# Can water vaporize without reaching 100 degrees celsius? [duplicate]

This question already has an answer here:

I think many of you just let water sit somewhere outside, on a surface it can not be absorbed in. What happened to the water? Did it vaporize? Can water still vaporize if it is not on 100 degrees celsius? If not where did it go? If yes then how come we weren't taught about it at school?

## marked as duplicate by ACuriousMind♦Feb 2 '17 at 21:06

• It's called evaporation; I'm afraid the question of why you weren't taught about it in school is a question you'll have to take up with your local education system, not us. – ACuriousMind Feb 2 '17 at 20:41
• Humidity and relative humidity are relevant here. – Jon Custer Feb 2 '17 at 20:43
• I think if this question is not a duplicate, then its not a bad one. – AHB Feb 2 '17 at 20:51
• What do you mean "we?" – Chet Miller Feb 3 '17 at 1:02

The quantity typically used to characterize this effect is the so-called vapor pressure: At finite temperatures all liquids have a non-zero vapor pressure, meaning the equilibrium partial pressure of free molecules of the liquid is greater than zero. This means that if you have liquid sitting in a closed environment (of finite volume), eventually (in equilibrium) the partial pressure from free molecules of that liquid will equal the vapor pressure. However, if you remove these free molecules somehow (say, wind blowing over a shallow puddle), or if the environment has essentially infinite volume so that the partial pressure of a liquid's molecules above the fluid's surface is less than the vapor pressure, then fluid molecules will continue to leave the body of the liquid, meaning they continue to evaporate.

The Wikipedia article I linked has more information on this. Specifically for water, here is a plot of its vapor pressure as a function of temperature (linear and logarithmic, source):

It's a legit question to ask your school teachers... I encourage you to go and do that, especially if you payed for your education... Yessss, water and most of other liquids do evaporate even being a room temperature=)

You can look up the details, but in simple words, the force that "holds" the liquid together is surface tension. Liquid by itself is an ensemble of many molecules with different velocities. Occasionally, molecules gain enough velocity to overcome surface tension and to escape the surface of the water. Some molecules can come back to the water too. It is a stochastic process of going back and forth through the surface barrier.

Usually, you can observe this process in nature being near the lake on the sunny morning. The temperature starts rising when the morning comes, and the Sun starts heating up the surface of the lake. The water molecules start evaporating, creating mist. If you wait for half an hour more, you would notice, that the reverse process happens as well: condensation on the grass=)

All liquids evaporate at any temperature. It varies by substance and drops if it's colder. At $0 \text{K}$ it stops.

We can say he effective parameters are:

• The contact surface area (+ effect)
• Temperature (+ effect)
• Existence of wind or breeze and its speed (+effect)
• As @JonCuster mentioned: How saturated the environment is also affects the evaporation (negatively).
• Properties of the substance which covers surface tension, viscosity and other things.

the reason can be studied by molecular theory. Assuming that molecules exist, you can say that liquid molecules, moving in a Brownian motion, hit the surface molecules randomly. Some of them get enough energy to escape the bond of surface tension.

Now:

• More surface area means more probability to take off (so + effect).
• More liquid temperature means more energy and more impulse on the surface molecules, leading to more ejections.
• Wind decreases the air pressure above the surface, molecules will more tend to get out of the liquid.
• But it the environment isn't much willing to accept any more molecules, less molecules will detach from the surface.
• And the strength of the prison walls (surface tension) has negative effect of evaporation rate.