Final answer:
Most substances are denser in their solid state due to the tight, orderly arrangement of particles that are only capable of vibrational motion. Liquids and gases have more kinetic energy, which results in less tightly packed arrangements for liquids and large separations in gases, hence lower densities.
Step-by-step explanation:
Most substances are more dense in their solid form than in their liquid or gaseous forms. This is primarily due to the way particles are arranged in solids. The particles of most solids are packed tightly together in an orderly, fixed arrangement. Such close packing results from the atoms, ions, or molecules being bound together by strong forces, and having only limited motion – mainly vibrational. This structure makes solids almost completely incompressible, which in turn contributes to their high density.
In contrast, the kinetic energy of particles in liquids is higher, which allows them to overcome some of the attractive forces between them and move past each other, leading to a less orderly and more fluid arrangement. As for gases, the particles have enough kinetic energy to completely overcome the attractive forces between them, causing them to be separated by large amounts of empty space, thus resulting in much lower densities compared to solids and liquids. An exception to this rule is water, which is less dense as a solid than as a liquid because of the special way its hydrogen bonds arrange when freezing.