Consider the equilibrium system below.
NH4Cl(S)
ẠNH, (g)
If the system is at dynamic equilibrium, which statement is true?
Sublimation of the NH4Cl crystals stops.
Solidification of the NH4Cl vapors stops.
The rate of sublimation of the NH4Cl crystals is equal to the rate of solidification of the NH4Cl vapors.
The rate of sublimation of the NH4Cl crystals is higher than the rate of solidification of the NH4Cl vapors.

Explanation:

The equilirium system under consideration is:

A dynamic equilibrium means that it is not static: some changes are happening, but the net change is zero, and thus the changes are not observed with the naked eye.

The chemical equilbrium NH₄Cl(s) ⇄ NH₄Cl(g) shows that NH₄Cl(s) crystals, on the left side, sublimate (pass from solid state to gas state) and the NH₄Cl(g) vapors, on the right solidify (pass from gas state to solid state).

To be in equilibrium, the rate at which the crystals sublimate must equal the rate at which the vapors solidify.