Which is it? Does the 'mass' (don't you mean 'weight'?) increase or decrease when you put a cover over it? It can't do both. Does the cover come into contact with the container, or is it just held above the container? If so, how far above?
The density of water changes as the temperature changes but I thought this meant the volume changes rather than the mass.
But as douglas says, we don't know the answer. I'm not sure we understand the exact question either.
I wish that Angellina would come back and explain the question a bit better. But, while we're waiting, I'd like to ask Angellina who says that the mass of the cold water increases and decreases when you place a cover over the container of cold water? If it were anything to do with atmospheric pressure, you wouldn't need to put a cover over the container.