|MadSci Network: Chemistry|
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 http://members.aol.com/profc 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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