9: Gases

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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. 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.

9.1: Straws!
Straws work because sucking creates a pressure difference between the inside of the straw and the outside. If you formed a perfect vacuum within the straw, the pressure outside of the straw at sea level would be enough to push water to a total height of about 10.3 m.
9.2: Kinetic Molecular Theory
The physical behavior of gases is explained by the kinetic theory of gases. An ideal gas adheres exactly to the kinetic theory of gases.
9.3: Gas Pressure
Pressure is a force exerted over an area. Pressure has several common units that can be converted.
9.4: Pressure and Volume
Boyle’s Law relates the pressure and volume of a gas at constant temperature and amount.
9.5: Volume and Temperature
Charles’s Law relates the volume and temperature of a gas at constant pressure and amount. In gas laws, temperatures must always be expressed in kelvins.
9.6: Pressure and Temperature
Gay-Lussac's Law relates the pressure and temperature of a gas at constant volume and amount. Gay-Lussac's Law is very similar to Charles's Law, with the only difference being the type of container. Whereas the container in a Charles's Law experiment is flexible, it is rigid in a Gay-Lussac's Law experiment.
9.7: The Combined Gas Law
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.
9.8: For Future Use
9.9: Exercises

Thumbnail Chapter 9: Movement of gas molecules. (Julio Miguel A Enriquez and Monica Muñoz via Wikimedia Commons)