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The boiling point of a given liquid varies directly with pressure. This relationship is crucial in understanding how temperature and pressure interact in phase changes of substances. Under increased pressure, the boiling point of the liquid rises. This is because the higher pressure forces the molecules of the liquid to stay closer together, requiring more heat energy (and thus a higher temperature) for the liquid to escape into vapor.

For instance, in a pressure cooker, the pressure is elevated, which allows water to reach higher temperatures before boiling occurs. Conversely, at lower atmospheric pressures, such as at high altitudes, liquids boil at lower temperatures since less heat is required to allow the molecules to escape into the gaseous state.

This relationship is a fundamental concept in thermodynamics and fluid mechanics, particularly in aviation contexts, where variations in altitude can significantly affect boiling points and, consequently, the behavior of liquids in aircraft systems.