Why does H2O expand when it gets cold?

“Most liquids have a quite simple behavior when they are cooled (at a fixed pressure): they shrink. The liquid contracts as it is cooled; because the molecules are moving slower they are less able to overcome the attractive intermolecular forces drawing them closer to each other. Then the freezing temperature is reached, and the substance solidifies, which causes it to contract some more because crystalline solids are usually tightly packed.

Water is one of the few exceptions to this behavior. When liquid water is cooled, it contracts like one would expect until a temperature of approximately 4 degrees Celsius is reached. After that, it expands slightly until it reaches the freezing point, and then when it freezes it expands by approximately 9%.

This unusual behavior has its origin in the structure of the water molecule. There is a strong tendency to form a network of hydrogen bonds, where each hydrogen atom is in a line between two oxygen atoms. This hydrogen bonding tendency gets stronger as the temperature gets lower (because there is less thermal energy to shake the hydrogen bonds out of position). The ice structure is completely hydrogen bonded, and these bonds force the crystalline structure to be very “open””

Source

http://en.wikipedia.org/wiki/Ice It has to do with the hydrogen bonds between oxygen and hydrogen molecules. Looking at the article just reminded me of why I hated chemistry class so much.

Because it is all pissed off.

Precisely the opposite affect on testicles.Just sayin.

@MissAusten
I’m still not quite understanding what you’re explaining.

So the H2O molecules are connected together with hydrogen bonds. somehow when these bonds get stronger they expand could you explain this?

When water is in its liquid state, it has too much kinetic energy to form the hydrogen bonds it does in solid form. The molecules can take any orientation as they slide past each other. When it loses this energy though, the hydrogen bonds lock the molecules into a hexagonal lattice which has a greater volume than the random orientation of the molecules in the liquid state. See this diagram.

@FireMadeFlesh
Thanks

@FireMadeFlesh took the words right out of my mouth. That’s exactly how I was going to explain it. Really. To be honest, I looked up that wikipedia entry because I didn’t know the answer, although I remember discussing it in some class ages ago. I’m glad someone who knows what all that means was able to answer your question better!

Once the molecules lose sufficient kinetic energy (get cold enough), it’s thermodynamically more favorable for the molecules to link up in the open hegxaonal crystals typical of ice, even though it puts the individual molecules slightly farther apart than in liquid state, hence less dense than water. This unusual property of water is made possible by hydrogen bonding of its molecules. The only other substance I know that’s less dense as a solid than liquid is bismuth.