When boiling water, the total mass of the system (which includes both the liquid water and the vaporized water or steam) remains constant, assuming there is no loss of mass to the environment. This is in accordance with the law of conservation of mass.
In a closed system, when water reaches its boiling point, some portion of it transitions from the liquid phase to the gas phase (steam) without any loss in total mass. For example, if you start with 100 grams of water, and some of it boils off into steam, the sum of the mass of the remaining liquid water and the mass of the steam produced will still equal 100 grams, as long as no water escapes from the system.
In an open system (like a pot of boiling water without a lid), some steam may escape into the air, leading to a decrease in the mass of the water in the pot over time. However, the relationship holds for the closed portion of the system.
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