How come when cooking pasta – a slow boil keeps the water level down, but once you turn up the heat for?
a faster boil why does it raise up and spill over?
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You can bring it CLOSE to the boil or JUST to the boil, then turn down the heat so it doesn’t boil over.
Boiling at high heat produces too much steam in a short time, which has trouble escaping, because the content of the pasta in the water forms a film around the steam bubbles. Thus, it all rises as bubbles which cannot burst, and boils over. Reducing the heat reduces the volume of steam forming bubbles, and allows them more time to burst early.
Reducing the heat does not increase the cooking time, as water, once boiled, does not increase in temperature. Reducing heat also conserves more water, so the pot does not boil dry.
If you think pasta is bad for boiling over, try milk sometime!
Boiling water is a phase change from liquid to vapor. By doing so, you expand it to nearly a thousandth the density, and the flow rate of the steam is buoyantly getting unboiled water to push its way out.
By changing the heat, you aren’t changing the temperature. You are changing the mass phase change flow rate. You are changing the kilograms per second of steam trying to exit the pot. More heat means more flow rate.