Why Does Heating a Flask Change Its Weight?

[qrt388]

If you heat up a glass flask why will the weight of the flask decrease slightly?

The lab we're doing deals with the ideal gas law, so maybe it has to do with the mass being inversely proportional to the temperature? Or is there another reason?

Also, what would happen if you heat up a flask with no air in it that's sealed off from the outside (so inside the flask is like a vacuum)? Would the mass decrease?

 

[Dick]

What do you think? Why on Earth do you think the weight of the flask will decrease, even slightly? The ideal gas law doesn't say mass will decrease. It says the ratio of mass to volume will decrease under constant pressure.