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Why does water expand when cooled, unlike other substances which contract when...

ustaad's profile pic

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Why does water expand when cooled, unlike other substances which contract when cooled?

As we all know, substances on Earth contract when cooled, except for water which expands when cooled.  What is the scientific reason for expansion of water on being cooled?

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enotechris's profile pic

Posted (Answer #1)

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Water's uniqueness arises from the fact that the water molecule is polar.  The orientation of the two smaller hydrogen atoms is such that they occupy one side of the larger oxygen atom.  Because the water molecule is held together by a covalent bond, the electrons, although being shared between the oxygen and hydrogens,  tend to congregate nearer the oxygen side of the molecule.  So one side of the molecule is slightly negative, resulting in the other side being slightly positive.

Because each water molecule is polar, the interaction between them allows for a hydrogen bond to be created, whereby the negative end of one molecule aligns with the positive end of another.  In liquid form, the molecules bounce off each other, but when water cools, the polarity and hydrogen bonds orient the molecules in a rigid pattern, where they are actually further apart from each other than in a liquid state.  On a macroscopic level, we see then that water expands rather than contracts when cooling into ice.

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Posted (Answer #2)

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the water molecules have the simplist atom that is hydrogen and easily compressable essential element oxygen. hydrogen has the tendensy to occopy more volume under low temperatures than the other elements.so with hydrogen oxygen also does the same.

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