Why does pasta water more readily boil over (and out of the pot) when it’s covered vs when it’s uncovered?
In: Chemistry
Heat is trapped, water is trapped. Steam is hotter than boiling water and convection currents also become more rapid.
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Bubbles disappear because they dry out, losing their structural integrity until they pop.
When your pot is covered humidity doesn’t escape as much, and in a humid environment bubbles don’t dry as much. With actual condensed water in the air (that otherwise would just rise away from the bubbles) you have water coming in the bubbles, so they don’t lose water as fast.
More bubbles appearing while the existing ones don’t disappear as fast is how your pot boils over.
Covering the pot traps heat inside the pot, mostly because you trap the steam so that it recondenses instead of escaping and the cooler air in the kitchen isn’t sucked into the pot by the lower pressure caused by hot air rising out of an uncovered pot.
Pasta water itself boils over because as the pasta is cooked, it releases starches into the water (which is why the water looks cloudy afterwards). The starches make the gas bubbles stronger and more stable, which allows them to build up rather than popping quickly. Covering the pot also means the water film of the bubbles lasts longer because vapor can recondense on it (the net loss from evaporation is slower) and so the bubbles linger.
It makes no difference whether it’s pasta water or any other kind of water. The lid keeps the heat trapped inside the pot.
The bubbles in boiling water consist of steam (that is, vaporized water), not air. When the bubbles come to the surface, they contact the air, and since the air is cooler than the water, the bubbles cool off, and the steam turns back into water, and the bubbles collapse. If the lid is in place, the air in the space above the water gets hotter until it is the same temperature as the water. Then it doesn’t make the bubbles collapse anymore.
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