A liquid boils when its vapour pressure becomes equal to
Correct answer: C. the external atmospheric pressure
- A. zero
- B. its surface tension
- C. the external atmospheric pressure
- D. the vapour pressure of water
Explanation
Bubbles of vapour can only form and rise through the bulk of the liquid when the vapour pressure inside them matches the pressure pushing down from outside. Because atmospheric pressure falls with altitude, water boils below 100 degrees Celsius on a mountain, which is why food takes longer to cook there. A pressure cooker uses the same relationship in reverse.
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About Evaporation, Boiling Point and Vapour Pressure
Evaporation is the escape of higher-energy molecules from a liquid surface, while vapour pressure is the pressure exerted by vapour in equilibrium with that liquid. The topic distinguishes evaporation from boiling and relates both to temperature, intermolecular forces, surface area, atmospheric pressure and the boiling point.
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