Why is solid water (ice) less dense than liquid water?
A - The hydrogen bonds between water molecules are formed and held (or frozen) farther apart than in liquid water
B - Hydrogen bonds are not able to be formed because of the cold temperature
C - Liquid water does not form hydrogen bonds
D - It is actually more dense
Answer: A - The hydrogen bonds are farther apart when frozen.
There are fewer water molecules per volume (so less mass/volume) for solid water than for liquid water. Due to the hydrogen bonds being held farther apart, and therefore occupying more volume, the overall density has decreased.