What you have to remember about kids is that there are many different kinds of learners. Some need to touch and feel, some need to see things written down, some need to hear and some need all three. When you design an experiment, try to include all three areas so each and every student is involved, feels there is something for them in the exercise and can understand what is going on.
Air is amazing stuff, but we cannot see it so how do you explain to a bunch of high school students about its power, its contents, its importance and how it affects everything? How do you explain that although we cannot see it, air is a mix of different gases, may contain particles so tiny you need and electron microscope to see them and that you cannot tell whether air is good or bad simply by smelling it because even toxic gases can be disguised?
Well, explain the above for a start. Make them realise air is not just stuff we breathe but that different places have different air. Caves, roofs, jungles, beaches, all these places will have different things in the air due to the location and outside effects.
Show them how plants respire, just like us. Put a diagram on the board showing the mitochondria and respiration equation in plants. Then, explain also that plants make their own oxygen as a by-product of photosynthesis. Take some plants and pop plastic bags over them. Seal them. Ask the kids what should happen because plants need to breathe? They will say, they will die due to lack of oxygen. Show them the plants a week later, still living and then tell them that Dr Davis Ward discovered this remarkable ability of plants ot survive using air they circulate and oxygenate and this led to the creation of the Wardian case, used to bring plant material on long sea journeys from all over the world. making it possible to get live specimens here in one piece instead of pressed or dried as they were before the 19th century. This allows the kids to relate air to real life and how understanding what is in the air (in this case oxygen) helps them understand processes in plants.
Also, get hold of a pop air gun – one of those amazing toys which shoots a blast of air. Get a student to stand at the 10 m line and ‘shoot’ them with the air. They will feel the blast as it hits them – they are popular toys in the UK! That is air under pressure and shows the students just how powerful air can be.
Put a piece of bread into a container and seal it. Ask the students what might happen. Tell them no air can get in or out. Most will say nothing will happen. A week later show them the bread. It will be covered in mould, proving that in the air in the container there were spores of mould which have now grown on the bread.
Air is used to move particles too. Break open an onion about 5 feet from the group. Ask them if they can smell it. Why is this? It must be because the air carries particles from the onion to their noses.
Teaching about air can lead to other learning such as how noses detect particles and the brain responds, how plants photosynthesise to use air and how air movement, even not apparent such as in a class room, is going on all the time. Of course, the power of air can also be discussed with the air pop experiment. However you teach about air, make it interesting, visual and be sure that you can explain it all- the students will ask lots of questions as they consider the wonder that is air!