Do you recognize the object in Figure 1? It is a carpenter's level. A level can be used to see whether a surface is perfectly horizontal. The level has one or more transparent tubes inside a metal or wooden frame. Inside each tube is a clear liquid, such as alcohol, and an air bubble. When a carpenter places the level on a surface that is perfectly horizontal, the air bubble stays in the middle of the horizontal tube. The bubble moves to the high end of the tube if the surface is slanted.

Figure 1  Carpenters use a level to find out if a surface is perfectly horizontal. In the level shown, three clear plastic tubes are set into an aluminum frame. Each tube contains a liquid and a gas.

The metal, alcohol, and air in a carpenter's level represent three states of matter. At room temperature, most metals are solids, alcohol is a liquid, and air is a gas. In this chapter, you will learn why the appearance and behavior of solids, liquids, and gases are different.