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Science for Kids/Why does water expand when put in the freezer?

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Expert: Dan Fink - 6/25/2007


Question
I am learning that the particles in a matter expand when heated and contract when cooled. But how come when you put water in the freezer to make ice cubes, the ice expands? Shouldn't the water have contracted because its being cooled?

Answer
Hi Kirra -- that's  a great question!
Water is really strange stuff, because of the very strong hydrogen-oxygen bonds. One odd thing is that water is most dense at 4 deg C -- that's why lakes freeze from the top down; the warmer water sinks.

Every substance expands when you heat it -- because the average space between the atom's nuclei increases when the atoms have more energy (vibrating faster, more heat). So water DOES expand when you heat it, like anything else -- it's only when it freezes and crystallizes that the weirdness (expansion of ice) happens.

The reason water expands when frozen is the crystalline structure that forms, and those strong O-H bonds. The hydrogen atoms have a very strong attraction for the unbonded electrons in the nearby molecules. In an ice crystal, each oxygen atom  has it's own 2 electrons, and grabs hold of 2 more electrons from the water molecule next door. And so they crystallize into a big hexagon shape, which takes up more space than the same molecules do when the water is in liquid form. An ice crystal's form is called a 'network structure.' Same molecules, but takes up more space.


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Dan Fink

Expertise

Though my experience is mostly in the fields of electricity, magnetism, and physics, I have a broad science background. My career is in the field of alternative power sources -- solar, wind, water and battery power. But any questions about electricity, magnetism, energy conservation, power generation, electric motors, and even general physics are very welcome--especially from kids. They ask the best questions of all! I pride myself in answering science questions accurately, with ideas for SAFE, easy experiments that kids can perform by themselves--and that let them prove the answers to their own satisfaction. I think science should be fun, and available to everyone, regardless of age.

Experience

I have volunteered in our local public schools for 5 years. I currently make presentations at our schools about electricity and magnetism, with a focus on solar, wind, water and other alternative power sources. I try to demonstrate at our schools how easy it is to make electricity, with simple devices using spinning magnets and coils of wire--powered by wind, water, bicycles, gerbils...etc. And of course solar panels! I am the webmaster of Otherpower.com, an alternative energy website. I have lived 10 miles from the nearest power pole for 11 years--I make all my own electricity from scratch with sun, wind and water.

Publications
Otherpower.com
Wondermagnet.com
Co author of the book "Homebrew Wind Power" ISBN 978-0-9819201-0-8
My articles appear regularly in such magazines as Home Power and Back Home
Education/Credentials
BA Technical Journalism


Education/Credentials
BA Technical Journalism

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