|When water boils the "air bubbles" rising from the bottom of the pan seem to be created spontaneously out of nothing. I told my son that they are not air bubbles but rather water vapor. Is that correct? — JG, Austin, TX
Yes, these bubbles contain water vapor, not air. The reason that you don't see them until the water reaches its boiling temperature is that a bubble containing only water vapor isn't stable at lower temperatures—the surrounding air pressure will crush it. But once the water temperature is high enough, the water vapor bubbles are stable and they grow while rising to the top of the water.