The density of an object or substance is its mass divided by its volume:
The units of density depend on the units used for mass and volume, but are usually:
The more dense a substance is, the heavier it feels for its size. The table shows three examples:
|Substance||Density in g/cm³|
Notice that the solid (steel) is the most dense, the gas (air) is the least dense, and the density of the liquid (water) is in between.
The particles in solids are very close together. They are tightly packed, giving solids high densities.
The particles in liquids are close together. Although they are randomly arranged, they are still tightly packed, giving liquids high densities. The density of a substance as a liquid is usually only slightly less than its density as a solid. For example, the density of solid aluminium is 2.72 g/cm3 and the density of liquid aluminium is 2.38 g/cm3. This means that liquid aluminium floats on top of solid aluminium.
Water is different from most substances: it is less dense as a solid than as a liquid, because its particles move apart slightly on freezing. This is why ice cubes and icebergs float on liquid water.
The particles in gases are very far apart, so gases have a very low density.
You need to know two things to measure the density of a substance:
The mass is measured using a balance. The volume of a liquid is easily measured using a measuring cylinder. The volume of a solid can be measured by: