Liquid to Gas and Vice Versa Water undergoes the process of evaporation when changing from the liquid to a gaseous state. According to the molecular theory of matter, all matter consists of molecules in motion. The molecules in a bottled liquid are  restricted  in  their  motion  by  the  walls  of  the container. However, on a free surface exposed to the atmosphere, the motion of the molecules in the liquid is restricted  by  the  weight  of  the  atmosphere  or,  more precisely, by the atmospheric pressure. If the speed of the  liquid  molecules  is  sufficiently  high,  they  escape from the surface of the liquid into the atmosphere. As the temperature of the liquid is increased, the speed of the molecules is increased, and the rate at which the molecules   escape   from   the   surface   also   increases. Evaporation takes place only from the free or exposed surface of a substance. During the process of evaporation, heat is released. This heat is absorbed by the water that has vaporized. The amount absorbed is approximately 539 calories per gram of water at a temperature of 100°C. On the other hand,  the  amount  is  597.3  calories,  if  the  evaporation takes place at a water temperature of 0°C. This energy is required to keep the molecules in the vapor state and is called the latent heat of vaporization. Since the water needs to absorb heat in order to vaporize, heat must be supplied or else evaporation cannot take place. The air provides this heat. For this reason, evaporation is said to be a cooling process, because by supplying the heat for vaporization, the temperature of the surrounding air is lowered. Condensation    is    the    opposite    of    evaporation because water vapor undergoes a change in state from gas back to liquid. However, a condition of saturation must exist before condensation can occur. That is, the air  must  contain  all  the  water  vapor  it  can  hold  (100 percent relative humidity) before any of it can condense from the atmosphere.   In the process of condensation, the heat that was absorbed in evaporation by the water vapor is released from the water vapor into the air and is called  the  latent  heat  of  condensation.  As  you  might expect, condensation warms the surrounding air. Solid to Gas and Vice Versa Sublimation  is  the  change  of  state  from  a  solid directly    to    a    vapor    or    vice    versa    at    the    same temperature.  In  physics  and  chemistry,  sublimation  is regarded as the change of state from solid to vapor only, 2-7 1 gm ICE 0  C 1 gm ICE 0  C 1 gm ICE -10  C 1 gm ICE 0  C 1 gm INVISIBLE WATER VAPOR 100  C 1 gm INVISIBLE WATER VAPOR 100  C 1 gm WATER 0  C 1 gm WATER 0  C 1 gm INVISIBLE WATER VAPOR 0  C TOTAL CALORIES ADDED TO THE AIR 727 723 677 TOTAL CALORIES TAKEN FROM THE AIR 727 723 677 80 597 50 4 80 100+539 677 NOTE: 1. EVAPORATION COOLS AIR. 2. CONDENSATION HEATS. 3. CALORIES SHOWN TO NEAREST WHOLE FIGURES. 1 gm INVISIBLE WATER VAPOR 0  C AG5f0205 Figure 2-5.—Thermal history of 1 gram of ice during changes of state.