MadSci Network: Chemistry

Re: If water expands as it freezes, why does my water bottle contract?

Date: Tue Jun 3 04:52:55 2003
Posted By: Gareth Evans, Industrial R&D practitioner and manager ( retired )
Area of science: Chemistry
ID: 1053105579.Ch

Hello Katharine, your question was: 
I understand that water expands as it freezes.  What I don't understand is 
why when I fill up my plastic water bottle with tap water, put it in the 
freezer, and then take it out, the bottle has contracted?  

You are right about water expanding when it freezes.  It is an unusual 
property.  Most materials contract as the temperature is lowered. As the 
temperature is lowered the vibrations and movement of molecules is less 
energetic and the room they need is reduced.  This is also to water, both 
liquid and solid, except between 4 degrees C and zero, freezing point when 
it expands as the temperature drops.

When you filled your bottle you probably used water at a temperature of 
15C to 20C at a guess.  As the bottle of water cooled, the water 
contracted, then expanded until frozen and then contracted again.  The 
amount of contraction was more than the amount of expansion.

I’m assuming that all the air was excluded from the bottle.  If you left 
some air in, then there would be another reason why the bottle 
contracted.  The air would contract on cooling more than the water and its 
solubility in the water would increase.  Both effects would reduce the 
volume of air.  You might like to try half filling the bottle and seeing 
what happens.

Let’s look a little further for the reasons why water behaves in this 
way.  Water molecules are made up of two hydrogen atoms and an oxygen 
atom.  The hydrogen atoms are bound to the oxygen atoms by bonds formed by 
the electrons associated with the hydrogen and oxygen nuclei.  There’s no 
room here for a full explanation of chemical bonding.  Suffice it  to say 
that the hydrogen atoms can form weak bonds with oxygen atoms of other 
water molecules while at the same time retaining a strong bond with the 
oxygen in their own molecule.  These weak bonds are called “hydrogen 
bonds”.  They are made and broken continuously as the molecules tumble and 
bump together.  As the temperature drops to around 4C the energy in these 
movements of water molecules is sometimes insufficient to break the bonds 
and as the temperature drops further, more and more molecules form semi-
permanent assemblies of molecules.  The ordered structure of these 
assemblies occupies a greater volume than the collections of unconnected 
molecules at the same temperature.  At freezing point, when all the 
molecules are in a structured, now solid arrangement, the volume of solid 
ice as about 10% greater than the liquid a few degrees warmer.

The consequences of this behaviour are enormous.  Some consequences are to 
be welcomed while some are no so good.  For example, ( see
hm/water_ex.html )  if ice was denser than 
water, icebergs would sink and accumulate in the oceans until all the 
oceans were solid.  The freeze-thaw cycle is a major contribution to 
weathering of rocks.  On the other hand because we can melt ice with 
pressure we can skate and ski !

Thanks for the question.  Keep observing and asking.  

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