Aluminum/Uranium and SF6/Air/Helium comparisons.
What It Shows
The concept of mass per unit volume is punctuated by having several different substances on hand for comparison. In solid materials, we have equal size chunks1 of aluminum (2.7 g/mL) and uranium (18.7 g/mL) for comparison. For gases, we typically use balloons filled with helium (0.18 g/L), air (1.29 g/L), and sulfur hexafluoride (6.50 g/L). Being 5 times heavier that air, the SF6balloon noticeably feels like it weighs more than the air-filled one and drops (almost) like a rock! We all know what happens when you drop the helium balloon; this can lead into a discussion of buoyancy.
1 They are cubes measuring 5 cm on a side. Most demonstration collections do not have chunks of uranium. Ours has a story attached which can be found in the Uranium Block demo under Quantum Physics and Relativity, Radiation and Radioactive Decay. Lead (11.4 g/mL) or a bottle of mercury (13.5 g/mL) could be substituted for the uranium.