18.1 Solids, Liquids, and Gases
In Part V we will study the properties of matter itself, as opposed to the motion of matter. We will focus on a macroscopic description of large quantities of matter. Even so, part of our modern understanding of matter is that macroscopic properties, such as pressure and temperature, have their basis in the microscopic motions of atoms and molecules, and we’ll spend some time exploring this micro/macro connection.
As you know, each of the elements and most compounds can exist as a solid, liquid, or gas—the three most common phases of matter. The change between liquid and solid (freezing or melting) or between liquid and gas (boiling or condensing) is called a phase change. Water is the only substance for which all three phases—ice, liquid, and steam—are everyday occurrences.
NOTE This use of the word “phase” has no relationship at all to the phase or phase constant of simple harmonic motion and waves.
MODEL 18.1 Solids, liquids, and gases
A solid is a rigid macroscopic system consisting of particle-like atoms connected by spring-like molecular bonds. Solids are nearly incompressible, which tells us that the atoms in a solid are just about as close together as they can get.
The solid shown here is a crystal, meaning that the atoms are arranged in a periodic array. Elements and many compounds have a crystal structure in their solid phase.
A liquid is a system in which the molecules are loosely held together by weak molecular bonds. The bonds are strong enough that the molecules never get far apart but not strong enough to prevent the molecules from sliding around each other.
A liquid is more complicated than either a solid or a gas. Like a solid, a liquid is nearly incompressible. Like a gas, a liquid flows and deforms to fit the shape of its container.
A gas is a fairly simple system in which each molecule moves through space as a free, noninteracting particle until, on occasion, it collides with another molecule or with the wall of the container. A gas is a fluid. A gas is also highly compressible, which tells us that there is lots of space between the molecules.
Note that a gas completely fills its container.