The Watched Pot

You have probably heard the expression “A watched pot never boils.” Is this statement correct physics? That is, when would this statement be good physics? (Hint: One should interpret the phrase “never boils” here to mean that the cooking takes a longer time).

Answer

Put a pot of water on a flame atop a stove. The thermal energy from the flame raises the water temperature. If the pot has no cover, soon the water vapor pressure above the water surface equals the pressure of the ambient atmosphere. The water is now boiling. If we are high in the mountains, the boiling has occurred at a lower temperature than when we are near sea level. So at higher elevations potatoes may not cook as quickly in the open pot, and the lukewarm water will not make good tea or instant coffee. In the mountains we would be wise to use a pot with a lid so that the total pressure acting downward on the water surface can be higher vapor pressure plus atmosphere and so that the water boils at a higher temperature than without the lid, hopefully almost at 100°C.

Suppose the cooking at sea level is done in a pot with a lid. Now the action of lifting the lid to “watch the pot” reduces the thermal energy in the air above the liquid surface as some molecules escape. These escaping water molecules are among the most energetic in the vapor, so they can carry away much thermal energy. The pressure above the liquid is now lower than before, the boiling occurs at a lower temperature, and the cooking takes significantly longer. One should replace the lid and let the food cook undisturbed! Hence the expression “A watched pot never boils.” This statement actually refers to the extended cooking time and involves some good physics.

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