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First, I think I should make it clear that when water boils, the bonds in the water molecule linking the hydrogen and oxygen atom are not broken. During boiling, the intermolecular bonds in water are the ones that get broken, that is the bonds that link the water molecules together. At room temperature, there is evaporation I wouldn't call it excitation. This is because there are a few molecules of water which can manage to muster enough energy to escape from the large body of molecules and escape into air.
This can be explained through a graph depicting the distribution of speed among water molecules worked out by Maxwell and Boltzmann. As you can probably see, there are a lot of water molecules with lower kinetic energy than with higher kinetic energy.
Those that have the higher kinetic energy are the ones that are able to break through the water surface to become vapour. Even at low temperatures, there are some water molecules are have enough energy to escape and that's why evaporation in water can occur at any temperature yes, even if the water is in ice. When the temperature increases, there are more molecules with higher kinetic energy and thus, more water can evaporate.
To add to Jerry's answer, the amount of evaporation of water also depends on pressure. Infact, one way of defining boiling point is the temperature at which the vapour pressure equals atmospheric pressure.
The temperature at which water in liquid form is converted into gaseous form. Then how it possible for water to evaporate at room temperature?
Think of temperature as average kinetic energy of the water molecules. While the average molecule doesn't have enough energy to break the inter-molecular bonds, a non-average molecule does. Water is a liquid because the dipole attraction between polar water molecules makes them stick together. However, at the surface of the liquid, lone molecules may end up getting enough kinetic energy to break free due to the random nature of molecular motion at basically any temperature.
On the flip side, water molecules in the atmosphere may enter the liquid at the surface as well, which is measured by equilibrium vapour pressure. Imagine spinning a roulette wheel, but instead of dropping in one ball, you drop in They all rattle around at different speeds, like the molecules in water.
You can cool them down by spinning the wheel slower, so they bounce about less; heat them up by spinning faster so they bounce more; you can freeze them by stopping the wheel and waiting till they're all stationary; and you can boil them by spinning the wheel so fast that they all fly out of the top.
Now pick up all the balls and throw them back in with the wheel spinning at a moderate speed. If you watch for a while you'll see that although the average speed of the balls is below the "boiling point" where they all fly out the top, every now and again one ball will ricochet off another with enough force to send it flying out of the wheel. If you watch for long enough eventually all the balls will be gone. Your balls just evaporated. Temperature is a measure for how much kinetic energy the molecules in a substance have.
If the temperature is high, they are moving pretty fast, if the temperature is low, they are moving a lot slower. If molecules are moving slow, they bundle up and you get a solid. Once you heat it up a bit, the substance starts to become liquid. When you heat it up even more, the molecules will start to move so fast they will spread out into the entire space gas.
However, this is all averages. In a liquid all molecules are moving, some faster than others. If a molecule happens to break through the 'surface' of the water, it'll have escaped the inter-molecular forces holding the water together and it'll be evaporated.
This can also happen with solids, there it is called sublimation. If you're heating up water, you're adding energy so this process will start to go faster. Then at boiling point, you'll reach the point where molecules will want to start moving so fast they start to form gas bubbles inside the liquid. The boiling temperature of a liquid is not the temperature at which it can enter the gaseous state.
This is why, for example, water boils at lower temperatures at higher altitudes. Furthermore, water is always evaporating.
There is a variety, because the molecules in a liquid can move around. The molecules can bump into each other, and when they hit A little bit of energy moves from one molecule to another. Since that energy is transferred , one molecule will have a little bit more and the other will have a little bit less. With trillions of molecules bouncing around, sometimes individual molecules gain enough energy to break free. They build up enough power to become a gas once they reach a specific energy level.
In a word, when the molecule leaves, it has evaporated. The rate of evaporation can also increase with a decrease in the gas pressure around a liquid.
Molecules like to move from areas of higher pressure to lower pressure. The molecules are basically sucked into the surrounding area to even out the pressure.
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