Liquid water and water vapour reach equilibrium only in an enclosed container because the rate in which the liquid vaporized is equal to the rate in which the vapour condenses.
Also, it could be said that at that situation, the rate of vapourization is equal to the rate of condensation or the amount of gas is equal to the amount of liquid.
What is Vapourization
Vapourization is the transitioning of an element or compound from liquid to vapour.
What is Condensation
Condensation is the conversion of vapour or gas to liquid.
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