How do things dry when it isn’t hot enough for water to evaporate?
In: Physics
Water can evaporate at virtually any temperature; it can even turn to vapor from solid ice (this is called sublimation). It’s a common misconception that evaporation only happens at the boiling point. Water boils at 100C, but it can evaporate (at a slower rate) at room temperature.
For the same reason salt can dissolve in water even though it isn’t at the salts melting point. Water dissolves into the air up until it reaches a certain saturation. That’s what humidity is.
When water boils away, it doesn’t dissolve into the air, it **becomes** a gas on its own.
It’s basically always hot enough. Heat makes water evaporate faster, but it still happens at basically any temperature.
Evaporation happens when water molecules randomly bump into each other. Some molecules happen to get enough energy to get bumped out entirely and into the air. High temperature means stronger and more bumps, so it evaporates faster. But there’s always going to be bumping at any temperature.
Edit: ice can also evaporate
Water has a thing called a triple state. This is a temperature at which water can exist as a solid, liquid and a gas. This is at 0.01C.
Consequently water evaporates, becoming a gas, all the time at temperatures above that.
When it is warm the rate of evaporation increases and we notice it.
When we say temperature, it’s only the average temperature of a substance. Individual molecules may be much hotter or colder. In the case of water, the molecules closest to the surface are a lot more exposed to heat and therefore can just evaporate away without the rest of the water boiling.
One of the sources of your confusion is the conflation of evaporation and boiling – evaporation occurs at all temperatures, and always from the surface. Boiling happens at 100°C (at sea level), and happens throughout the liquid, not just the surface.
Evaporation happens at virtually all temperatures. The boiling point is not the point where water starts to evaporate, it is rather the point where water can get no hotter as a liquid, so any additional heat is used for evaporation until the water completely evaporates to steam.
The molecules of water in any state are always moving (even well below freezing) to some extent. Those molecules near the surface may actually move fast enough to escape the hydrogen bonds that hold solid and liquid water together and enter the vapor state so they become gas.
The more heat you have in the water, the faster it moves, the more escapes. The boiling point of water is basically the temperature at which all the water will move fast enough to escape and not return to the liquid state.
Also, the less water vapor you have over or around the water in liquid or solid form the more likely it is to have an escape and net loss. So assuming you don’t have saturating humidity most water, especially in small puddles or thin films, will evaporate.
Water likes to go from crowded to not crowded place.
Wet things is crowded with water. Dry air is not crowded with water. Water moves to air. Air is not dry anymore.
Some water molecules are moving faster than others. Sometimes the fast ones can break free and escape into the air as water vapor. That’s what we call evaporation. It happens faster with higher temperature, but it can happen any time you have liquid water with not too humid air.
Water vapor molecules in the air also move around, and if they hit the surface of liquid water, or other surfaces, especially cool surfaces, they can get stuck. This is why your bathroom mirror gets foggy after a hot shower.
It doesn’t, but what you might think isn’t hot enough is actually enough most likely. How hot depends on the humidity and pressure mainly, so if for example you have 100% humidity then things don’t dry. Also vapor is not the same as boiling.
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