The Kinetic Molecular Theory of Liquids and Solids

Liquids and solids are quite a different story. The principal difference between the condensed states (liquids and solids) and the gaseous state is the distance between molecules. In a liquid, the molecules are so close together that there is very little empty space. Thus, liquids are much more difficult to compress than gases, and they are also much denser under normal conditions. Molecules in a liquid are held together by one or more types of attractive forces, which will be discussed in Section 11.2. A liquid also has a definite volume, because molecules in a liquid do not break away from the attractive forces. The molecules can, however, move past one another freely, and so a hquid can flow, can be poured, and assumes the shape of its container. 

TABLE 11.1 Characteristic Properties of Gases, Liquids, and Solids  

State of Matter Volume/Shape Density Compressibility Motion of Molecules 

Gas Assumes the volume and Low Very compressible Very free motion 

Liquid Has a definite volume High Only slightly compressihle Slide past one another freely 

---

We begin by applying the kinetic molecular theory to liquids and solids and compare their properties with those of gases. (11.1)... 

The Kinetic Molecular Theory of matter attempts to describe all the states of matter and the conversion between the states by considering the structures of molecules comprising matter and how those molecules interact. There are three commonly encountered states of matter solids, liquids, and gases. There are a few other states of matter, such as plasmas, but these are encountered only under extremely high energy conditions. Therefore, we will restrict our conversation to the more mundane states. 

Several gas laws have been introduced in this chapter, but no explanation as to why those laws apply to all gases has been proposed. This section introduces the kinetic molecular theory of gases, which explains the gas laws and when extended, also explains some properties of liquids and solids. Five postulates explain why gases behave as they do ... 

LIQUIDS OR SOLIDS IN MANY WAYS. MOLECULAR MOTION IN GASES IS TOTALLY RANDOM, AND THE FORCES OF ATTRACTION BETWEEN GAS MOLECULES ARE SO SMALL THAT EACH MOLECULE MOVES EREELY AND ESSENTIALLY INDEPENDENTLY OF OTHER MOLECULES. SUBJECTED TO CHANGES IN TEMPERATURE AND PRESSURE, GASES BEHAVE MUCH MORE PREDICTABLY THAN DO SOLIDS AND LIQUIDS. ThE LAWS THAT GOVERN THIS BEHAVIOR HAVE PLAYED AN IMPORTANT ROLE IN THE DEVELOPMENT OF THE ATOMIC THEORY OF MATTER AND THE KINETIC MOLECULAR THEORY OF GASES. 

The answer is yes and we will digress a bit at this point to introduce these concepts as we did earlier in the chapter. The temperature and pressure conditions that govern physico-chemical behavior of liquids are defined in terms of thermodynamics. The Gibbs Phase Rule is a direct outcome of the physical chemistry of changes in the state of matter. The phase rule helps to interpret the physico-chemical behavior of solids, liquids, and gases within the framework of the kinetic-molecular theory of phase equilibria. 

Gases, the subject of this chapter, are simpler than liquids and solids in many ways. Molecular motion in gases is totally random, and the forces of attraction between gas molecules are so small that each molecule moves freely and essentially independently of other molecules. Subjected to changes in temperature and pressure, it is easier to predict the behavior of gases. The laws that govern this behavior have played an important role in the development of the atomic theory of matter and the kinetic molecular theory of gases. 

Scientists attempt to explain the various properties of gases, liquids, and solids in terms of the properties of the atoms, molecules, or formula units of which they are composed. The theory used to do this is called the kinetic-molecular theory, or in the case of gases, the kinetic-molecular theory of gases. It is like looking through the mythical microscope to see what the atoms or molecules are doing. This theory has the following key points ... 

The behavior of gases is quite different from that of liquids and solids. Gas particles are far apart, whereas particles of both liquids and solids are held close together. A gas has no definite shape or volume and will completely fill any container. Because there are great distances between gas particles, a gas is less dense than a solid or liquid, and easy to compress. A model for the behavior of a gas, called the kinetic molecular theory of gases, helps us understand gas behavior. 

For a quantitative description of the behavior of gases, we will employ some simple gas laws and a more general expression called the ideal gas equation. These laws will be explained by the kinetic-molecular theory of gases. The topics covered in this chapter extend the discussion of reaction stoichiometry from the previous two chapters and lay some groundwork for use in the following chapter on thermochemistry. The relationships between gases and the other states of matter— liquids and solids—are discussed in Chapter 12. 

