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.

  • $\begingroup$ Why are you surprised that water doesn't freeze at 4 degrees celsius? $\endgroup$
    – user253751
    Oct 9, 2020 at 17:55
  • $\begingroup$ I am not surprised for not getting freezed at 4 degree celsius. I want to know why water behaves strangely at that temperature. $\endgroup$ Oct 9, 2020 at 17:59
  • $\begingroup$ chemistry.stackexchange.com/questions/81756/… may provide some insight (full disclosure - answer is mine). $\endgroup$
    – Jon Custer
    Oct 9, 2020 at 18:01
  • 1
    $\begingroup$ It is currently totally impossible to calculate melting temperature etc. There are many anomalies in the physical properties of water. Martin Chaplin has an excellent overview: www1.lsbu.ac.uk/water/density_anomalies.html $\endgroup$
    – user137289
    Oct 9, 2020 at 18:18
  • 3
    $\begingroup$ Water behaves strangely at EVERY temperature, due to the fact that its molecules are all hydrogen bonded to their neighbors. When liquid, these hydrogen bonds constantly break and reform. Note that based on its molar mass of 18.02, water should have a normal boiling point that is close to methane (-258 deg F with no hydrogen bonding; molar mass of 16), but it obviously (and thankfully) doesn't. $\endgroup$ Oct 12, 2020 at 23:21

3 Answers 3


sou said, "water tend to expand instead of freezing just below and above 4 degree celsius".

Take a look at a chart Wikipedia.


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.


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


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.


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