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

11.6 Classification of Solids by Type of Attraction of Units

11.7 Crystalline Solids; Crystal Lattices and Unit Cells

11.8 Structures of Some Crystalline Solids

11.9 Calculations Involving Unit-Cell Dimensions

11.10 Determining Crystal Structure by X-Ray Diffraction

We look at the kinds of forces that hold the units of a solid together. Then, we look at crystalline solids and their structure.

Solid carbon dioxide is shown in Figure 11.1. It has an interesting property: at normal pressures it passes directly to the gaseous state without first melting to the liquid. This property, together with the fact that this change occurs at -78^oC, makes solid carbon dioxide useful for keeping materials very cold. Because solid CO₂ cools other objects and does not leave a liquid residue, it is called dry ice.

Liquid carbon dioxide is obtained by putting carbon dioxide gas under pressure; it can exist only at pressures greater than 5 atm. In most cases, carbon dioxide is transported as a liquid in high-pressure steel tanks or at low temperature (about -18^oC) and moderate pressure. When liquid carbon dioxide evaporates (changes to vapor), it absorbs large quantities of heat, cooling as low as -57^oC. Because of this property, it is used as a refrigerant. Liquid carbon dioxide is often used to freeze foods for grocery stores and fast-food restaurants. If the compressed gas from the evaporating liquid is allowed to expand through a valve, the rapidly cooled vapor forms solid carbon dioxide “snow.” (Most gases cool when they expand in this manner; for example, air cools when it escapes from the valve of an inflated tire.) This carbon dioxide snow is compacted into blocks and is the source of dry ice.

Carbon dioxide, like water and most other pure substances, exists in solid, liquid, and gaseous states and can undergo changes from one state to another. Substances change state under various temperature and pressure conditions. Can we obtain some understanding of these conditions for a change of state? Why, for example, is carbon dioxide normally a gas, whereas water is normally a liquid? What are the conditions under which carbon dioxide gas changes to a liquid or to a solid?

These are some of the questions we will examine in this chapter. They concern certain physical properties that, as you will see, can often be related to the bonding and structure of the liquid and solid states.

■ See pages 465–466 for the Media Summary.