}$, the motion of molecules ceases. Actually the motion of molecules ceases at 0 K not at $0^\circ {\rm{C}}$. Charles law states that molecular motion ceases at 0 K or $ - 273^\circ {\rm{C}}$ (absolute zero)
Hence, correct statements are A and C.

Note: It is impossible to achieve $ - 273^\circ {\rm{C}}$ or absolute zero for a gas under any condition because its volume becomes zero. In other words, we can say that there is no existence of gas at that temperature. Actually, a gas changes to a liquid state before attaining this temperature. Thus, absolute zero is only a hypothetical temperature for a gas.