The temprature at which liquids change into vapours is very high, for example, water vapourises at 100°C. Then how is it possible for water to evaporate at room temperature or at any other temperature?
Answers
The process is called evaporation.
Answer:
Water doesn't become steam before 100°C. It turns into water vapours.
There is a difference between steam and vapours. Though both are gaseous forms of water but steam is when water reaches its boiling point and water vapours are formed during evaporation.
Boiling point is when the pressure of a particle (or water in this case) in liquid state exerted to its condensed form (liquid water) equals the atmospheric pressure. Or in easy words, the point at which water molecules have enough energy that they break the cohesive force between them (formed by the vapor pressure) and get loose to wander freely is boiling point.
Evaporation is a surface phenomenon, unlike boiling. If you mark a water molecule X when water is at room temperature (and sea level atmospheric pressure), it wont be converted into gaseous state unless it reaches surface. Whereas in boiling this water molecule X reaching its boiling point can be converted into gaseous state even if its not at the surface. Thats why you see bubbles popping up during boiling and not during evaporation (ever wondered what those bubbles are-those are bubbles of steam).
Now you might ask why evaporation takes place....to answer this you must understand energy. A glass full of water at room temperature has some energy. Because of this energy, water molecules are randomly hitting each other in proper chaos (called brownian motion). As they hit each other, their energies are transferred from one to another.
Now this transfer of energy is very normal until the energy reaches the surface. At the surface, the topmost molecule doesn't have any neighbouring particle to transfer its energy. If this energy is sufficient to overcome surface tension of the liquid, it bails out.
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