Gases

The States of Matter

The term state can be defined as a set of conditions that describe a person or thing at a given time. It is in this sense of the word that scientists divide matter into the three states shown in the figure below.

The three states of matter have characteristic properties. Solids have a distinct shape. When they melt, the resulting liquid conforms to the shape of its container. Gases expand to fill their containers.

There are two reasons for studying gases before liquids or solids. First, the behavior of gases is easier to describe because most of the properties of gases do not depend on the identity of the gas. We can therefore develop a model for a gas without worrying about whether the gas is O₂, N₂, H₂, or a mixture of these gases. Second, a relatively simple, yet powerful, model known as the kinetic molecular theory is available, which explains most of the behavior of gases.

Elements or compounds that are Gases at Room Temperature

Before examining the chemical and physical properties of gases, it might be useful to ask: What kinds of elements or compounds are gases at room temperature? To help answer this question, a list of some common compounds that are gases at room temperature is given in the table below.

Common Gases at Room Temperature

There are several patterns in the table above.

• Common gases at room temperature include both elements (such as H₂ and O₂) and compounds (such as CO₂ and NH₃).
• Elements that are gases at room temperature are all nonmetals (such as He, Ar, N₂, O₂, and so on).
• Compounds that are gases at room temperature are all covalent compounds (such as CO₂, SO₂, and NH₃) that contain two or more nonmetals.
• With only rare exception, these gases have relatively small molecular weights.

As a general rule, compounds that consist of relatively light, covalent molecules are most likely to be gases at room temperature.