Correct Response: A. Unlike most other substances, water becomes less dense as it is cooled below freezing to form ice. The rigid crystal structure of ice makes it less dense than liquid water because molecules in liquid water are constantly changing position and maximizing hydrogen bonding between water molecules. This allows liquid water molecules to pack more closely than water molecules that are frozen into a crystal structure. Viscosity is a measure of the internal friction of a substance and the viscosity of water (B) is not a factor directly responsible for the relatively low density of ice. Although the water molecules in ice are constantly vibrating (C), it is not that characteristic of ice that is responsible for its low density. The high heat capacity of liquid water (D) results from the hydrogen bonding between water molecules and is not directly the cause of ice's low density.
