Remember when your carrots were done before the rest of your meal, so you turned off the heat, only to discover that in ten minutes you could not remove the lid from the pot? Charles's Law and Boyle's Law, taken together, explain that kitchen phenomenon. In simplified terms, the combined law would read: At a constant volume, the pressure of a gas is proportional to its temperature.
Of course, if you cover a partially filled pot of liquid and bring it to the boiling point, the air space inside the pot experiences little increase in pressure because the built-up pressure created by the heat lifts the lid, allowing most of the expanding gas to escape. However, once you turn off the heat source, the pressure inside the pot gradually decreases - along with the temperature. If the lid and pot are precision-match (as a quality brand should be), the outside air has less than an open invitation to squirm inside. Water molecules, forming an airtight seal, assist in keeping outside air molecules from entering. Within five minutes or so, the difference between the two pressures becomes so pronounced that it creates a "suction", making the separation of the lid and pot a Herculean task.
Some novice cooks try to pry open the lid with a tool, a tactic that usually damages the pot, the lid, and the ego. Other neophytes - under the impression that if the food inside cools, the lid will promptly loosen - place the pot in cold water. This last method runs contrary to the laws of Boyle and Charles and therefore is exactly the opposite of what the cook should do.
When you encounter an unyielding lid, place the pot over moderate heat for a short time. The temperature of the air inside the pot will soon rise and, in accordance with the laws of physics, so will its pressure. When the pressure nearly equals that of the outside, the lid and pot will easily part.
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