Density

Density
Density measures the mass of cubic centimetre of a material.  This is an important physical measurement for designers, scientists and engineers.  For instance the density of iron is around 7.8 g/cm3 (so a cube one centimetre in all three directions has a mass of 7.8 grams) but the density of Aluminium is only 2.7 g/cm3.  If you replaced all the aluminium parts of an aeroplane with iron it would not get off the ground because the mass of each part would be around three times higher.

Density is also important in predicting if things will float.  A material will float if it is less dense that the liquid or gas it is in.  Water has a density of around 1 g/cm3 (well exactly 1 g/cm3 at 4 degrees centigrade) so materials with a density of less than 1 g/cm3 will float but those with a density of more than 1g/cm3 will sink.  This is also why Helium balloons float, they are less dense than air.

 If you put a helium balloon into a room full of hydrogen what would it do? Well it would sink. Helium is more dense than hydrogen.

Heating and cooling things can change their density.  Hot air balloons work by this principle heating the gases of the air gives the particles more energy so they move apart.  This means there are fewer of them in the same volume of space so compared to the cold air outside the balloon the density drops and the balloon starts to rise.  This is also how convection cells or current work.  Cooling most things makes them smaller so their density rises.  This does not happen with water!  When water freezes it expands and so its density actually goes down which is why icebergs float.

To work out the density of a material  we need to know how to find it mass and volume.  There are different ways to do that shown on these videos:  Density of a rectangular solid<>,  Density of a liquid<>, Density of an irregular solid<>