A liquid boils when its vapour pressure becomes equal to
- 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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