You have an evacuated container of fixed volume and known mass and introduce a known mass of a gas sample. measuring the pressure at constant temperature over time, you are surprised to see it slowly dropping. you measure the mass of the gas-filled container and find that the mass is what it should be-gas plus container-and the mass does not change over time, so you do not have a leak. suggest an explanation for your observations.

Chemical or physical reaction

Explanation:

Two reasons for reduction in pressure which have been discounted in the question are reduced temperature or leakage. As both possibilities have been discounted in the question, it means another possibility has to be considered.

Two other possible reasons for reduction in pressure are a chemical or physical reaction. Pressure exerted on a container by a gas sample is related to the number of gaseous moles of the sample. Either a chemical reaction has occurred causing a reduction in the number of gaseous moles; or a physical reaction such as condensation/reverse-sublimation has occurred, reducing the number of gaseous moles.