All substances, at least in principle, can exist in one of three states: solid, liquid, and gas. A gas is a substance that is normally in the gaseous state at ordinary temperatures and pressures. In a solid, molecules are held close together in an orderly fashion with little freedom of motion. Molecules in a liquid are not held so rigidly, so they can move past one another but not totally break loose. In a gas the molecules are separated by distances. Movement of gas molecules is totally random.

Match the following descriptions of gases to the name of the gas. Click here to show the list of gases in a small window that you can print or view next to the questions.