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Which has greater mass, mercury, air, or water?
That would depend how much of each you have. The real question is: which one is the most dense?
To figure that out, you need to take the same amount of air, water, and mercury, to compare them. Let's take one cubic meter, because it's standard. One cubic meter of water weighs 1,000 kg (2,205 pounds); one cubic meter of air weighs 1.2 kg (2.6 pounds); and one cubic meter of mercury weighs 13,545 kg (29,861 pounds).