vapor pressure equilibrium

In a closed container, the molecules of the liquid are in equilibrium with the gas. Click here for animation.

Vaporization occurs as molecules escape the liquid. FORWARD REACTION

Condensation occurs as molecules in the gas collide with the liquid. REVERSE REACTION

Both the forward and reverse reaction occur at equal rates, so the conditions remain constant.

The number of molecules in the gas stays the same and so does the vapor pressure.

 

The pictures in a - c shows the changes as the system reaches equilibrium.

The graph below shows the changes in rates as the system reaches equilibrium.