In liquids, the molecules of the liquid are always in a state of random motion, within its surface. Some molecules may have sufficient kinetic energy to escape from the surface of the liquid. This process is known as evaporation. Evaporation takes place at all temperatures. Rate of evaporation increases with rise in temperature and becomes maximum at the boiling point of the liquid. The process of evaporation also increases with increase in surface area of the liquid.
Evaporation requires energy. A liquid draws heat energy from the surrounding thereby cooling the surrounding.
Example 1: Water placed in a porous pot becomes very cool after some time. This is because water molecules draw energy from the water itself for evaporation and hence, the temperature of water in the pot falls.
Example 2: If we smear our hand with spirit that portion feels cold because spirit evaporates quickly using heat energy from the skin.
Example 3: During summer process of perspiration keeps the body cool. When we perspire, the sweat evaporates using heat from our body thereby keeping it cool.
Example 4: Water cycle in nature is initiated by the evaporation of water from lakes, ponds, rivers, sea, etc. Water evaporates due to sun's heat. Water vapor rises to the sky to from clouds. Clouds condense to form raindrops, which fall on the earth. And the water cycle continues.
Boiling point of certain liquids
CondensationThe change of state from the gaseous form to the liquid form is called condensation. It is also called liquefaction. A good example of this process is that of water vapour in atmosphere condensing to form rain.
By applying pressure to a gas the particles come closer and start condensing to the liquid state. By applying pressure and reducing temperature one can liquefy gases. A good example is the liquefaction of petrol gas from oil refineries into LPG cylinders (liquefied petroleum gas).
ExperimentA 100 mL syringe is taken whose nozzle is plugged or closed by inserting a rubber cork. The plunger is drawn out such that the whole space is occupied by air. When the plunger is pushed back into the syringe we notice that the air inside the syringe decreases i.e., it is compressed.
The experiment is repeated with water replacing the air in the syringe. It is found that the plunger cannot be pushed into the syringe. This is because liquids are hardly compressible. This shows that pressure does not produce much effect upon liquids.
Accordingly, temperature and pressure determine the state of a substance either solid, liquid or gas.