Why does water expand when it freezes?

11 October 2009


Ice Shear Zone



Why does water expand when it freezes?


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.


this is the first comment since Covid started.

Is this a Joke? .. maybe it would satisfy as an explanation for a 3 year old, but this is insufficient. If you do not know what's going on, why make an article on the matter...

If you are such an expert on this topic: a) why not use some of your (self-proclaimed) extensive knowledge to make suggestions regarding what is missing from the piece, rather than just offer sarcastic criticism? And b) why are you even reading this page in the first place if you know so much about the subject? 

I've reviewed the article above and it actually presents a very clear and accessible description suitable for a range of audiences. Please take your head out of the clouds (it's icy up there). Meanwhile, maybe devote some time to learning about civility too? 


I agree with OP. I couldn't believe the article just...ended like that. He basically said water molecules are a certain shape, and because of that they tend to join together and form bigger shapes.

OP is correct, only a child would be satisfied with that!

Quote from the article: 

"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."

That's a perfectly valid answer. I'm not clear what else you are searching for, other than some kind of quantum mechanical interpretation of the ice configuration.

As pointed out by those (like myself) in agreement with this article, those who are deriding its explanation fail to give any constructive comment. And that’s happened twice now. We’re waiting for your insights...

Hello! Nice website! I ice skate frequently. Here, when the ice is 12 inches thick or more, our city plows the snow off the lakes and uses a hot mop to smooth the surface. They do this daily. Our temperatures have been around 24 degrees Fahrenheit for several weeks, and the ice has had great integrity and smoothness. The temperatures dropped in one night to 0 deg. F. and stayed in the single digits the next day and night. It rose up to 10 deg. during the next day, and I went skating that evening. The lake was completely fractured. Deep splits ran throughout the skating area. As my blades ran over the ice, I could hear it crackling. My runner broke through the surface and chipped out a wedge of ice the length of the blade, and about an inch wide. My question: Did the ice continue to expand with the lower temperatures, or did the ice contract? What caused the deep cracks in 12-inch + ice? Why is it chipping to pieces under my skates?

Sorry, I made an error in wording. The density reaches a maximum, not a minimum, at 4 deg C.

It would seem to be two (or more?) effects at work here, one causing the water molecules to contract, the other, to expand. At 4 deg C they balance and thus the density-temperature function reaches a minimum. If they do exist, what are these competing effects? Do they explain the minimum-energy configuration of the molecules?

This is a great website very good at describing why water expands when it freezes.

Last lines are not clear can you please explain them in a different way

When someone likes another they will tell someone else they like them but the person they tell will maybe shout it out to there class in my case is it be cuz of jealousy or other reasons ? it happened in my class today and I thought I would ask ..it was quite funny cuz my teacher started a hole conversation about it cuz he is a funny teacher ...I would appreciate and answer to my question..thank u ;)

What? Who knows why people do what they do. PS Stay in school so you’ll learn how to spell complete words. *because, not cuz. * whole, not hole.

can this property of water be use for artificially condense or evaporate water?

?? 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

This reminded me of the rubberband trick, stretch a rubberband almost to the breaking point for 10 seconds, let it contract back to it's normal state quickly, still holding onto the rubberband you can feel the rubberband absorbing the heat energy from your hand. It "feels" like the rubberband is cold, but it's just the transfer of heat.

The shrinking elastic band is "doing work" to re-compact and coil the stretch rubber polymer chains. Doing work consumes energy, so the average energy of the polymer chains - and hence the temperature of the rubber band falls i.e. it gets colder. For more information see https://www.thenakedscientists.com/get-naked/experiments/rubber-fridge-w...

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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