If the displayed mass gradually drops while a liquid is being weighed, that liquid is probably volatile. The mass falls because the liquid is evaporating, causing the amount of substance in the weighing vessel to decrease. Take special precautions, such as a placing a rubber stopper in an Erlenmeyer flask, to seal the weighing vessel when weighing a liquid that evaporates quickly. Make sure to account for the mass of whatever you use to seal the weighing vessel.
