Matter is anything that takes up space and has mass. Gases are one state of matter.
Physical Properties of Gases
Gases are neither close together nor fixed in place. This means that the shape and volume of a gas depends on the shape and volume of its container. 
Air is the mixture of gases in earth’s atmosphere. By volume, air contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.039% carbon dioxide, and small amounts of other gases. Air also contains water vapor. The average amount of water vapor at sea level is about 1%. Considering the entire atmosphere surrounding earth, there is about 0.4% water vapor.
Water Vapor is water in its gas state at a temperature below its boiling point.
Humidity is the amount of water vapor in the air for a specific region.
Activity: Gas takes up space.
Purpose: To demonstrate that air takes up space, thus it is an example of matter.
Use an empty plastic see-through bag. With the end open, swish the bag through the air, then squeeze the opening closed. The bag is filled with air. Air in the room is not visible nor is the air in the bag visible. The air molecules in the bag are pressing against the inside of the bag, thus there is a shape. You might change the shape some if you press against the bag, but basically the volume and shape of the gas inside the bag remains constant.
Gases are a state of matter because they have mass and take up space.
Science Challenge
Stuff a balloon inside an “empty” plastic bottle. Strectch the mouth of the balloon over the mouth of the bottle.
Make a Prediction: Can the balloon be inflated inside the bottle? Explain your answer.
Investigate: Hold the bottle and blow into the mouth of the balloon. Note: The balloon’s mouth must remain over the top of the bottle so that the boottle’s mouth is sealed.
What happened? Can you explain the results with the knowledge you have about gases?