Why is ice less dense than liquid water?

Ice is less dense than liquid water primarily due to the unique structure of the water molecule and the hydrogen bonding that occurs between its molecules. Water (H₂O) consists of one oxygen atom covalently bonded to two hydrogen atoms. When water freezes into ice, the molecules arrange themselves into a crystalline lattice structure that maximizes the hydrogen bonds between them.

This lattice structure causes the molecules to be held farther apart than they are in the liquid state, where the molecules are more closely packed due to the constant movement and weaker interactions among them. As a result, the volume of ice increases compared to liquid water, leading to a decrease in density. This is why ice floats on water, as substances that are less dense than the liquid in which they are placed will float.

The other explanations do not accurately reflect the reasons for ice's lower density compared to liquid water. Molecular size or absence of interactions does not account for the specific behaviors and arrangements that arise due to hydrogen bonding in water.