If atmospheric pressure decreases, what happens to the boiling point?

• A. The boiling point will go down
• B. The boiling point will go up
• C. The boiling point will stay the same
• D. The boiling point could change unpredictably

Answer: (A)

Boiling point is the temperature at which a liquid’s vapor pressure equals the pressure surrounding it. Atmospheric pressure is that surrounding pressure, so when the outside pressure drops, the liquid only needs a lower temperature for its vapor pressure to match it. That means the boiling point decreases. This is why water boils at lower temperatures at high altitudes, and in a vacuum it can boil at room temperature. So, a decrease in atmospheric pressure leads to a lower boiling point. The other options don’t fit because the boiling point does not rise with lower pressure, it does not stay the same, and it changes in a predictable, not unpredictable, way.