– Why does putting the top on a pot of boiling pasta make it frothy and boil over?
In: Chemistry
The starch released into the cooking water stabilizes the froth that you get if you boil it too vigorously. Putting the lid on keeps more heat in so effectively it’s like turning up the electricity or gas too high.
The lid traps heat and steam ~~, building a little pressure, which can lower the boiling point~~ . It may not be a lot, but can be enough.
This then causes the pot to boil more, as it would if just cooked over a higher temperature.
The frothy bubbles come from the starches in the water, giving a little extra strength to the surface tension. This doesn’t have anything to do with the lid, though.
Edit: pressure increases increase boiling point, but pressure on stove top pot not likely to amount to much
This doesn’t answer your question but put some butter or oil in the water to prevent this from happening…
Changing boiling-temp water into steam takes a ton of energy, at least as much as heating liquid water up to boiling temp. The phase change itself needs huge energy input.
If the steam is free to escape, it carries all that energy away with it.
If the steam is trapped by a lid, then when it condenses back into liquid it dumps all that energy right back into the pot. It’s like doubling your element’s heat setting!
Possibly the equilibrium is reached in the small section between the liquid level and the lid. This could either allow gas bubbles to re-entrain in the liquid and form an emulsion like layer of bubbles (once you start making bubbles there will be a chain reaction.)
Alternate the gas bubbles rising the the top cannot escape the liquid because equilibrium has been reached (gas/liquid phases) and no More gas can enter the gas phase and so are forced to develop a bubble under the liquid surface
Once the lid is removed the whole room above is unsaturated gas (air) and can absorb all the highly saturated gas vapour (eliminating the bubbles)
With the lid on the vapor pressure of water in the limited atmosphere reaches its saturation point. Evaporation is now in approximate equilibrium with condensation. Without efficient evaporation there is no easy way for the system to shed heat from the burner. Water skyrockets toward maximum heat capacitance.
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.
It’s partially because the inside of the pot fills with steam instead of air, and the bubbles are a lot harder to pop by evaporation when they’re exposed to steam compared to when they are exposed to air. Starchy pasta water forms bubbles fairly easily.
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