Question

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

Answer 1

The boiling point is defined as the temperature...

The boiling point is defined as the temperature at which the vapor pressure of the liquid is equal to the ambient pressure, and bubbles of vapor form directly in the liquid. The normal boiling point is specified when the ambient pressure is 1 atm

Of course, if we reduce the ambient pressure, the boiling point will substantially be reduced, and this is the example of the principle of vacuum distillation, where an otherwise in volatile liquid is distilled under high vacuum.

Answer 2

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.