to show that water vapor is present in the air experiment

Without seeing the experiment, it is hard to tell, but you add salt and crushed ice when you want to make a really cold slurry. I imagine that would cause any water vapor in the air to condense on the outside of the container.

Well, some people use gin and vermouth...with three olives. But that often gets mixed up with a martini...so they go with salt and crushed ice.