Most materials can exist as a solid, a liquid, or a gas. These are the most common phases of matter.

Starting with this chapter and continuing through Part V you will come to understand that the macroscopic properties of matter, such as volume, density, pressure, and temperature, can often be understood in terms of the microscopic motions of their atoms and molecules. This micro/macro connection is an important part of our modern understanding of matter.

《 LOOKING BACK Sections 14.1, 14.2, and 14.3 Fluids and pressure

You’re familiar with temperature, but what does it actually measure? We’ll start with the simple idea that temperature measures “hotness” and “coldness,” but we’ll come to recognize that temperature measures a system’s thermal energy. We’ll study the well-known fact that objects expand or contract when the temperature changes.

We’ll model a gas as consisting of tiny, hard spheres that occasionally collide but otherwise do not interact with each other. This ideal gas obeys a law relating four state variables—the ideal-gas law:

You will use the ideal-gas law to analyze what happens when a gas changes state.

Heating or compressing a gas is a process that changes the state of the gas. An ideal-gas process can be shown as a trajectory through a $\mathit{p}\mathit{V}diagram\text{.}$ We’ll study three basic processes:

• Constant-volume process

• Constant-pressure process

• Constant-temperature process

Physicists, chemists, biologists, and engineers all work with matter at the macroscopic level. Everything from basic science to engineering design depends on knowing how materials respond when they are heated, compressed, melted, or otherwise changed by factors in their environment. Changing states of matter underlie devices ranging from car engines to power plants to spacecraft.