Why Is Milk White?: & 200 Other Curious Chemistry Questions (2013)
Why does adding oils to cooking water keep it from boiling over?
Plain water by itself does not usually boil over the edge of the pot. The reason for this is that the bubbles pop when they reach the surface.
If you are cooking pasta, the proteins from the pasta change the surface tension of the water. Proteins often have one side that is charged and attracts water and another side that is not, which sticks out into the air. In this way proteins act like the soap in a soap bubble. They stabilize the bubble, so it can live longer before popping.
If the bubbles don’t pop, then new bubbles form underneath them and lift them up. Eventually they will spill over the edge of the pot and make a mess.
Adding a little oil changes this. The ends of the proteins that avoid water end up sticking into the oil instead of into the air. This makes the proteins unavailable to stabilize the bubbles. They are all locked up in the oil, making tiny oil droplets out of bigger ones, the same way soap does when you wash the dishes.
Adding oil to pasta coats the pasta with oil when you lift it out of the pot. This can interfere with the sauces, so they no longer coat the noodles. You can pour out the surface water first, so the oil goes down the drain, or you can stop putting oil in the pasta and just watch the pot more carefully. If you turn down the heat when the water starts to boil over, you can prevent the mess without chemistry.
As long as the water is boiling at all, it will be at the temperature of boiling water. Boiling the water faster only makes the water evaporate faster. It does not speed up the cooking, since the temperature cannot get higher than the boiling point as long as there is still water in the pot.