Gas

A gas is one of the states of matter, consisting of a collection of particles without a definite shape or volume with random motion.

Gases have freely moving particles. A gas is unlike a solid or a liquid because a solid has tightly packed particles that are vibrating while a liquid has particles that have more free space to move. Density is a really important property of gas. Because the molecules are free to move about in a gas, the mass of the gas is normally characterized by its density. Density is the mass per volume of a substance or simply, the opposite of specific volume. For gases, the density can vary because the molecules are free to move. Density is a variable of a gas, and the change in density during any process is governed by the laws of thermodynamics.

We are learning about matter and what gas is because it is part of life. Matter consists of solids, liquids ,gases, and plasmas. The air in a hot air balloon is a gas.  Nitrogen and hydrogen are examples of gases.  In science,  matter is defined as the substance that makes up physical objects, not counting energy or force-fields.  They are not usually considered to be matter though they may contribute to mass. Matter makes up most of what you can see and what you can't see too.  You can't usually see gases, but they are still matter. Scientists study matter to see how it works, and how it can be used for machines, technology, health, the environment.