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Q & A: boiling pasta in Denver and New Orleans

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Most recent answer: 10/22/2007
Q:
Why is more time required to boil pasta in Denver, Colorado, than in New Orleans, Louisiana?
- Cynthia
BPJH, California
A:
Denver, Colorado is called the "mile-high city", and most places in New Orleans are actually below sea level. The air pressure is significantly higher in New Orleans than in Denver.

Water boils at the temperature when its vapor pressure is the same as the atmospheric pressure -- bubbles of vapor are then free to form and grow. Actually, bubbles usually form at the bottom of the pot, because that's where the heat comes in, so the pressure is actually a little higher down there under the water.

The vapor pressure increases with temperature, and so the boiling point increases with the ambient air pressure. If water is sitting in a pot over a flame or heating coils, it will heat up until it boils, and the boiling carries heat away rapidly enough to ensure the temperature does not go beyond the boiling point (it takes 540 calories per gram to evaporate water). Increase the external heat, and you just boil the water faster, without raising its temperature.

So the water in New Orleans will be hotter than the water in Denver, when it is boiling. The rate at which pasta takes up water increases with temperature, so the pasta will be done sooner in New Orleans.

Pasta recipes usually recommend adding salt to the water in Denver to raise its boiling point to compensate for this effect. I'm not such a fan of doing this, as I never really want to calculate how much salt I need to get it right, and besides, I don't always want salty pasta, and I usually check it for proper al dente doneness anyhow. I'm patient and am willing to wait for my pasta to cook.

Tom

(published on 10/22/2007)

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