When water boils, what are the bubbles made of?
At 80 oC, small bubbles can be observed in water. These bubbles are air that was dissolved in the water at room temperature coming out of solution. Air becomes less soluble in water as the temperature goes up. If you continue to heat the water, bubbles form then collapse. The water on the bottom is heated to the point where it converts to vapour but the vapour cools and expands as it encounters the slightly cooler water just above it. When the water is uniformly hot the bubbles rise from the bottom and break free at the top. This is often called a rolling boil and is the point of true boiling. The definition of boiling is the existence of sustained bubbles that break free of the surface. For a bubble to exist the pressure of the vapour in the bubble must be pushing against the water with exactly the same force that the water is pushing back. With a few cm of water, almost all of the pressure being exerted by the water is air pressure. For the bubble to exist the vapour pressure must be
