Particles in a solid are tightly packed together (fixed shape) and often arranged in a regular pattern; in a liquid, they are close together with no regular arrangement (no fixed shape); in a gas, they are far apart with no regular arrangement (no fixed shape). Particles in a solid vibrate about fixed positions (cannot flow) and do not generally move in relation to one another; in a liquid, they move past each other (can flow) but remain in essentially constant contact; in a gas, they move independently of one another (can flow and expand) except when they collide.

The differences in the properties of a solid, liquid, or gas reflect the strengths of the attractive forces between the atoms, molecules, or ions that make up each phase. The phase in which a substance exists depends on the relative extents of its intermolecular forces (IMFs) - electrostatic forces of attraction existing between the atoms and molecules of a substance - and the kinetic energies (KE) of its molecules. While IMFs serve to hold particles close together, the particles’ KE provides the energy required to overcome the attraction and thus increase the distance between particles. For example, in a liquid, attractive intermolecular forces hold the molecules in contact, although they still have sufficient KE to move past each other. Due to this, liquids flow and take the shape of their container.

According to kinetic-molecular theory (KMT), the temperature of a substance is proportional to the average KE of its particles. Changing the average kinetic energy (temperature) induces changes in the physical state along with associated changes in intermolecular forces. For example, when gaseous water is cooled sufficiently, or the average kinetic energy of molecules is reduced, the increased attraction between H₂O molecules will be capable of holding them together when they come into contact with each other; the gas condenses, forming liquid H₂O. When liquid H₂O is further cooled, the attractive forces become stronger, and water freezes to form solid ice.

In cases where the temperatures are not too high, gases may be liquefied by compression (high pressure). Gases exhibit very weak attractive forces due to which the particles are spread apart at large distances. The increased pressure brings the molecules of a gas closer together, such that the attractions between the molecules become strong relative to their KE. Consequently, they form liquids. Butane, C₄H₁₀, is the fuel used in disposable lighters and is a gas at standard temperature and pressure. Inside the lighter’s fuel compartment, the butane is compressed to a pressure that results in its condensation to the liquid state. Moreover, if the temperature of a liquid becomes sufficiently low or the pressure on the liquid becomes sufficiently high, the molecules of the liquid no longer have enough KE to overcome the IMF between them and a solid forms.

This text is adapted from Openstax, Chemistry 2e, Chapter 10: Liquids and Solids.