oxygen

[elderly]

Is a oxygen cylinder more heavy when it is full?

Does oxygen actually weigh anything?

[Ray The Grey]

Yes it is quite heavy really. It forms the majority of the weight of water and is a constituent in rust.

Air pressure relates to the force of gravity on air molecules (21% of which are oxygen). An oxygen cylinder is a lot heavier as the gas is compressed into liquid form and there are a lot more atoms of oxygen in a cylinder than you would get in air - imagine the difference in weight between water and steam.