Step 1: Understanding the Concept:
Most substances become denser when they freeze. Water is an anomaly. This anomalous behavior of water is solely due to the specific nature of hydrogen bonding in the solid state.
Step 2: Detailed Explanation:
In liquid water, molecules are close together and move randomly.
When water freezes to form ice, each oxygen atom is tetrahedrally surrounded by four hydrogen atoms—two by covalent bonds and two by hydrogen bonds.
This results in a highly ordered, three-dimensional "open cage" like structure.
Because of this open structure, there is a significant amount of empty space between the molecules.
For the same mass, the volume increases upon freezing, which means the density decreases (\( \text{Density} = \text{Mass}/\text{Volume} \)).
Step 3: Final Answer:
Hydrogen bonding interactions lead to the open cage structure of ice, resulting in its low density.