Condensation

CONDENSATION IS THE change of phase of a substance from vapor to liquid. It is the opposite of evaporation, the change of phase from liquid to vapor. The condensation of water is one of the most important physical processes of the Earths climate system. Condensation forms cloud particles and precipitation. It is the main sink of atmospheric water. On Earth, water can be found in the solid, liquid, and gas phases. Evaporation (and sublimation) of water substance, the transport of water vapor, and condensation away from its sources, is the most important heat transport mechanism in the Earth's climate system. Condensation is an extremely important process in the earth's water cycle because it is responsible for the formation of clouds and precipitation.

Weak forces between molecules cause them to stick to each other and produce the various phases of a substance. Random thermal motions cause some molecules to overcome these inter molecular forces and escape from the liquid and form a gas phase around it. The number of molecules that leave the liquid phase increases with its temperature because of the increase in the thermal energy, and, therefore, kinetic energy, of the substance. The molecules in the gas phase jiggle and move randomly because of their thermal energy. Some of the gas molecules stick to the liquid when they strike it. The numbers of molecules that return to the liquid phase increases with their concentration. Thus, the concentration of molecules in the gas phase increases until a balance between molecules leaving and returning to the liquid surface is reached. This is called thermodynamic equilibrium. The concentration of molecules in the gas phase, in equilibrium with the liquid phase at a given temperature, is defined as the saturation value.

Condensation is integral to the most important heat transport mechanism in the earth's climate system.

The molecules that leave the liquid phase are the ones moving faster than the average molecule in the liquid. That is, they have larger kinetic energy than the average molecule. This is the reason a liquid cools while it evaporates. There is a sudden large attraction when a molecule of water vapor approaches the surface of liquid water. This speeds up the incoming molecule and increases its kinetic energy. Thus, a liquid releases heat while it condenses. This is known as the latent heat of condensation. The saturation vapor pressure depends on the curvature of the surface of the liquid phase.

This has important implications for climate processes. In a curved surface, such as that of a cloud [Page 276]droplet, each water molecule has fewer nearby neighbors than on a flat surface. Thus, the intermolecular attractive forces holding them together are smaller; a water molecule can escape a curved surface easier than a flat surface. At equilibrium, the concentration of water molecules in the gas phase has to be larger than over a curved surface over a flat surface to compensate for the fact that a larger number of molecules leave the liquid at a given temperature. Therefore, the saturation water vapor pressure is larger over curved surfaces.

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