Water exhibits several properties distinct from most other liquids. These unique characteristics allow it to play an important role in natural processes.
Water’s Unique Expansion
Most substances behave predictably when cooled; their molecules move closer together, causing them to contract and become denser. This phenomenon is why solid forms of most materials are denser than their liquid counterparts. Water, however, presents a significant departure from this general rule.
As water cools, it initially follows the typical pattern, contracting and increasing in density. However, below a specific temperature, its behavior becomes anomalous. Below this point, water expands instead of contracting, leading to a decrease in density.
The Temperature of Lowest Density
Water reaches its maximum density at approximately 4 degrees Celsius (39.2 degrees Fahrenheit). As the temperature drops below this point, water’s density decreases. This means that water is at its least dense state when it solidifies into ice, which is why ice floats.
This unusual behavior stems from the unique structure of water molecules and the hydrogen bonds they form. Above 4°C, water molecules are closely packed but in a somewhat disordered state, constantly breaking and reforming hydrogen bonds. As the temperature falls from 4°C towards 0°C, the water molecules begin to arrange themselves into a more open, hexagonal crystalline structure. This arrangement takes up more space than the more randomly packed liquid water molecules. This increase in volume for the same mass results in a lower density for ice and for water just above freezing.
Why This Matters: Impact on Nature
Water’s anomalous density has a significant impact on aquatic environments. When bodies of water like lakes and ponds cool in winter, the denser 4°C water sinks to the bottom. Water colder than 4°C, being less dense, remains closer to the surface.
This stratification ensures that ice forms first on the surface of the water, rather than at the bottom. The resulting layer of ice then acts as an insulating barrier, protecting the deeper, warmer water below from freezing solid. This allows aquatic organisms to survive the cold winter months in the unfrozen depths. Without this property, many aquatic ecosystems would freeze solid from the bottom up, making life in colder climates unsustainable.