Heat - Part III

Evaporation and boiling :  There is a difference between the process of evaporation and the process of boiling. In evaporation, the liquid molecules escape the liquid surface without any external heat source. The process by which liquid directly changes to gaseous state is known as evaporation.

Evaporation : If you keep water in a saucer and expose it to air, you will see that the water has disappeared after sometime. The water is converted into gaseous state without giving it an extra energy or heating it up to its boiling point. 

To understand the process of evaporation, do the following experiments :

• Take two vessels whose mouths have different areas. Fill the vessels with water and keep them in open air. Note their levels with a marker pen. After a few hours, note their levels again. You will observe that the level in vessel with a wider mouth and larger exposed water area has gone down more than what has happened in the other vessel. This shows that the rate of evaporation depends on the exposed area of the liquid.

• Take equal amounts of water at different temperatures. Put then in exactly similar vessels with similar mouths. Notice the water levels after a short passage of time. You will observe that the water at higher temperature evaporates faster. Thus the rate of evaporation depends on the temperature of the liquid.

• Take equal amounts of water and alcohol at the same temperature. Put them in identical vessels and observe the levels over a short period of time. You will observe that despite identical conditions, alcohol evaporates faster than water. This shows that the rate of evaporation varies with the nature of the liquid.

• Take equal amounts of water in identical vessels. Mark the level of water with a marker pen. Cover on of the vessels with a bell-jar. Note the level of water with respect to time. You will see that the uncovered water level goes down faster. This shows that the rate of evaporation depends on whether it takes place in the open or in a closed space.

Inverse process of evaporation is called condensation. Hold your hand high over a vessel of boiling water. You will feel that your hands are wet. The steam coming out of the boiling water condenses on your hand. Similarly, if you take out a cold utensil from the fridge, you will notice water droplets starting to cling on its outside. The moisture from the air condenses on the surface of the vessel because of the coldness of the vessel.

From the microscopic explanation given above, all the observations regarding the phenomena of evaporation can be explained neatly.

• The number of molecules on a larger surface will be more than those on a smaller surface. Since evaporation takes place due to molecules escaping from the liquid surface, it is obvious that the rate of evaporation will be faster from a vessel with a larger mouth.

• The higher temperature of a liquid means higher molecular kinetic energy. Hence the rate of evaporation is larger for a liquid at higher temperatures.

• The nature of the attractive cohesive forces will determine the rate of evaporation. The cohesive forces in water is much larger than that in alcohol. This is the reason why the rate of evaporation in alcohol is higher than that seen in water.

• We have observed that the rate of evaporation in a closed vessel is slower than that in the open vessel. The molecules escaping from the surface of the liquid in a closed vessel collect in the space above the liquid. Some of the molecules evaporated can return and condense back to the liquid, after losing their energy in collision. Thus the evaporation-condensation goes on till equilibrium is reached. That is the rate of molecules evaporating is the same as the rate of molecules condensing back. Once the equilibrium is established, no more extra evaporation occurs. (In this situation, the space above the liquid is said to be saturated with vapor. The pressure exerted by the vapor on the liquid is said to be saturated vapor pressure. The saturated vapor pressure of a liquid increases with the increase of temperature).
  But the same phenomena does not happen in an open vessel. Because molecules escaping from its surface get scattered away into the atmosphere, they are unable to condense back in the same liquid surface. New molecules come to the surface, get evaporated and the process continues. 

Put some drops of water in your hand, When the water evaporates, you feel cool. The heat required by the water molecules is taken from your skin itself. The temperature of the surrounding skin is lowered a bit. This makes you feel cool. Ý

We now know that energetic molecules escape from the surface of a liquid during evaporation. This lowers the average kinetic energy of the molecules left behind in the liquid. This lowers the temperature of the liquid. Thus evaporation lowers the temperature of the liquid. This is the reason why you put tea/coffee in a saucer to cool it. Also blowing cools a hot liquid, because blowing makes the energetic molecules leave the surface quickly.

The boiling occurs in the entire volume of the liquid, unlike what happens in evaporation. From the above discussions, we can now say that a liquid boils at a temperature at which its vapor pressure becomes equal to the atmospheric pressure. This temperature at normal atmospheric pressure of 76 cm of Hg is called the boiling point of the liquid.Ý
Ý
Try and do the following experiment. Keep water for boiling in two identical vessels. Close one vessel with a lid. You will see that the water in the vessel with a lid boils faster. This is because the water vapor escaping in the closed vessel collects in the space above the liquid. The steam returns back into the water and heats the water faster. We are creating a condition of saturated vapor in the closed vessel. This is the reason why water in a closed vessel boils faster, that is it attains the temperature of 100ƒC faster.

Anders Celcius (1701-1744). Celcius was a Swedish astronomer, physicist and mathematician. He was born in Uppsala, Sweden, where he succeeded his father as professor of astronomy in 1730. It was there also that he built Sweden's first observatory in 1741. Celsius was in an expedition that measured and showed that the Earth was flattened at the poles. Celsius' fixed scale for measuring temperature defines zero degrees as the temperature at which water boils, and 100 degrees as the temperature at which water freezes. The scale, an inverted form of Celsius' original design, is adopted as the standard and is still used in almost all scientific works. The name "centigrades" for the Celsius temperature scale became obsolete when the SI system was declared international standard in 1954-1960. The correct name is "degrees Celcius." Anders Celcius died in 1744.

Next        Main        Previous