Overview

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.

Procedure

All matter is composed of a very large number of molecules that move in a constant, random motion. Even in ice and metals, atoms still move.

There are three different states of matter: solids, liquids, and gases. In all these states molecules, atoms, or ions are in motion with a specific amount of kinetic energy, which determines the state of matter.

In the gaseous state, molecules have a high kinetic energy. The high energy molecules move quickly past each other and are widely separated, making it harder for them to intersect frequently. As a result the attractive forces in gases are weak.

With this negligible force of attraction, gases have no definite volume or shape and expand freely to fill in the entire volume of its container. With a lot of space around the molecules, gases have low densities making them easy to compress.

When gases are compressed or cooled, their kinetic energy is reduced and consequently the molecules slow down. The molecules move past each other more often and come closer together.

This increases the amount of intermolecular forces and transforms the gas to a liquid state.

Molecules within liquids are bound by stronger attractive forces, leaving little space around them. Thus, liquids are denser than gases making them difficult to compress. A liquid also has a definite volume and assumes the shape of the container.

Despite stronger attractive forces, molecules in liquids can still move freely about one another, and so the liquids can flow or be poured.

When liquids are further cooled, the kinetic energy reduces to a point where the particles almost stop moving due to the strong intermolecular forces and can only vibrate at their fixed position. At this point, molecules transform into a three-dimensional conformation state called solids.

In any solid, molecules are densely packed, leaving almost no empty space around them. Thus, solids are incompressible, having a definite volume and shape.