What causes water to boil over


What are the mechanics of water boiling over? How can you stop it from happening? Is it more likely to happen with certain ingredients? Which? How does the amount of water effect the likelihood of water boiling over? What role does burner temperature have? Will it only occur during a rolling boil?

This is brought up by this comment by MeltedPez in one of the cooking pasta questions:

The only other argument for using more
water is that less water is more
likely to cause the pot to boil over.
Basically the starch in the pasta
makes it easier for the water to form
bubbles that collect and spill over
when not paying attention. I've found
that with a very low pasta to water
ratio can end in a messy kitchen.

This seems very counter intuitive to me initially, as I feel less water should make it harder to boil over the edges.

Best Answer

Adding a starchy substance such as rice or noodles to boiling water increases the surface tension of the water. When it's just plain water boiling the surface tension of the water can't hold back the force of the steam rising and the bubbles burst. Starches increase this surface tension making the bubbles more elastic/pliable (essentially creating a foam), thus requiring more force for them to burst. This makes the bubbles last longer which allows them to build up and eventually boil over.

It's more likely to occur with anything starchy like rice and pasta.

A lower temperature should help to control a boil over as it will allow the water to cool somewhat and slow the boil.

I'd assume that, from your quoted text, that if you use less water you're still imparting the same amount of starch making a thicker solution. More water would dilute this and bring it closer to normal water. In my own experience, small amounts of water allow lots of bubbles to form but nothing really boils over, it just splatters a bit. But reducing temperature and tilting the lid to let some air in has helped me in the past.