Question

A mixture of gases in a container acts as if it is a single gas unless there is a chemical reaction going on. What does this say about a sample of air? Do we have to measure the properties for the nitrogen portion separately from the oxygen, carbon dioxide, and water vapor portions?

Answer 1

I hope, your question is "you have a mixture of gasses in a container and behaving like a single gass until they react. So to say it is air what else you have to measure? Do you need to determine compositio of the gas?"

So, my answer is composition you have to measure definitely. Let's say, if there N₂ is 50%, you can't say that is air. But how can you decide composition of air? Air contains lots of gasses. So you just check the compostion range of the components present in the air significantly like N₂, O_2, CO₂, CO, water vapor etc. Remember compotion of air is not fixed, it vary from place to place. Like sea beach will have more vapor quantity when that at desert area is less. Industry area will have SO₂ like gases more. So once you see composition of the gas misture is almost same to typical air then can measure density of the gas mixture and compare with air density of your locality. You can study difussion also and can check if there is any exchange between the gas misture and air. If there is no exchange, then you can say, your gas misture is air.