Why does water expand when it freezes?

11 October 2009

Question

Why does water expand when it freezes?

Answer

Usually, when things freeze - in other words turn from a liquid into a solid - they shrink or get smaller.

This is because, normally, if you make something hotter, it vibrates more. When it vibrates more, it tends to take up more space, so it tends to expand.

So, logically, if you cool something down, then the particles should move more slowly, collide and bounce off one another less hard and less frequently, and therefore, on average, spend more time closer together, making the material shrink.

Ice, on the other hand, is very unusual in that, as it gets colder, although the particles are certainly vibrating less for the reason explained above, it nonetheless expands or gets larger.

The reason for this is due to the strange shape of water molecules.

If you've ever seen a picture of a water molecule you'll know that it looks like a "Mickey Mouse" head, with an oxygen atom where Mickey Mouse's face is, and then two hydrogen atoms where his ears are.

The oxygen atom is slightly negative, and the hydrogens are slightly positively charged, so water molecules tend to stick together forming what are called hydrogen bonds.

Owing to the water molecule adopting that shape, the way water molecules tend to link together in the liquid state is to form a very open structure with big holes. That means, there's quite a lot of extra "empty" space.

When water freezes, the molecules get themselves into the most stable configurations or positions that have the minimum amount of energy in the resulting ice crystal.

It so happens that the arrangement of water molecules that best satisfies this requirement is one that takes up even more space. And so ice expands when it freezes.

Comments

?? Since oxygen forms 4 bonds with hydrogen during ice formation I have always pictured the reason for expansion as the return of the electron to the hydrogen atom. In ice, oxygen has 4, 1/2 strength bonds each shared equally with a neighboring oxygen. The equal opposite pull on the electron puts it in the middle. Right where the hydrogen is hanging out. Giving hydrogen back its electron cloud triples the number of electron clouds compared to water and gas and the bonds become more covalent like (rigid). The structure then becomes the most logical arrangement and the neg neg interactions cause the expansion. Thoughts??

Thanks you for that

Would you say that this is a correct statement
"The hydrogen bonds of water expand during the freezing process, creating more volume and decreasing density of the ice."

The hydrogen bonds do not expand; the way that the water molecules arrange themselves in three-dimensional space is a more open, less compact structure.

good explanation

The last sentence explains almost nothing... please explain further

Thank you for highlighting that; I've altered the wording slightly to improve the clarity.

There is no such thing as cooling. There is removal of heat, or , energy. The energy must travel through a medium in order to get to whatever is pulling the energy out. Since Hydrogen and Oxygen are the media, they get pulled and flow outward and apart. The bond between these molecules, due to their positive (H) and negative (O) charges, creates an opposing force to the force that is removing the energy. As the process progresses, and the water expands further, the force of the molecular bond and of whatever is taking the energy from the water, cancel out, creating the frozen water. AKA Ice. The molecules are suspended between two forces with little motion in either direction. The outer molecules move slower than the innermost molecules. This is why you see lakes with liquid water covered by ice in the winter. Correct me if I'm wrong

So, pretty much when the particles or the solid substance gain energy, they get hot so the particles tend to expand causing the solid to expand?

That's correct.

Your explanations are great!
my professor doesn't know how to talk to the students in simple terms!
thank you so much for this!

I love ur explanations they are good

I love your explanation .They are very clear

Thank you,I exactly got what i needed to know

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