How is the mass of a liquid substance affected after it boils?

When a mass of a liquid boils its molecules pass from liquid state to vapor state. This is done gradually, that is, before the boiling point is reached some molecules of the liquid evaporate forming a bubble and are liberated to the surrounding air, at the same time vapor molecules condensate and come back to the mass of liquid; the quantity of liquid and vapor states an equilibrium. As the temperature of liquid increase, more and more molecules evaporates, so it's more difficult to condensate them at the same velocity, so that the equilibrium is reestablished. Finally, at the boiling point the vapor is formed but no vapor is condensed.

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