By Harold McGee
It isn’t always easy for the cook to recognize and maintain a particular cooking temperature, and reproduce the same temperature reliably. Thermostats, thermometers, and our senses are all fallible. So one of the great advantages of water as a cooking medium is that its boiling point is constant—212°F/100°C at sea level—and it’s instantly recognizable. The sure sign of boiling water is bubbling. Why? When the water in a pan is heated near boiling, molecules at the bottom, where the pan is hottest, vaporize and become steam, and form regions that are less dense than the surrounding liquid. (The small bubbles that form very early on are pockets of air that had been dissolved in the cold water but became less soluble as the temperature rose.) Because all the pan heat at the boil goes into vaporizing the liquid water, the temperature of the water itself stays the same. It’s only slightly higher at a full, rolling boil than in a gently bubbling pot, and will not get any higher until the phase change from liquid to gas has been completed.