(a) Initial conditions—dry air at 20°C with no observable vapor pressure. (b) Evaporation generates measurable vapor pressure. (c) As more and more molecules escape from the water surface, the steadily increasing vapor pressure forces more and more of these molecules to return to the liquid. Eventually, the number of water-vapor molecules returning to the surface will balance the number leaving. At that point the air is said to be saturated. (d) When the container is heated from 20° to 30°C, the rate of evaporation increases, causing the vapor pressure to increase until a new balance is reached.