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Hello Diana,
This is likely too late for this lesson, but here are some ideas when you might teach this in the future:
- perfumes: Open a bottle of liquid cologne and obviously the students will catch the scent. They can even watch it evapourate if you spill it onto a dark surface or plate - showing the change in state from liquid to gas.
- solid room air fresheners: Here I'm talking about the waxy material inside a container that you open - not the car deodorizers which are blotter paper dipped in a liquid. This is a scented material in solid form and, over time, you will see the wax disappear. This is sublimation - the change in state from solid directly to gas. By the way, don't use urinal cakes or mothballs - they contain paradichlorobenzene and aren't the safest materials to have around children.
- 'ghost in a bottle' demo: (You can have some fun with theatrics on catching ghosts) get a large pop bottle and put it in the fridge before class. Bring it, place a coin over the opening and ask the students to be quiet and listen. In a very short time you will hear the coin rattling as the gas inside the bottle heats and expands. An easier way to demonstrate that gases contract when cooled and expand when warmed is to put balloons in and out of a freezer. Introduce the idea of a control run by just putting empty balloons in and out of freezer and observe the behaviour of the latex.
- crushing can demo: This is the opposite of the previous demo - in this one we use condensing gas to create a partial vacuum. You'll need some empty pop cans, water tongs, hot plate (or open burner) and a pan with ice water. Place the hot plate and ice bath close together. Put a few mL of water in the pop can and heat on the hot plate until you see steam coming out the opening. Grasp the can using the tongs and immediately turn it upside down and dip the end with the opening into the ice-cold water. The trick is to cover the opening with water so no air can enter the can. The can will almost immediately and rather violently be crushed due to the condensation of steam back to water inside. Atmospheric pressure outside is now greater than the steam pressure inside the can, causing the can to crush. (Definitely PRACTICE this before you ever try it before a class and keep the students back a little.)
- breaking a stick/ruler using air. Get some plain, wooden rulers or long, thin pine strips, about the thickness of a ruler. Lay the stick on a flat desk hanging over the side. Lay a full sheet of newspaper flat across the stick and flatten the paper so it sits smoothly on the desk. Quickly and forcefully hit the stick hanging over the edge and you should be able to break it. The large surface area of the newspaper adds more of the atmosphere above the stick, holding the stick in place while you strike it. To prove that the newspaper was not the reason - simply crumple the paper and place it on the stick. This time when you try to break the stick the paper should fly up.
Hope this helps!
Gabe Kraljevic
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