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Why does water freeze at 32 degrees Fahrenheit? 

Asked by: Paul Ajak
School: St. John the Evangelist, BCCS
Grade: 5
Teacher: Anu Rai
Hobbies/Interests: Sports, reading
Career Interest: Pharmacist, basketball player


Answer from Matthew Lundquist

PhD Candidate in Biological Sciences

Research area: Entomology, aquatic ecology, stream ecosystems, urbanization, biodiversity Interests/hobbies: Hiking, fishing, running

The freezing temperature of water is 32 oF because of the unique characteristics of the water molecule, H2O. Molecules are always moving. As temperature goes up, they move faster; as temperature falls, they move more slowly.

Liquids boil when their molecules heat up to the point that they start moving so quickly that they can vaporize and turn into a gas. For water, this happens at 212 oF. Freezing happens when the molecules of a liquid get so cold that they slow down enough to hook onto each other, forming a solid crystal. For pure water, this happens at 32 oF, and unlike most other solids, ice expands and is actually less dense than water. That is why ice cubes float!

The fact that ice floats on water is important for the survival of many freshwater organisms that live in places with cold winters. Have you ever gone ice fishing? Ice on the top of lakes and ponds in the winter actually insulates the water underneath, keeping it just above freezing so fish can survive even the harshest winter chills without having to worry about freezing solid!

The freezing temperature of water can be changed by adding other compounds and impurities to the water. Ever wonder why plows add salt to roads and sidewalks in the winter? Deicing salt

actually lowers the freezing temperature of water, down to -6 oF. This stops ice from forming on the road, even on really cold days. This keeps cars and pedestrians from slipping and sliding in the winter. 

The freezing point of saltwater in the ocean is also lower than freshwater, about 28.4 oF. Cold winters in the arctic are extremely important because polar bears and other arctic wildlife hunt for their food by walking out onto the floating sea ice. Warmer arctic winters in recent years have reduced the amount of sea ice and has made life much more difficult for these animals.

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Last Updated: 6/10/16