Ask A Scientist

When my mom makes pasta, she puts salt in the boiling water. After a few minutes, the water bubbles. Why is this?

Asked by: Finbarr Huff
School: Seton Catholic at All Saints School
Grade: 6
Teacher: Mr. Martinkovic
Hobbies/Interests: Sports, reading and music
Career Interest: Sports player or work at my dad’s office

Answer from Douglas W. Green, EdD

Adjunct Lecturer

Former principal at Woodrow Wilson Elementary in Binghamton, NY
Research area: Leadership, Learning Theory & Social Media
Interests/hobbies: Playing banjo, biking, golfing and reading
Family: Daughter Lena, age 27, who is an animator for Nickelodeon in New York City
Web Page Address
Check out Doug's blog, a site for educators who don't have as much time to read as he does http://www.drdouggreen.com.  

All food contains some salt. If you heat food containing salt in water without salt, some of the salt in the food will migrate out of the food and into the water. If too much salt leaves the food it can taste bland. When you add salt to water, the temperature at which the water boils will go up so the food will cook a bit faster. Mom may not realize this, but this explains why many people add salt to pasta water.

As for boiling, it is important to understand that water molecules move faster as you heat them. This is true for all materials. If water molecules move fast enough, they can go from being a liquid, where the molecules are close together, to being a gas where the molecules are far apart. In the case of water, this happens when the temperature reaches about 212 degrees Fahrenheit or 100 degrees Celsius. Temperature is just a measure of the average speed to the molecules in a sample.

Water molecules are a bit like little magnets, which is why they are attracted to each other. If they get cold enough, they lock in to a crystal formation as solid ice. If you heat ice, the molecules vibrate until the are no longer “frozen” in place and become liquid water. When the molecules move fast enough, they can leave the surface and become water vapor, which is a gas like the air you breath. The air around you always contains some water vapor in addition to oxygen, nitrogen, small amounts of other chemicals, and dust. This is a good thing as water in the air is necessary for good health.

Below the water’s surface in your pasta pan, the rough edges of a pasta pan that are too small to see help gas bubbles form. Since these bubbles are much less dense than the surrounding water, they rise to the surface and become part of the air in the room. As a pan of water boils, water molecules gradually leave. If you don’t pay attention to your boiling pasta, all of the water can boil into the room. This is dangerous and smelly as once the water is all gone, the contents will get much hotter and may catch fire. Water is so important to life on Earth that understanding it is one of the most important things that scientists and citizens can do.

Dr. Doug Green blogs at DrDougGreen.Com for educators and parents who don’t have as much time to read and surf as he does.  

Last Updated: 9/18/13