Why density of water decreases below 4 degree C ?

Because below that temperature the water molecules start to form networks of hydrogen bonds, which causes them to all move slightly apart. The spacing between molecules in the hydrogen-bonded network is larger than the spacing between molecules not in the network.

I thought water has hydrogen bounds regardless of the temperature. So, water at 25 degree C does not have hydrogen bounds? Or it doesn´t have the network?
Here (http://www.lsbu.ac.uk/water/explan.html#density) is a link that may be useful.

Hey, it was worth reading that article about water just to pick up the word ebullioscopic. I can't wait to drop that one in a conversation with my friends. :)

I thought water has hydrogen bounds regardless of the temperature. So, water at 25 degree C does not have hydrogen bounds? Or it doesn´t have the network?
Here (http://www.lsbu.ac.uk/water/explan.html#density) is a link that may be useful.

No, he explained it kind of funny. Water in the liquid state also has some hydrogen bonding going on....thus you see the "beading effect" of water....or how water striders can walk on water...it's because of the surface tension which is created from the hydrogen bonding.

But when the water freezes....the water molecules are aligned into a crystal network which increases their distance from one another and makes all the molecules evenly spaced out...taking up more room. So....ice floats.

Lucky thing, too. Otherwise ice would form on the bottoms of ponds and lakes, instead of on the surface!

Why density of water decreases below 4 degree C ?

At first, for some reason I thought you had this around the wrong way. But no...

Water is most dense at 4 degrees. Make it hotter and it gets less dense (hot water rises). Make it colder than 4 degrees and it ALSO gets less dense.

Freaky.

Water is most dense at 4 degrees. Make it hotter and it gets less dense (hot water rises). Make it colder than 4 degrees and it ALSO gets less dense.

Freaky.

Yeah, odd, but if it wasn't that way, try and imagine how different the landscape would look without the cracking and erosion caused by ice expansion.

I thought water has hydrogen bounds regardless of the temperature. So, water at 25 degree C does not have hydrogen bounds? Or it doesn´t have the network?
Here (http://www.lsbu.ac.uk/water/explan.html#density) is a link that may be useful.
Above 4 C the hydrogen bonds are constantly breaking and reforming in different orientations because of the thermal motion of the water molecules. The water molecules never get a chance to really order themselves because the hydrogen bonds are breaking too quickly.

Yeah, odd, but if it wasn't that way, try and imagine how different the landscape would look without the cracking and erosion caused by ice expansion.

Not only that, but importantly bodies of water would freeze from the bottom up, with serious biological and climate implications.

And for whatever reason, water is the only thing that works that way in density. It is the only thing which has the property that its solid form will float in its liquid form.

This is why I love water so much. Hell, we're around 60% water anyway....one liter weighing one kg so that's a lot of water in the average person. Water is here, water is there, water is everywhere and yet it's nowhere in sight for many people in rural China and in Africa and in many South American villages.

I love this molecule....it's the molecule of life. I like to swim in it, wash myself in it, wash other things with it, stand in it in thunderstorms, squirt it at other people. It's great. I can't imagine a world without something as basic at H20. If I could marry it, I would.....although the sex wouldn't be much fun. :)

That is very interesting, that it decreases with density lower or high than 4°celcius.
I looked it up on google, and my only explanation would be that, of course, it's high density above 4° because of the heat making the molecules more energetic and bouncy, but below 4° it may be that it's in a state between solid and liquid. That liquids don't necessarily jump instantly from liquid to solid. the changing states of matter is not necessarily an orderly, exact thing, sublimation of ice to gas for example occurs on sunny days to quite an extent.

So my answer would be that it's in a quasi-solid/crystalline state between 4° and 0°



If I could marry it, I would.....although the sex wouldn't be much fun.

KY Jelly is 90% water.