Density

Density measures how "heavy" a material is for it's size (volume)

Density (g/cm^3) = Mass(g) / Volume (cm^3)

Mass = Density * Volume

Volume = Mass / Density

Measuring the density of an object requires 2 figures; the volume and the mass.

Metals expand when they are heated:

When metal is heated, the particles expand as the solid prepares to change into a liquid.

When a material is heated the particles within it move faster. This causes the particles to push one another apart which causes the material to expand. As the volume of the object has increased but the mass has stayed the same the density of an object has decreased.

Assuming that pressure remains constant, the volume and absolute temperature of a certain quantity of a gas are directly proportional.

Temperature = Constant x Volume
or
Volume = Constant x Temperature
or
Volume/Temperature = Constant

Assuming that pressure does not change, a doubling in absolute temperature of a gas causes a doubling of the volume of that gas. A drop of absolute temperature sees a proportional drop in volume.