# How is evaporation a surface phenomenon?

## Evaporation of water

### Science song for kids with lyrics - nursery rhymes from The Learning Station

\$ \ begingroup \$

What are the conditions for water evaporation?

I know that in order to boil water, the vapor pressure of water has to be equal to the partial pressure of water vapor.

But what about evaporation? Is it the same condition as above, in addition to the requirement that the relative humidity is not 100%?

Can we cook when the relative humidity is 100%?

\$ \ endgroup \$

\$ \ begingroup \$

Evaporation is a surface phenomenon, cooking is a mass phenomenon. Evaporation always occurs in liquids with an exposed surface. Water molecules in a liquid sample have a distribution of kinetic energies. It takes less work for a surface water molecule to break away from its neighbors than for a water molecule in the bulk to escape since the former has fewer neighbors (no molecules on top). Those molecules with enough kinetic energy to break away from their neighbors will escape.

In order for water to boil, its vapor pressure must correspond to atmospheric pressure.

The only requirement for evaporation is an exposed liquid surface. I don't think the relative humidity affects the boiling point, but I suspect it does affect the rate at which a liquid evaporates from an open container. Evaporation always occurs, but to notice a change in the amount of material in an open system, more material has to leave the surface and then return to the surface, and the return rate is affected by the composition (water content) of the air above the open sample .

\$ \ endgroup \$ 3
• \$ \ begingroup \$ So is delta G always negative for evaporation? \$ \ endgroup \$
• \$ \ begingroup \$ @Dissenter No, \$ \ Delta G \$ is not negative for metals, or if so, it must be very close to zero. \$ \ endgroup \$
• 1 \$ \ Begingroup \$ \$ \ Delta G \$ is negative if the process takes place spontaneously, ie if the partial pressure of the substance vapor at the given temperature is below the vapor pressure of the substance. \$ \ Delta H \$ for evaporation is of course always positive; However, since the entropy of a gas is greater than the entropy of the same substance in the liquid phase, the value of \$ \ Delta G = \ Delta H - T \ Delta S \$ for the evaporation can be positive, negative or zero. If negative, net evaporation occurs; if positive, net condensation; if zero, equilibrium. \$ \ endgroup \$
\$ \ begingroup \$

Water evaporates (in the sense that more water leaves a body of liquid or solid water than comes back) when the partial pressure of water vapor in contact with liquid or solid water is less than the vapor pressure of water at the given temperature. This is the same as saying that relative humidity is less than 100% according to the definition of relative humidity.

Can we cook when the relative humidity is 100%?

You can. For example, suppose you have a bottle of water with a narrow neck that is surrounded by thermal insulation. When you heat the water to the boiling point you will soon displace all of the air so that both the liquid and gas phases are 100% water and the water will continue to boil as long as you continue to provide heat.

\$ \ endgroup \$ 2
• \$ \ begingroup \$ Would there be net evaporation at a relative humidity of 100%? \$ \ endgroup \$
• \$ \ begingroup \$ No net evaporation at a relative humidity of 100%; The speed with which molecules leave the liquid corresponds to the speed with which (other) vapor molecules enter the liquid. \$ \ endgroup \$