Science Questions

Why does water expand when it freezes?

Sun, 11th Oct 2009

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Question

Martin asked:

Why does water expand when it freezes?

Answer

Basically, the default thing for things to do is to shrink when they freeze because normally, if you make something hotter, it’s vibrating more.  When it vibrates it tends to take up more space, so it tends to expand.  Ice is very unusual that as it gets colder, although it’s essentially vibrating less, it does expand.  And the reason for that is due to the strange shape of water.  If you’ve ever seen the picture of a water molecule, it looks like a Mickey Mouse head with an oxygen molecule where Mickey Mouse’s face is and then two hydrogen atoms where his ears are, it’s bent basically.  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.  And because  of that bend, the way they tend to link together is actually a very open structure with big holes in it and that means, there’s quite a lot of extra  empty space in that structure.  So when water freezes it, it releases a load of energy because  lots of extra strong bonds can be made.  But it does take up more space.  And so, ice expands when it freezes.

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I can't answer all your questions, but i can answer some.

Why does water expand when it's freezing: because it's maximum density is at 4°Celcius.
This is also the reason why water is fluid under great pressure. You won't find any frozen water pockets on the bottom of the ocean neither, because the pressure will make the water be at it's maximum density.

Maybe someone else can explain why the density of Ice is lower than 4°C water... Nizzle, Mon, 17th Aug 2009

I'll give it a shot, see if I'm correct

As you know, water is a really characteristic compound in comparison to other compounds. Many compounds contract when they are frozen, just like your thermometer drops when it gets colder. Water differs because, as Nizzle correctly stated, it's maximum density occurs at 4 degrees celcius. This means that below 4 degrees celcius it can do nothing but decrease its density and expand, as it could not have a greater density (The exact reason why ice floats).

Another thing which plays a role is its polar nature. What basically goes on here is related to its structure and polarity. Water has an sp3 hybredization, but due to what we call "Valance Shell Electron Pair Repulsion" (or short VSPER Theory), the angle between the centered Oxygen atom and two hydrogen atoms is about 105 degrees. I will return to the structure in a moment. Water is also polar. Oxygen is more electronegative than Hydrogen and greedily pulls Hydrogen electron towards itself. This makes Oxygen slightly negative, and hydrogen slightly positive. This enables water to form what we call 'Hydrogen Bonds' (Process: Hydrogen Bonding). In water these bonds will form, break and form again on a new. Freezing causes less molecular activity because it means it's a drop in energy. Hydrogen bonds become more stable, and order themselves as a quite spacy hexagonal structure, due to its structure (It expands, taking up more space).

I am not sure what force it exerts when expanding though 

P.s. I should add that this is for 1 atm (Atmospheric Pressure), which is the equivalent of 101325 Pa. Also as Nizzle stated, greater pressure ensures water as liquid. DrChemistry, Mon, 17th Aug 2009

The answer was helpful, though I have a follow up question. If the water is in a confined space, say a bottle, before freezing, then once all the water has frozen the air pressure inside the bottle rises. This must meen that the air inside the bottle is not filling in that 'space' inside the ice crystals. Does this meen that inside these cystals is a vacuum or is it just that air is inside these crystals but at a lower pressure to the surrounding atmosphere? If not, what exactly fills these crystals? Richard, Wed, 10th Feb 2010

Can anybody please answer this question not according to Chemistry? Haider Riaz, Mon, 3rd Feb 2014

If by non-chemical you mean simpler, maybe I can help. In general, liquids, and other states for that matter, expand as they get hotter and contract as they get cooler. That happens with water, too, but once it gets to freezing temperature, the molecules basically lock in place in a crystalline structure that actually has more space in it than there would be if the molecules we're still floating freely in liquid form. I'm not sure if I added anything useful to the above explanations, but it's simpler at least. Rob , Wed, 19th Mar 2014

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