Of state. Contents and Concepts 1. Comparison of Gases, Liquids, and Solids Changes of State

FIGURE 11.1. Dry ice

Dry ice is solid carbon dioxide, CO₂, whose temperature is about -78^oC. It passes directly from the solid to the gaseous state. The white plumes are composed of ice crystals (H₂O, not CO₂) formed by condensation from the cold air.

11.1 Comparison of Gases, Liquids, and Solids

The different states of matter were defined in Section 1.4, and the gas laws and the

kinetic theory of gases were defined in Chapter 5. Here we want to recall the salientfeatures of those discussions and then compare gases, liquids, and solids. We especiallywant to compare how these states are viewed in terms of kinetic-molecular theory.Gases are compressible fluids. According to kinetic-molecular theory, gases arecomposed of molecules or single atoms that are in constant random motion throughout mostly empty space (unless the gas is highly compressed). A gas is easily compressed because the molecules can be pushed into a smaller space. A gas is fluid because individual molecules can move easily relative to one another.

Liquids are relatively incompressible fluids. According to kinetic-molecular theory, the molecules of a liquid are in constant random motion (as in a gas) but are more tightly packed, so there is much less free space. Because the molecules can move relative to one another as in a gas, a liquid can flow (it is fluid). But the lack of empty space explains why a liquid, unlike a gas, is nearly incompressible. Solids are nearly incompressible and are rigid, not fluid. Kinetic-molecular theory explains this by saying that the particles making up a solid (which may be