What’s the difference between water boiling and it evaporating normally? Don’t both end up the same, ie. water turning into gas form?
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Yes, both evaporation and boiling will produce the exact same thing: water vapor. The only difference is the speed with which it happens, and the amount of energy you need to input. Vapor from boiling and vapor from evaporation are 100% chemically identical, and indistinguishable from each other.
Whenever you have a surface of liquid water, some water molecules will break free and fly away from the normal jostling around of molecules. That’s evaporation. And also, some water molecules that are flying around will collide with the surface and stick to it. If there are as many molecules colliding and sticking as there are breaking loose, then the overall evaporation stops. We call that 100% humidity, and it depends on how many molecules are flying around in the air.
Boiling is different – that happens when the temperature is too high for the water molecules to stick together at all. They are wiggling so much that it overcomes the stickiness that holds the whole blob together.
Air is able to hold a little bit of water vapor at room temperature (think of humidity). If you have an open container of liquid water, as long as the nearby air isn’t totally saturated with water vapor, water will very gradually evaporate from the container and go into the air. Think about dipping the edge of a paper towel into water; it will suck up a small amount of the water until it can’t hold any more. If you had a lot of water in an airtight room, eventually you would not get any more evaporation because the air is already holding all the water it can. But usually due to wind, air conditioning, etc., the water gets continually exposed to new, drier air, so it can keep evaporating until it’s all gone.
Heating up water to the point where it boils means that *all* the water wants to become vapor *right now* and is willing to shove the air out of the way to do so. Instead of the previous analogy of dipping a paper towel into the water to soak a little bit up, this is like sticking a vacuum hose into the water and pulling it all out of the container really fast.
One way you can visualize water involves thinking about its molecules like a big ball pit. For the most part those molecules are inclined to stick to each other. But if some effort is made, balls can leave the pit and end up stuck to air molecules instead.
Evaporation is a really slow process where two things do the “work”. One is a temperature differential: if the air is warmer than the water, heat “wants” to move between the two and the movement of that heat constitutes energy transfer. That extra heat hitting the water can do the work of “knocking loose” a molecule. (The visualization is more like air molecules are bouncing around and hit the water molecules.) The secondary factor is warmer air has more “space” for water molecules to fit in (due to its molecules moving faster and being less dense) so the natural tendency of diffusion means the water molecules are sort of “pulled” into the air more easily. Colder air and colder water means slower evaporation, because there’s less energy being transferred and it takes energy for evaporation to occur.
Boiling is a much more violent process because it involves adding a ton of extra heat to the water. Once there is enough heat, the water molecules are bouncing around so fast they break their own bonds. That *forces* water molecules into the air, and part of why we see steam is there is temporarily so much water escaping the “ball pit” there’s not enough space for the air to accept it all. This is a fast process because energy is being delivered to the water very quickly.
So the same things are happening in boiling and evaporation, the main difference is whether the energy transfer that breaks molecular bonds is happening fast or slow.
They’re more or less the same thing.
Temperature is an average. Even though what I’m about to say is technically wrong, it’s accurate enough for the purposes of this question: for a cup of water at 30C, if you zoom in to the molecular level, you could observe bits that are 20C, 50C, 10C, and once in a while some that hit 100C and boil off just as if the whole cup of water were at 100C.
For completeness: The technically incorrect bit is that, again, temperature is an average. More specifically, it measures average kinetic energy. Molecules are constantly moving. Solids are made of molecules linked to each other wiggling about. Liquids are made of molecules sticking together but sliding around. Gases are molecules that fly around freely. And the act of boiling or evaporation is water molecules that happen to slide around fast enough to escape the sticky liquid bit and fly free as gas. Water molecules are much more likely to do this at 100C than 30C, so it happens fast enough to look like bubbling and steam. At 30C you still get individual water molecules escaping into gas speed, but since most of the water molecules are moving slower as they slide around, it happens too slow for you to really notice with your naked eye (unless you wait a while).
Isn’t water evaporating basically the same as a really slow boil?
Imagine water molecules as some sticky, bouncy balls. They roll around on each other, but try to stick together. Sometimes a molecule will randomly move in a way that it bounces more than it sticks and leaves the liquid. That’s what we call evaporation. If you heat up the water, the molecules will get faster and the chance for them to bounce off increases. When the liquid boils, that means these molecules are moving so fast, they break free while still inside the liquid, which creates bubbles. If you cool the steam down, it will slow the movement of molecules, so they start to fall and stick together again, turning back into liquid. Cool it down even more, and they will eventually lose so much of their energy, they can’t even roll around on each other anymore, just stick together and kind of vibrate. That’s what ice does. If you heat it up, the vibrations get stronger and cause molecules to break off, but most of them won’t be fast enough to evaporate, they will be liquid again
Water vapor wants to expand – all gasses do that – and the pressure it exerts to do that is called the “vapor pressure.”
Liquids *also* have a vapor pressure, called the “saturation vapor pressure” that depends on the temperature of the liquid. More temperature means that water vapor will “leak out” of the surface of the liquid at a higher temperature.
At room temperature this saturation vapor pressure is low, only about 1/50th of atmospheric temperature. So water vapor leaks out slowly. And if you have humid air, the pressure of water vapor going back into the liquid can balance it out.
If water is hot enough its saturation vapor pressure reaches the total absolute pressure. Newly formed water vapor is capable of pushing other things out of the way. So it pushes water away from the bottom of a kettle and makes steam bubbles, or a jet of steam can push through air and blow a whistle. Those things don’t happen during low-temperature evaporation.
If the ambient pressure is lower then you need less saturation vapor pressure and the boiling temperature is also lower.
There are several other interesting effects, like what happens when a container contains *only* water vapor and water with very little air or other gases. If you do that the water will quickly boil-evaporate at any temperature between about 0 and 370 and very quickly spread that heat to the rest of the container by condensing on cool spots. This effect is used to cool electronics.
Evaporation is the movement of a liquid to a gas below its boiling point. This happens because some water molecules will naturally have enough kinetic energy to escape into gas phase. But yes, the outcome is the same.
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