Why do some materials contract when heated? Our teacher told us that some materials contract when heated, but I could not understand why?
Help is appreciated.
 A: The volume occupied by a material is related to the way its individual molecules are arranged. Usually, as a material is heated the molecules becomes less organized in a way that required there to be greater separation between them; but occasionally, the lower-temperature, ordered state has more space between the molecules.
Such is the case, for example, with water. You may know that when water freezes, it expands: this has to do with the arrangement of water molecules in the ice. I explained this in more detail in this answer
For liquid water between 0 C and 4 C a similar thing is true: there is significant hydrogen bonding between the liquid water molecules, which imposes a certain amount of "local structure". As the water heats up, more of these bonds are broken and the other factors (the "jiggling" of the molecules) becomes more important. This is why water has its greatest density at 4 C - it shrinks as it heats because it loses some of the "open-structured order".
There are some variations on this theme in other materials but I believe the basic principle is the same. If anyone know counter examples I invite them to comment.
