-d- Inter-Conversion

Evaporation and Condensation

Melting and Freezing

\[\text{Evaporation is the process which water changes from liquid to its} \]

\[\text{gaseous form without boiling.}\]

Evaporation can be explained using the kinetic model of matter. All particles in a liquid are in constant random motion. Some particles are moving faster than the others. On the surface of the water, those particles that are moving faster will escape from the water. Since the faster molecules escaped, what is left in the water are those particles that are moving slower. Since we have concluded earlier that the kinetic energy of a system of particles is directly proportional to the average velocity of the particles, we can deduce in this case that the water after evaporation occurs, has a lower temperature i.e. evaporation causes cooling.

Using the above mechanism for evaporation, we can see that any circumstances that promote the escape of the molecules from the surface will increase the rate of evaporation. Hence,
  1. high temperature results in molecules with higher velocities. Such molecules have greater tendencies to escape from the pool of water. High temperature causes higher rate of evaporation.
  2. bigger surface area of water allows more molecules to escape per unit time. Hence, evaporation rate can increase if we spread out the water over a larger surface area.
  3. high draught increase rate of evaporation. Draught is a current of flowing air. High draught removes molecules that have already escaped, thus exposing more of the dry air for further evaporation.

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