## Density

Usually, when we refer to density we mean a mass density which would be mass per unit volume.  This is usually expressed by the equation

where the Greek letter ρ stands for density, m is for mass, and V for volume.  Some texts will use the letter D for density.

In the metric system we might use units of grams per milliliter, grams per cubic centimeter or kilograms per cubic meter. It is worth noting that the c.c or cubic centimeter is equal to one milliliter.  You often hear the term c.c. on bad television shows about hospital emergency rooms.

However, we could also have a charge density, current density, energy density among many others!

The density in a solid object does depends not only on the mass of the individual atoms, but on how those atoms are arranged.  What kind of crystal lattice creates spaces between the atoms, or is the solid an amorphous blob?

It is worth noting that the density of water happens to be 1 g/ml or 1000 kg/cu.m  This is largely due to the historical context and the role water played in defining the gram.

Most metals have a density about 10 times greater than water. This table of densities is in units of g/ml

 Solids Magnesium 1.7 Aluminum 2.7 Copper 8.3-9.0 Gold 19.3 Iron 7.8 Lead 11.3 Platinum 21.4 Uranium 18.7 Osmium 22.5 Ice at 0 C 0.92

I’ll end this with a couple thoughts.  Why is it impossible to sink in the dead sea?

And what is happening in this picture?  Why don’t the colours mix in this cylinder?

Reminds me a bit of a Tequila Sunrise.

One last thing to share is a great YouTube video from a Museum up in Canada.