Question

Explain the various factors affecting the rate of evaporation of liquid
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Answer 1

Evaporation is the slow process of liquid molecules turning into gas and escaping from the fluid. Because the molecules need a place to escape evaporation can only occur at the surfaces of the fluid that are not contained, e.g., on top of a glass of water which is exposed to the air. Therefore,

(1) the exposed surface area is important in determining the rate. Volume or the amount of fluid does not change the rate of evaporation. Granted it will take longer for a larger volume of fluid to evaporate, but the rate at which it loses mass will be irrespective of volume assuming other influencing variables are held constant. Again, this is because the liquid molecules are not escaping from the volume everywhere at once, they are escaping only at the surface exposed to air.

(2) Heat will affect the rate of evaporation. The molecules of a hot fluid are vibrating more frequently and with more energy than in a cool fluid. The added energy of heat, therefore, makes it easier for a given molecule to escape the fluid.

(3) What the fluid is comprised of will influence the rate of evaporation as well. If the fluid is made up of large charged molecules, the molecules will escape at a slower rate because more energy is required to lift their mass and overcome their electromagnetic interactions with each other to allow the molecules to escape. Also, a mixed fluid can evaporate faster or slower depending on how the different molecules interact with each other.

(4) The amount of the molecules already in the gas phase surrounding the liquid will also influence the rate of evaporation. For example, at the surface of the water, there are molecules going from liquid to gas but ALSO from gas back to liquid. The rate of evaporation is determined by the difference between these two processes. If there are a lot of water molecules in a gas phase surrounding the area of fluid that is exposed, a greater number of chance will replace the molecules that are escaping the fluid. So the if you measuring the rate at which the mass of water changes over time (which is the same as measuring the rate of evaporation), it will be slowed down if the fluid is surrounded by more water molecules in the gas state. Accordingly, the fastest way to evaporate a fluid would be to have it in a vacuum that immediately removes the gas before it could go back into the liquid state.