drying to constant mass
Imagine drying a wet towel and weighing it again and again until two weighings come out the same — at that point you know there is no more water hiding inside. Drying to constant mass is that simple discipline applied with a balance and an oven.
Drying to constant mass means heating (or otherwise drying) a sample, cooling it, and weighing it, then repeating the cycle until successive weighings agree within a tiny set limit. Agreement signals that all the moisture or volatile material that the conditions can remove has gone.
It matters because a precipitate that still holds water weighs too much, inflating the result. Constant mass is the analyst's proof — not a guess — that drying is genuinely complete and the recorded mass reflects only the substance, not leftover moisture.
A crucible is heated, cooled in a desiccator, and weighed; the cycle repeats until two successive masses differ by no more than 0.3 mg — only then is the crucible declared at constant mass.
Repeat heat-cool-weigh until the mass stops changing.
Always cool the sample in a desiccator before weighing. A hot object stirs convection currents and absorbs moisture as it cools on the pan, so weighing it warm gives a false, unstable reading.