Gas is one of the four fundamental states of matter, along with solid, liquid, and plasma. A gas is a substance that is in a gaseous or vaporous state of matter, and it has no fixed shape or volume. The atoms or molecules that make up the gas fill the container that holds them, and the gas expands until it is uniformly distributed throughout the container, even in the presence of gravity. The particles in a gas are more distant from each other than in liquids, and gases condense into liquids or ionize into plasma.
Gases can be pure substances, such as oxygen or helium, or they can be mixtures, such as air or natural gas. Examples of gases include air, water vapor, and helium. Natural gas, which is a fossil fuel energy source, contains many different compounds, with the largest component being methane.
It is important to note that the term "gas" can also refer to the gas inside the digestive tract, which is made up of air and other gases. Gas typically leaves the digestive tract through the mouth when a person belches or through the anus when they pass gas.
