Density of water decreases with decreasing temperature. Why?

[qabawe]

Hi People,

 

we have taken today, in chemistry class, wot is called the phase diagram and as we all know that the density of water decreses as it changes to ice.

a question came up in my mind...... why is that?

why the density of water, and other substances of course, decrases with decresing temperature??

When i asked my teacher this question.... he explaind.... but i didnt understand:embarass:

 

So, if someone can explain that to me please.

[Darkblade48]

As far as I know it, water is the only substance that has a decrease in density when the temperature decreases.

 

All other substances should have an increase in density when the temperature decreases. For example, at STP, nitrogen is a gas, and is not very dense when compared to something like liquid nitrogen. Liquid nitrogen exists at 77 K, so at low temperatures, its density increases (as it's a liquid now, and liquids are obviously denser than gaseous nitrogen).

 

For these "normal" substances, as the temperature decreases, what occurs is that the molecules come closer together, as they do not have the energy to "fly away" from one another. In a solid, the molecules are quite close together, so you have more mass per unit volume (the definition of density). This means that the density is high. In a gas, the molecules have more energy to "fly away" so in the same unit volume, there will be less molecules and thus less mass. The density is then less.

 

In water however, as temperature decreases, the density decreases as well, which is an unusual characteristic of this substance. Why this occurs, I'm not sure of though

[encipher]

I think Silica also becomes less dense as a solid.. not sure about that through.

 

Usually, one would expect, when a liquid freezes it would become more 'compact' because molecules are in fixed positions within a solid but require more space to move around as a liquid.

 

The case with water however, is the structure it forms when it begins to freeze. The molecules within Ice's structure have a low packing efficiency. They water molecules form 4 straight tetrahedral oriented bonds. (I'm not sure about this, that's what I can remember, so if I am mistaken please correct me.) The reason water does this is because of the Hydrogen bonding that occurs between the molecules.

[insane_alien]

yeah there are only a few compounds that decrease in density when frozen (i think the number can be counted on one hand). its all to do with the arrangement of molecules. one analogy i've heard is for scaffolding. you can fit enough scaffolding poles in a truck to cover the face of quite a large building. the only difference is the way they are arranged.

 

after the water molecules ar in their expanded form(ice) then the density will increase with further cooling as with all substances.

[John Cuthber]

Gallium and tin are probably the 2 commonest (after water) things that expand on freezing. The answer involves the lattice formed from water, being held together by relatively weak hydrogen bonds, has quite big gaps between the molecules. When it melts the molecules can slip into those gaps.(And I'm afraid I haven't a clue why Ga and Sn do this, it may be related to the fact that they have unusually long liquid ranges)