Temperature can affect density in certain ways. If the temperature decreases, what effect can that have on the density of a substance?

Liquid water at 25 deg is so rapidly breaking bonds between H2O units and reforming them that extra water molecules get trapped inside the water lattice. This is the reason why liquid water is more dense than solid water.

The bonds in water break more slowly as temperature decreases and the structure tend to trap fewer extra water molecules. At low temperature, more of the water has the same lattice as ice.  

Wikipedia, Water Density

It is possible to have liquid water at temperatures well below 0 deg C. Molecules in this supercooled water are free to move. Bonds are made and broken. The long range structure is not perfect but the short range structure of supercooled water is very much like ice. Adding a crystal to supercooled water causes instantaneous ice formation.

Explanation: :P