eli5: Why do some liquids dry faster than others?


Like, why do alcohols evaporate faster?

Are liquids that take longer to dry ‘wetter’ than others?

In: Chemistry

The fancy word for that is volatility. The more volatile it is the faster rate evaporates. Why does it happen? Because it does LOL. I don’t have a better answer. Sorry.

Liquids have weak bonds which hold them together and create surface tension.

These bonds are caused by the liquids having unequal charges distributed throughout their molecules, causing the molecules to stick together.

Some have very lopsided charges (like water) while others have less lopsided charges (like ethanol) and so they are less ‘sticky’

This means it takes less energy to free a molecule from the collection.

Another factor is that fluids try to diffuse, move from a high concentration to a low concentration. The concentration of water in the atmosphere in the air varies a lot, but it is almost always higher than the concentration of, say, ethanol in the atmosphere.

Water molecules really like to stick together. Alcohol molecules not as much.

Imagine a group of people dancing. That’s a liquid. Each person moves around within the group, but mostly everyone stays inside the group. Sometimes, especially if the dance is very exuberant (maybe imagine a mosh pit), a person goes shooting out of the group. That’s evaporation: a liquid molecule has joined the air. The air (or other gases or gas mixtures) is more like a bunch of strangers running around at great speed in a big empty room. They don’t stick together, just sometimes bump into each other.

When the liquid is water, imagine the people dancing are very fond of each other. Maybe imagine them holding hands or hugging a lot and then letting go again. It’s hard for a single person to break free from a group like that. Alcohol, or other more *volatile* liquids, is like people who don’t like each other very much. Yeah they’re dancing, but they don’t hold hands, or not as often. So when the dance gets exuberant (people moving around at high speed), it’s much more likely in this less lovey-dovey group for a person to come shooting out.

(An exuberant dance, with people/molecules moving around at high speed, corresponds to a higher temperature. A person can break free of the group at any temperature, but it’s more likely if everyone is moving a lot anyway. If the temperature gets hot enough, then no amount of hand-holding will keep the group together. That’s boiling. When the group boils, they have to break apart and all will turn to gas (eventually). Below the boiling point, some people will leave the group, and some will come back in, and there’s a balance of dancing and running people, with more dancing and fewer running people as the temperature falls. If it gets very cold indeed, then the people stop dancing and just stand there shaking and holding hands continuously.)

Volatility occurs because different substances have different boiling points (the temperature where liquid turns to gas). Liquid water turns to gas at a relatively high temperature (100 Celsius) however something like diethyl ether “boils” at room temperature, aka if you leave the liquid exposed it will evaporate without any added energy.