The kinetic-molecular theory states that gas particles are in constant rapid, random motion. The theory also states that the particles of a gas are very far apart relative to their size. This idea explains the fluidity and compressibility of gases. Gas particles can easily move past one another or move closer together because they are farther apart than liquid or solid particles. 

You will use the kinetic-molecular theory to explain the physical properties of gases, liquids, and solids. 

Apply kinetic-molecular theory to the behavior of liquids and solids. 

Although the kinetic-molecular theory was developed to explain the behavior of gases, the model can be applied to liquids and solids. When applying the kinetic-molecular theory to these states of matter, you must consider the forces of attraction between particles as well as their energy of motion. 

According to the kinetic-molecular theory, a mole of solid particles has as much kinetic energy as a mole of liquid particles at the same temperature. By definition, the particles in a solid must be in constant motion. So why do solids have a definite shape and volume For a substance to be a solid rather than a liquid at a given temperature, there must be strong attractive forces acting between particles in the solid. These forces limit the motion of the particles to vibrations around fixed locations in the solid. Thus, there is more order in a solid than in a liquid. Because of this order, solids are much less fluid than liquids and gases. In fact, solids are not classified as fluids. 

Most substances can exist in three states depending on the temperature and pressure. A few substances, such as water, exist in all three states under ordinary conditions. States of a substance are referred to as phases when they coexist as physically distinct parts of a mixture. Ice water is a heterogeneous mixture with two phases, solid ice and liquid water. When energy is added or removed from a system, one phase can change into another. As you read this section, use what you know about the kinetic-molecular theory to help explain the phase changes summarized in Figure 13-22. 

You can t understand gases without understanding the movement of gas particles. Remember from your study of the kinetic-molecular theory in Chapter 13 that gas particles behave differently than those of liquids and solids. The kinetic theory provides a model that is used to explain the properties of solids, liquids, and gases in terms of particles that are always in motion and the forces that exist between them. The kinetic theory assumes the following concepts about gases are true. 

The properties of gases, liquids, and solids differ in a number of respects. How would you use the kinetic molecular theory (see Section 5.7) to explain the following observations (a) Ease of compressibility decreases from gas to liquid to solid, (b) Solids retain a definite shape, but gases and liquids do not. (c) For most substances, the volume of a given amount of material increases as it changes from solid to liquid to gas. 

As we learned in Chapter 10, the molecules in a gas are widely separated and in a state of constant, chaotic motion. One of the key tenets of kinetic-molecular theory is the assumption that we can neglect the interactions between molecules. (Section 10.7) The properties of liquids and solids are quite different from gases largely because the intermoiecuiar forces in liquids and solids are much stronger. A comparison of the properties of gases, liquids, and solids is given in TABLE 11.1. 

We can investigate the role of temperature in kinetics by referring to the most useful model for describing the motion of molecules—the kinetic-molecular theory. Let s consider the events that lead to a chemical reaction between two gas-phase molecules. (The same ideas apply to reactions involving solids and liquids, too. The gas-phase case is just easier for us to discuss.)... 

Careful scientific studies of the behavior and properties of gases were begun in the seventeenth century by Robert Boyle (1627-1691). Flis work was carried forward by many investigators, and the accumulated data were used in the second half of the nineteenth century to formulate a general theory to explain the behavior and properties of gases. This theory is called the kinetic-molecular theory (KMT). The KMT has since been extended to cover, in part, the behavior of liquids and solids. 

After their excursion through gases and the kinetic-molecular theory (Chapter 12), liquids and solids (Chapter 13), and solutions (Chapter 14), students will have the appropriate background for a wide variety of laboratory experiments. These chapters also provide a strong presentation of the molecular basis of the physical behavior of matter. 

You first considered the particulate character of matter in the kinetic molecular theory in Section 2.2. The particles of a solid were described as being held in fixed position relative to each other. A degree of freedom is reached in the liquid state, in which particles move about among themselves, but still remain together at the bottom of the container that holds them. As a gas, the particles gain complete independence from each other and fly about randomly to fill their containers. 

When you think of Earth s oceans, lakes, and rivers and the many liquids you use every day, it is hard to believe that liquids are the least common state of matter in the universe. Liquids are less common than solids and gases because a substance can exist in the iiquid state only within a relatively narrow range of temperatures and pressures. In this section, you will examine the properties of the liquid state and compare them with those of the solid state and the gas state. These properties will be discussed in terms of the kinetic-molecular theory. 

The common expression solid as a rock suggests something that is hard or unyielding and has a definite shape and volume. In this section you will examine the properties of solids and compare them with those of liquids and gases. The properties of solids are explained in terms of the kinetic-molecular theory, as are the other states of matter. 

---