For example, suppose we have a closed system, a closed container, and inside this container we have pure water. But that simply means that the only types of molecules found within our system or water molecules. Now, what will happen to our system if it's left untouched? Well, eventually, some of the water molecules found on the surface of the liquid will escape into the gas state and will become gas molecules. And this is called evaporation. Now, on the rate of evaporation and condensation are equal.