Of the three basic phases of matter—solids, liquids, and gases—only one of them has predictable physical properties: gases. In fact, the study of the properties of gases was the beginning of the development of modern chemistry from its alchemical roots. The interesting thing about some of these properties is that they are independent of the identity of the gas. That is, it doesn’t matter if the gas is helium gas, oxygen gas, or sulfur vapors; some of their behavior is predictable and, as we will find, very similar. In this chapter, we will review some of the common behaviors of gases. Let us start by reviewing some properties of gases. Gases have no definite shape or volume; they tend to fill whatever container they are in. They can compress and expand, sometimes to a great extent. Gases have extremely low densities, one-thousandth or less the density of a liquid or solid. Combinations of gases tend to mix together spontaneously; that is, they form solutions. Air, for example, is a solution of mostly nitrogen and oxygen. Any understanding of the properties of gases must be able to explain these characteristics. 6.1: Prelude to Gases 6.2: Kinetic Theory of Gases The physical behavior of gases is explained by the kinetic theory of gases. An ideal gas adheres exactly to the kinetic theory of gases. 6.3: Gas Pressure Gases exert pressure, which is force per unit area. The pressure of a gas may be expressed in the SI unit of pascal or kilopascal, as well as in many other units including torr, atmosphere, and bar. Atmospheric pressure is measured using a barometer; other gas pressures can be measured using one of several types of manometers. 6.4: Gas Laws The behavior of gases can be modeled with gas laws. Boyle’s law relates a gas’s pressure and volume at constant temperature and amount. Charles’s law relates a gas’s volume and temperature at constant pressure and amount. In gas laws, temperatures must always be expressed in kelvins. 6.5: Other Gas Laws There are other gas laws that relate any two physical properties of a gas. The combined gas law relates pressure, volume, and temperature of a gas. 6.6: The Ideal Gas Law and Some Applications The ideal gas law relates the four independent physical properties of a gas at any time. The ideal gas law can be used in stoichiometry problems whose chemical reactions involve gases. Standard temperature and pressure (STP) are a useful set of benchmark conditions to compare other properties of gases. At STP, gases have a volume of 22.4 L per mole. The ideal gas law can be used to determine densities of gases. 6.7: Gas Mixtures The pressure of a gas in a gas mixture is termed the partial pressure. Dalton’s law of partial pressure says that the total pressure in a gas mixture is the sum of the individual partial pressures. Collecting gases over water requires that we take the vapor pressure of water into account. Mole fraction is another way to express the amounts of components in a mixture. 6.8: Gases (Exercises)