Why does ice have a lower density compared to water at low temperatures?

Ice has a lower density compared to water at low temperatures primarily due to the unique arrangement of water molecules in its solid form. When water freezes, it forms a crystalline structure in which each water molecule is hydrogen-bonded to four neighboring water molecules. This arrangement creates a highly organized, open lattice structure that occupies more space than the arrangement of molecules in liquid water.

In contrast, liquid water has molecules that are in constant motion, allowing them to be packed more closely together. As a result, the density of liquid water is higher than that of ice because the liquid's structure can accommodate more water molecules in a given volume than the solid structure of ice can.

The other options do not accurately describe this phenomenon. Ice’s density is not related to the number of hydrogen atoms; rather, it is about how these atoms are arranged in the respective states of the substance. The color of ice has no impact on its density. Similarly, while it's true that water may be more compact than ice, this statement does not explain why ice is less dense. The key concept here is the role of hydrogen bonding and the resulting molecular structure.