Density changes with the state of matter. In general, solids are denser than liquids, while liquids are denser than gases. There are a few exceptions to this rule. For example, solid water is less dense than cold liquid water and, as a result, ice floats above liquid water in cold seas.
When a material changes phase or state of matter, its volume and, therefore, its density usually change. Although pressure can influence the change in state of a material, temperature is the main factor. Gas volumes, as you may already know, are very sensitive to temperature and, of course, so are their densities. In general, gases have the lowest densities, but these densities are highly dependent on pressure and temperature, which must always be specified.
Densities are widely used to identify pure substances and to characterize and estimate the composition of many types of mixtures. Students will be able to explain, at the molecular level, how heating and cooling affect the density of water. Metals, whose atoms are grouped together quite compactly, have the highest densities, although that of lithium, the highest metallic element, is quite low. These very high densities represent the final limits of the amount of mass that can be packed into a given volume.
The range of densities found in the world spans a remarkably wide range, from practically zero in outer space to the unimaginably enormous values found in stellar bodies. In addition, certain temperatures and pressures can cause a change in the state of matter, which also affects density.