The liquid metal, mercury, makes for dramatic demonstrations of density and buoyancy. You can, for example, float heavy objects such as bricks and dumbbells in a sufficiently large container of mercury because its density is greater than that of lead. Some elements, however, are denser still, and objects made of these substances will sink in mercury.

All substances possess a property called density, which is mass divided by volume. If you know the substance from which an object is made, you can look up the density, measure the volume and find the mass by multiplying volume by density. Elements at the top of the periodic table tend to have lower densities; those toward the bottom have the greatest densities. The density of mercury, which is element number 80 out of the 118 listed in the periodic table, is 13.55 grams per cubic centimeter.

Examples of things floating abound in nature and everyday life: you may see clouds floating in the sky or a cork floating on water. Whenever you see a floating object, its density is lower than that of the substance beneath. According to Archimedes' principle, an object placed in a liquid will displace some of it; the weight of the displaced liquid produces a buoyant force whose strength equals the weight. If the force is greater than the object’s weight, the object floats. Because mercury’s density is very high, objects that you might consider heavy, such as lead weights or a steel ball bearing, will float in it.

Because mercury’s density is high, most other substances will float in it. This includes metals such as nickel, iron and copper as well as mixed substances such as most types of stone and organic materials such as plastics and wood. Those liquids and gases that are less dense than mercury will also float in it.

A handful of elements are denser than mercury and objects made of these substances will sink into it. Several precious metals -- including gold, with a density of 19.3 grams per cubic centimeter, platinum with 21.4, and iridium with 22.65 -- will sink in a mercury bath. Many of the actinide elements, radioactive substances that reside at the very bottom of the periodic table, also have very high densities and will sink in mercury. Plutonium, for example, has a density of 19.84 grams per cubic centimeter. Americium, another actinide element, is slightly denser than mercury at 13.84 grams per cubic centimeter.