Gas

I

Introduction

Gas, one of the three ordinary states of matter. The other two ordinary states of matter are solid and liquid. Both solids and liquids are made up of particles that touch one another. The attraction between the particles of a solid is so strong that the particles hold rigidly together. This rigidity gives solids a definite shape and volume. The attraction between particles in a liquid is great enough to hold the particles near each other but too weak to prevent the particles from sliding around. Liquids have a definite volume but take the shape of their container. The particles that make up a gas, however, are completely separated from one another. Empty space accounts for more than 99 percent of the total volume of air, for example. Because gas particles are separated, the attractive forces between them are extremely small and are insufficient to hold gases in a definite shape or volume. Gases expand freely to fill their containers.

II

Properties of Gases

The characteristics or properties of gases vary widely. Some gases are transparent, some have a strong smell, some dissolve in water, and some react violently with almost any substance. Other gases exhibit exactly the opposite properties. The chemical structure of gases also varies greatly.

A

Color

A number of gases have a characteristic color. For example, fluorine gas appears green, chlorine appears yellow-green, and nitrogen dioxide (a component of smog) appears red-brown. The majority of gases, however, are colorless.

B

Odor

Many gases, including nitrogen, oxygen, and hydrogen, are odorless. Ammonia, however, has a sharp, pungent odor. Because fuel gases such as methane, propane, and butane are odorless, an intensely odorous sulfur compound is added to them to ensure early detection should these gases leak from their containers.

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