Evaporation
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Evaporation

The Evaporation Process

You know that water in an open vessel goes into the air eventually,as shown in the picture below. When you see the slow disappearance of an uncontained liquid like a puddle of water, you probably say it evaporates. The scientific term for the conversion of a liquid to gas or vapor below its boiling point is evaporation or vaporization. In evaporation, molecules go into the gas or vapor state. Only those molecules of the liquid with a certain minimum kinetic energy can break away from the surface. Some escaping particles collide with the air molecules and rebound back into the liquid, but others escape completely into the air, into the gaseous phase.

A liquid evaporates faster when heated because the kinetic energy of its particles increases. This enables more particles to overcome the attractive forces keeping them in the liquid. However, evaporation itself is a cooling process. The reason is that particles with the highest kinetic energy escape first. The particles left in the liquid have a lower average kinetic energy than the particles that have escaped. This is similar to removing the fastest runner in a race. The cules in your perspiration evaporate in you skin's surface, the remaining perspiration is cooler. Therefore the remaining perspiration cools you by absorbing body heat. 

This picture shows the evaporation cycle.

The evaporation of a liquid in a closed container is somewhat different. When a partially filled container filled container of liquid is sealed, some of the particles in the liquid vaporize. These particles collide with the walls of the sealed container and produce a vapor pressure above the liquid.As the container stands, the number of particles entering the vapor increases. Eventually some of the particles will return to liquid,or condense.

At equilibrium, the particles in the system are still evaporating and condensing,but there is no net change in either number of particles. One sign that equilibrium is established is that inner walls of the container "sweat". Liquid that once evaporated is now condensing ,but more liquid is evaporating to take its place.

An increase in the temperature of a contained liquid increases the vapor pressure because particles in the warmed liquid have increased kinetic energy. As a result, more of them have the minimum kinetic energy necessary to escape the surface of the liquid. The particles escape the liquid and collide with greater frequency with the walls of the container.

A manometer is similar to a barometer and is used to measure the pressure of gas sample. This illustration shows how the vapor pressure of a liquid can be measured at two different temperatures. The height difference in the U-tube equals the vapor to pressure at the temperature.

(a) Water molecules evaporate from the liquid and escape from an open container. No equilibrium is established.

(b)In a closed container, water molecules do not escape but collect as a vapor above the liquid. A dynamic equilibrium between the vapor and the liquid is established.

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