Why do liquids evaporate below their boiling temperature.


Water’s boiling temperature is 100C or 212F, when you spill some on concrete or leave a cup of water outside, it disappears without it reaching 100C even if it is in the shade. The water from the ocean also evaporates, but it is not boiling. This happens with other liquids to such as isopropyl alcohol, or gasoline.

How does this happen?

In: Chemistry

Boiling temperature is just the temperature at which material no longer exists in liquid form. All liquids evaporate at most temperature/pressure combinations – all it takes is enough energy to excite a molecule on the boundary layer between the liquid and its gas environment.

Because “temperature” is a property that only exists in collections of particles, each particle itself only has it’s own energy. So, all a particle on the surface needs, in order to evaporate, is to get hit from behind with enough force to overcome the bonding forces between it and the other particles.

All liquids have the ability to give off molecules in the gas phase: this is called the vapor. Boiling is a very specific event that occurs when the vapor pressure (the pressure of the vapor) equals the pressure of the air above the liquid.

Molecules can randomly exit the liquid phase and become gas. However, as long as the temperature is below the boiling point, those molecules will generally get pushed back into the liquid by the air pressure. As you increase the liquid’s temperature, those molecules get better and better at escaping, so they can “fight back” against the air pressure through their own pressure (vapor pressure). Once the temperature reaches the boiling point, their pressure is enough to counteract the air pressure, and the air pressure can no longer keep them in the liquid: they now have the strength to fight back and exit the liquid whenever they feel like it.

So when the vapor pressure (vapor molecules pushing on the air above) equals and then exceeds the air pressure (air pushing the vapor back down), the vapor is free to escape and the liquid starts to boil because there’s nothing keeping it in the liquid state anymore

think of boiling as forced evaporation, a temperature where no liquid phase can exist.

at all temperatures below that, the thing, say water, can exist in both a liquid and a gas form. and evaporation is just individual molecules getting enough energy to become a gas one by one.

Boiling is sending a bunch of energy in a forcing all the molecules to turn to gas.

Temperature is a measure of the average energy in a group of molecules. A water molecule on the water’s surface can gain enough energy to break off from the surrounding molecules and vaporize into the air. As a result, the temperature of the remaining liquid water will slightly decrease. This is why sweating cools down our skin during exercise. A chemical’s ability to evaporate is called volatility, and is linked to its boiling point. Alcohol and gasoline have lower boiling points than water; they are more volatile.

Liquids are made of molecules.

The “temperature” of a liquid is the average “speed” (more or less) that all the molecules are moving at.

This “speed” is actually a bell curve. The majority of the molecules in rhe liquid are moving around that speed, but a bunch are moving slower, and a bunch are moving faster.

“Boiling” isn’t actually about temperature, it’s about speed – it’s the speed that molecules need to be at, to leave the liquid altogether. Below that “speed”, something called “surface tension” pulls the molecules back. Imagine that all the molecules are something like magnetic and pulling on each other, but if you’re going fast enough, and there aren’t other molecules in the way, you can break free of the pull.

Once a liquid reaches it’s “boiling point”, the majority of the liquid’s molecules are moving fast enough to leave the liquid. But below that temperature, SOME of the molecules are moving fast enough to leave. If a molecule happens to be near the surface while it’s moving that fast… It leaves!

And that’s evaporation.

Incidentally this is also why boiling happens at different temperatures based on air pressure – higher air pressure means that more air molecules “get in the way” of a molecule leaving the water (again, more or less), even though there aren’t any actual water molecules “in the way”.

There are already some good descriptions in many comments. What I’d emphasize: evaporation always happens (as your question noted). Boiling is simply a quick evaporation process, that happens when the vapor pressure reaches that of the atmosphere. (At which point vapor can suddenly form inside the liquid, not just on the outer surface.) But this is not of a requirement for the slower evaporation that occurs at lower temperatures.

Fun fact: even [ice evaporates](https://en.wikipedia.org/wiki/Freeze-drying)! So frozen liquid would also “disappear” eventually, if exposed to dry enough atmosphere. That is, liquids evaporate (albeit very slowly) even below their freezing temperature.

Even funnier fact: you can [boil water at zero Celsius](https://en.wikipedia.org/wiki/Triple_point#/media/File:Water-triple-point-20210210.gif).