When water is taken in an open container at normal external pressure, vapours start coming out of its surface silently and steadily. It is shown by the decrease in the level of water surface in the open container. It is called evaporation and it is known to occur at all temperatures.
Increasing the temperature of water increases the rate of evaporation because the number of water molecules escaping the surface also increases. The temperature will keep on increasing until a stage will come at which the water molecules acquire the maximum value of kinetic energy while in a liquid state. The heat which is provided at this stage will be utilized to change the liquid water into gaseous water while keeping the temperature of water constant. This stage is called boiling. The whole process is being carried out at constant atmospheric pressure.
Rates of both evaporation and boiling are affected by the variation in the external pressure to which the liquid is subjected.
Rate of evaporation tends to increase by the decrease in external pressure and vice versa. If a liquid is in an open container, most of the molecules that escape into the vapour phase will not return to the liquid phase. Increasing the external pressure will force these molecules to return back to the liquid surface making evaporation difficult.
When a liquid boils, bubbles are formed inside the container, which then rise to the surface. At this stage, the temperature of the liquid remains constant till all the liquid boils over. This temperature is called the boiling point. If you measure the boiling point of water at Karachi it will be 100°C (1 atm pressure). However, if you will measure this boiling point at Murree it will be 98°C. The atmospheric pressure at Murree is lower than that in Karachi, so we find that the boiling point has decreased at lower atmospheric pressure.
Lesson outcomes: Students differentiate evaporation/boiling, relate pressure effects.