– Why does putting the top on a pot of boiling pasta make it frothy and boil over?
In: Chemistry
The froth you get from boiling pasta is caused by the starch in the pasta, and boiling over can happen even without the lid if the heat is on too high and the water is boiling too vigorously.
Here’s what I think happens, based on the my experience that blowing air on the froth quickly stops it: The froth bubbles are sensitive to dry and/or cool air. When the lid is off, there is a constant inrush of fresh air from the outside as the steam escapes upward, and this helps to pop the bubbles and slow the rate of froth formation. When the lid is on, air can no longer flow in, and the rate goes up and eventually overtakes the rim of the pot.
When it happens without the lid on, I think that the rate that fresh air can pop the bubbles is overwhelmed by the rate of bubble formation. That, and possibly the froth layering on top of itself protects the froth beneath from air.
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