How can you increase the rate of evaporation?

Answer 1

The evaporation rate can be increased by the increasing heat and mass transfer:

(a) Raising the temperature, which increases the vapor pressure

(b) Decreasing the concentration of the substance being evaporated in the vapor phase. For example, lowering the ralative humidity will allow more water to evaporate at a given temperature.

(c) Increasing the heat flow to the liquid

(d) Increasing the surface area of the liquid (For example, water in a wide pan will evaporate faster than water stored in an open bottle.),

(e) Reducing the pressure, which decreases the boiling point.

(f) Increasing the velocity of the vapor phase in contact with the liquid being evaporated

Answer 2

You can increase the water temperature, increase the water's surface area (spread it out on the floor), decrease the air humidity, increase the velocity of air across the water (blow it with a fan), or decrease the air pressure.

Answer 3

The evaporation rate of water is tied to the driving forces for evaporation - specifically to the difference between the fugacity of the liquid water and the atmosphere it is in contact with. Anything that lowers the fugacity of the liquid water or raises the fugacity of the water vapor in the air around it will decrease the rate of evaporation.

To lower the fugacity of the water you can lower the temperature or adulterate the water with another substance. A common example of the latter is adding salt to the water.

To increase the fugacity of the vapor in contact with the liquid water you can: 1) Confine it so that as it evaporates, the pressure goes up. Pressure cookers do this, 2) Prevent convection so that the area above the water becomes saturated and no more water can evaporate without a corresponding amount of vapor condensing. A sealed bottle of water will certainly follow this. The amount of liquid in the bottle stays pretty constant as long as it remains sealed.

You can also decrease the rate of evaporation by reducing the surface area available for evaporation. A puddle of water will evaporate more quickly than the same amount of water sitting in an open cup. Restricting the cross section of the area available to diffuse the vapor into the surrounding atmosphere will also reduce the rate of evaporation. The narrower and longer the neck of a bottle containing water, the slower the water will evaporate. In this case the reduced rate of evaporation is because the vapor next to the water becomes saturated and then the saturated vapor has to diffuse through the neck of the bottle into the unsaturated surrounding air. The rate of diffusion will be proportional to the cross section of the opening and inversely proportional to the path over which the diffusion must occur (i.e. the length of the neck of the bottle.)

Fugacity is a useful mathematical function that can be defined as:

ƒ = Psystem· exp{(1/RT)· ∫[(PV - RT)/P]dp} where the integral is evaluated on the interval from P=0 to P=Psystem and P is pressure, V is molar volume of the phase, R is the molar gas constant, and T is the absolute temperature. It is useful because when two different phases are in equilibrium, the fugacities of the two phases are equal.

At low pressures, the argument inside the integral reduces to near zero for gases, since they will be pretty nearly ideal gases (hence PV = RT so PV-RT=0) and the fugacity reduces to the vapor pressure of the liquid in equilibrium with the gas.

Answer 4

• Increasing the temperature or decreasing the atmospheric pressure will increase the rate of evaporation of water.

Answer 5

Heat increases the evaporation rate. Try it at home. Put two pots side by side on the stove with water of the same temperature. Turn one of the burners on. Which one evaporates first?

Answer 6

Change its temperature or the surrounding pressure or humidity.

Answer 7

Evaporation occur naturally at any temperature; increasing the temperature evaporation is faster.