How is Liquid Converted into Vapor?

            Wet clothes are put out in the open, under the sun to dry. Do you know where this water goes? This water gets converted into water vapor and goes into the atmosphere. Conversion of water into vapor is called evaporation. During the rainy season, the water falling on the ground disappears after some time. This is because the earth absorbs some water and a part of it evaporates into the atmosphere.

            How does evaporation take place? Every substance is made up of very small particles called molecules. These molecules are held together by strong forces of attraction called cohesive forces. The repulsive forces caused due to the motion of molecules oppose these forces. As long as the cohesive forces are far greater than the repulsive forces, the substance remains in a solid state.

            When the substance is heated up, it absorbs heat energy due to which the molecules are set in rapid motion. This motion stars counterbalancing the cohesive force, that is, this force tries to separate them apart. When the repulsive force generated by the motion of molecules equals the cohesive force, matter changes form solid to liquid state. If the liquid is heated continuously, the molecules move still faster and when the force duet o the increased motion exceeds the cohesive force, the molecules of the liquid become free and escape into the air. This is how a liquid is converted into vapor.

            The liberation of the molecules from the liquid surface into the air is called evaporation. Evaporation takes place at all temperatures.

            This explains the drying up of clothes in the air. The rate of evaporation increases with the rise in temperature. Wet clothes put in the sun dry up faster than in shade because at higher temperature the motion of molecules becomes faster and as such the process of evaporation is also accelerated. It also takes place more rapidly when the air is dry. That is why clothes get dry more quickly on a dry, sunny day than on a damp, cloudy day.

How is Dew Formed?

            You must have see small water droplets on the blades of grass and leaves of plats shining like pearls I the early hours of morning. These water drops are called dew. Many people still think that dewdrops are like rainwater, which fall on the earth form the sky at night, which is not true.

Dewdrops are formed when hot water vapors condense on cold surface

            The air around us contains water vapor, which we call moisture or humidity. Hot air contains more moisture as compared to cold air. At night, when the hot air comes in contact with some cold surface, water vapor present in the air condenses on the cold surface in the form of droplets. These tiny drops of water are called dewdrops.

            The process of dew formation can be see in a simple experiment. Take a tumbler and place it on a table. Now put some ice or ice cold water inside ether tumbler. You will notice that after some time, small drops of water appear on the outré surface of the tumbler. These water drops are formed by the condensation of the water vapors present in the air when it comes in contact with the cold surface of the tumbler. Similarly, the water vapor in the air condenses on the cold surfaces of grass, flowers and leaves to form dew drops.

            The dew formation is more conspicuous when the sky is clear, and less when it is cloudy. When the sky is clear, it becomes coiler at nights, therefore, there is more condensation of water and hence more dew formation. But when it is cloudy, trees and plants are not able to radiate and lose heat and therefore do not get cooled in the night. Hence, there is less dew formation. As the sun rises, it begins to get warmer and these dewdrops evaporate into air and disappear.