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Name: Samantha
Status: N/A
Age: 9
Location: N/A
Country: N/A
Date: Monday, January 20, 2003

All liquids will eventually evaporate. Some will evaporate faster than others. What are the factors that will determine how fast a liquid will evaporate?


Evaporation rate depends on the temperature of the liquid, how tightly the liquid molecules bond to each other, the surface area of the liquid, and the temperature, pressure, and air movements in the surroundings into which evaporation occurs.

The warmer the liquid, the faster the evaporation rate. The stronger the molecules bond to each other, the slower the evaporation rate. The larger the exposed surface, the faster the evaporation rate. The colder the surroundings, the faster the evaporation rate. The lower the atmospheric pressure above the liquid, the faster the evaporation rate. The faster the air above the liquid moves, the faster the evaporation rate.

ProfHoff 561

Several factors determine how fast a liquid will evaporate: 1. The vapor pressure. The vapor pressure is the intrinsic pressure of vapor of a substance that is in equilibrium with its liquid. For water at 25 C. the vapor pressure is 25 mm of mercury (that makes it easy to remember). At 0 C., the freezing point of pure water the vapor pressure of water/ice is about 4.5 mm of mercury. So the evaporation rate of ice + water is lower than water at either 25 C. 2. Increasing the temperature increases the vapor pressure and consequently increases the evaporation rate (other things being equal). So at 100 C. the vapor pressure of water is 760 mm of mercury, or 1 atmosphere. 3. The presence of a solute of lower or negligible vapor pressure reduces the vapor pressure and consequently tends to reduce the evaporation rate. So, for example, the evaporation rate of water from salt water will be somewhat less than from fresh water (again, other factors being the same). 4. In the case of water, the relative humidity, which is the percent water vapor in the air above the liquid compared to its vapor pressure at a specified temperature, reduces the evaporation rate. The evaporation rate of water is approximately a straight line from a maximum at 0% relative humidity to zero at 100% relative humidity. The evaporation rate of most liquids that do not absorb water is not affected very much be relative humidity. 5. An important factor is the speed of the air being swept across the liquid surface. The faster the speed of the air (wind) the more vapor is removed and the faster is the evaporation rate. But there can be conflicting factors if, for example, the air speed is so fast that it cools the water, which reduces the vapor pressure, which reduces the evaporation rate.

These are not all of the factors that influence the evaporation rate of a liquid, but you can see that your "short" question has a "long" answer!!!

Vince Calder

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