# Gases

## Overview

### Description

A gas is a form of matter without definite shape or volume. In an ideal gas, there are no intermolecular forces between the gas atoms or molecules, and the molecules do not take up space. The ideal gas law explains and predicts the behavior of a hypothetical ideal gas. Laws derived from the ideal gas law allow for calculations regarding the behavior of gases. These laws are based on kinetic molecular theory, which is a model that describes the macroscopic behavior of gases based on their microscopic components. This theory also explains how gases effuse through small holes in solids and diffuse from areas of high concentration to areas of low concentration. Dalton's law of partial pressures states that the pressures of each individual gas in a mixture add up to the total pressure of the mixture. This can be used to calculate stoichiometric reactions involving gases. Although the ideal gas law is commonly used for gases at low pressures, at higher pressures, real gases do not behave like ideal gases. At these pressures, the van der Waals equation can be used to correct for these changes to the ideal gas law.

### At A Glance

• Gases lack definite shape or volume. Gas pressure is measured in a number of scientific units, including atmospheres (atm), bars, and kilopascal (kPa).
• The ideal gas law, given by the equation $PV=nRT$, describes the behavior of ideal gases, in which no intermolecular forces act on the gas particles, and the particles do not take up space.
• Kinetic molecular theory, a model that describes the macroscopic behavior of gases based on their microscopic components, helps explain effusion of gases, in which gases move through small openings in solids, and diffusion of gases, in which gas particles move from an area of higher concentration to an area of lower concentration.
• Stoichiometry of gases relies on the partial pressure of gases—the pressure of one gas component in a mixture of gases.
• The ideal gas law is useful at very low pressures, but at higher pressures, real gases behave differently from ideal gases.