What is Evaporation and Sublimation? What are the effects on Evaporation and Sublimation. What does Evaporation depend on?
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Wet your hand with water and wave your hand about. How do you feel? What do you notice? Try this with alcohol. Which one of the liquids evaporates faster? Inside a liquid, the molecules are in motion. Some of them move faster than others. As they hit each other those near the surface may gain enough energy to escape into the air and become gas molecules.
Evaporation is to change from a liquid state into a gaseous state. Evaporation takes place at the surface of the liquid and at any temperature . It continues until the liquid disappears or the space above the liquid becomes saturated.
As only the molecules with high energy (and with high velocity) can escape into the air in this way, the average energy of the remaining molecules will be less than it was before. So the temperature of the liquid left behind falls. This evaporation effect is used to cool food and drinking water. When the weather is hot we perspire. As the perspiration evaporates our skin is cooled which helps to maintain body temperature.
Evaporation is a cooling process.
The rate of the evaporation depends on:
- a) temperature. Evaporation is faster at high temperatures.
- b) humidity. As the humidity of the air increases so evaporation becomes slower.
- c) size of the surface of the liquid. If the surface of the liquid is large then a greater amount of evaporation takes place.
- d) pressure. Evaporation is faster under low pressure.
- e) kind of liquid. Alcohol evaporates faster than water.
SUBLIMATION
When a particle on the surface of a liquid or solid gains enough energy to overcome the forces that hold it as part of the substance, it escapes and becomes a particle in the gaseous state. When this occurs in solids it is called sublimation. Sublimation is a direct change from solid to vapour without passing through the liquid state. Sublimation occurs more rapidly at higher temperatures and more slowly at lower temperatures.
A moth ball (naphthaline tablet) left out in the air becomes smaller and smaller, and eventually disappears, while its characteristic odor is noticed in the air near it. In this case, molecules of solid naphthaline become molecules of naphthaline vapour mixed with the gases in the surrounding air.
