Why water behaves strangely at 4 degree celsius? The basic physics concept I hope almost every one might be aware of i.e "water tend to expand instead of freezing just below and above 4 degree celsius".
My question is why exactly at 4 degrees this phenomenon occur? Why not at some other temperature also what makes it to behave that way?.
Any explanation for the reason might be greatly appreciated.
 A: sou said, "water tend to expand instead of freezing just below and above 4 degree celsius".
Take a look at a chart Wikipedia.
https://en.wikipedia.org/wiki/Properties_of_water#/media/File:Density_of_ice_and_water_(en).svg
This shows that the expansion of liquid water takes place between 0 and 4 degrees C, and that when ice is formed from liquid water at exactly 0 degrees C, it expands.
A: This is because of the hydrogen bond in water molecules. In order to understand why it happens at 4 degree, you have to study the behavior of the hydrogen bond in water molecules. A basic explanation about this is provided here, http://www.iapws.org/faq1/freeze.html
A: Water at 4 Celsius is more denser than any other temperature, because, water molecules jiggling and translational motion have greatly been hindered. If you lower the temperature further down, water molecules jiggling and translation motion is decreased, but at Zero Celsius, Water molecule stops jiggling/translation motion It had before ( when Zero Celsius), water molecules remains fixed in their location, now the frozen water molecule can make vibrations with some average amplitude. At 4 degree Celsius, water forms Hydrogen bonds, but time average picture shows that average number of water molecules for a given tiny volumes becomes larger than the zero degree Celsius, where H2O in ice forms nice symmetrical hexagonal structure. I hope this is helpful to get your head the question on water's density.
