Answer:
When the temperature drops from 0°C to 4°C, water expands instead of contracting, resulting in a reduction in density. In solid form, water molecules normally form open crystal structures, so as they freeze, their density decreases.
When liquids cool, they constrict and become denser. This is not the case with water. Upon cooling to 4°C, it contracts, but upon reaching 0°C, it begins to expand and becomes ice. Thus, it becomes denser as it cools.
