HOW COME? Water's evaporation act

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Why do some liquids, like alcohol wiped on your skin, seem to disappear right away, while water lasts much longer? asks a reader.

When water in the bottom of a glass does a disappearing act -- no drinking required -- that's evaporation. But it's also evaporation when brushed-on nail polish hardens into a glossy red coat. Evaporating liquids escape, bit by bit, into the air, wafting here, there, and everywhere.

Evaporation is always occurring at a liquid's surface. Where liquid meets air -- at the opening of a bottle, the surface of a lake, or on your sweaty skin -- some energetic molecules are breaking free from their companions and rising into the air.

All liquids evaporate, but some evaporate more quickly than others. How fast a liquid evaporates depends on its temperature (the average kinetic energy of its molecules). But evaporation rates also depend on how tight the bonds between those molecules are. Finally, a liquid whose molecules weigh more will tend to evaporate more slowly than one whose molecules weigh less.

Molecules at a liquid's surface are attracted only "down" and "sideways" to other molecules. So it's easier to skip out at the surface than from down further in the molecular bunch. The larger the surface, the more quickly and easily molecules make their escape. Water spilled in a pool on the floor will evaporate quickly. But the same amount of water in a long-necked bottle will take its time fleeing through the small, round surface.

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Why does water evaporate more slowly than alcohol (and many other liquids)? Water (H2O) molecules are strongly bound together. These "hydrogen bonds" between the atoms of neighboring water molecules make it hard for individual molecules to break free from the bunch. So even when water has a lower molecular weight than another liquid, it may evaporate more slowly.

Ethyl (rubbing) alcohol, with its more loosely bound molecules, evaporates almost five times as quickly as water. When energetic molecules depart from a liquid, they leave lower-energy, lower-temperature molecules behind. Which is why rapidly evaporating alcohol makes your skin feel cooler.

But water's disappearance is speedy compared to the average oil's. Spill a little water on the kitchen floor at night, and it will be gone by morning. Spill some vegetable oil, and watch out for a surprise wake-up slide.

Raise a liquid's temperature, and the vanishing happens sooner. Boiling water begins to evaporate from within, forming bubbles that rise to the surface, releasing vapor with tiny pops. So while a small pot of room-temperature water may take days to disappear, a rapidly boiling pot may be gone in less than 30 minutes.

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