How can water turn to vapor below the boiling point?

Una replies:

You’re correct – unless you’re the Human Sponge, most of the water on your hands evaporates and isn’t absorbed into your skin.

Your question is one many people wonder about. When water evaporates out of a glass or the bathtub or, in your case, off your body, clearly it’s well below the boiling point, typically 212°F (100°C) at sea level. To understand why this happens, we need to learn a bit about what’s occurring at the molecular level.

What we perceive as temperature is an average effect. In any given quantity of water, individual molecules are moving at different speeds. Low energy/low temperature molecules move slowly; high energy/high temperature ones move fast. While the average temperature of the liquid may be below boiling, some molecules build up enough speed as a result of random collisions to break free from the liquid’s surface and enter the surrounding air. This is the process we know as evaporation.

What’s the difference between evaporation and boiling? During evaporation, some hotter-than-average molecules escape the liquid, taking their heat energy with them. (As a result, what’s left behind on average is cooler, which is why evaporation of sweat helps to cool us). During boiling, on the other hand, so much heat has been added to the water that all the molecules are scrambling to escape, and the water turns to steam.

Evaporation isn’t the only thing happening at the molecular level. Some molecules of water vapor in the air slow down enough that when they strike your skin they stick. This process is called condensation. Evaporation and condensation occur simultaneously, but since the particles are so small we only see the net effect. Under certain conditions, e.g., if the air temperature is higher than your skin temperature (typically 35°C/95°F) and the relative humidity is 100%, as in in sauna, a considerable amount of water will condense on your skin. More commonly the net effect is evaporation – you lose more water than you gain. But in moderate heat and humidity, the net evaporation rate when you’re at rest is relatively low.

Now let’s focus on your example. The rate of evaporation from your skin is determined by four primary factors:

1. The temperature of your skin.
2. The temperature of the ambient (surrounding) air.
3. The amount of water in the air relative to the amount it can hold (the relative humidity).
4. The ambient air pressure.

When you dry your hands under a hot-air dryer, you’re doing three things:

• You’re increasing your skin temperature, speeding up the water molecules on your hands and making them evaporate faster.
• You’re increasing the local air temperature (i.e., as your hands perceive it).
• You’re blowing the moist air away from your skin, allowing hot, dry air to come into contact with it, in effect decreasing local humidity.

Net result: You greatly speed the evaporation rate and in so doing dry your hands.

Air pressure isn’t much of a factor when you’re using a hot-air dryer, but it definitely affects both evaporation and boiling, which is why water boils at a much lower temperature at high altitude than at low (and why it’s difficult to make proper English tea in the mountains). In simple terms, as the pressure of the atmosphere is reduced, water molecules need less energy to escape into it – which means a lower boiling point. Thus, water for tea on top of Pike’s Peak (14,110 ft / 4,300 m) boils at roughly 187°F (86.1°C).

References:

ASME Steam Tables, American Society of Mechanical Engineers, 1997.

ASHRAE Handbook of Fundamentals, American Society of Heating, Refrigerating, and Air Conditioning Engineers, 1972.

Engineering Thermodynamics – Fundamentals and Applications, Huang, Francis F., 1988.

Fundamentals of Heat and Mass Transfer, Incropera, Frank P. and De Witt, David P. 1990.

Engineer-in-Training Reference Manual, Lindeburg, Michael R., 1992.

Una

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