When water is heated, its density generally decreases. Density is defined as mass per unit volume, so any change in volume for a fixed mass of water impacts its density. Most liquids follow a predictable pattern of thermal expansion, but water exhibits an unusual deviation near its freezing point. Water’s behavior is highly dependent on the specific temperature range, which dictates how its molecules are arranged.
The General Rule of Expansion
For most liquids, heating causes expansion due to the transfer of thermal energy, which increases the kinetic energy of the molecules. As molecules move faster, the average distance between them increases. This greater separation results in a larger volume. Since mass remains constant while volume increases, the density decreases. Water follows this standard principle of thermal expansion when heated above 4°C.
Water’s Unique Density Peak
Water’s behavior changes dramatically near freezing, a phenomenon called the density anomaly. Liquid water reaches its maximum density at approximately 4°C, not at its freezing point of 0°C. When water is cooled from above 4°C down to 4°C, it contracts and its density increases, following the expected pattern. However, when cooled below 4°C toward 0°C, it begins to expand again, known as negative thermal expansion. This means water at 0°C is less dense than water at 4°C due to molecular forces.
The Molecular Explanation
The underlying cause of water’s unique density profile is the presence of hydrogen bonds between its polar molecules. A water molecule (\(H_2O\)) can form up to four hydrogen bonds with neighboring molecules. Above 4°C, molecules have enough kinetic energy to constantly break and reform these bonds, keeping them closely packed.
As the temperature drops below 4°C, the hydrogen bonds stabilize, forcing the molecules into a more rigid, open, and ordered tetrahedral arrangement. This expanded structure contains significant empty space compared to the liquid state at 4°C, causing the total volume to increase. When water freezes into ice at 0°C, the molecules lock into a highly organized, hexagonal crystal lattice. This crystalline structure is even more spacious, making solid ice about 9% less dense than liquid water, which is why ice floats.
Practical Effects in Nature
Water’s maximum density at 4°C and the fact that ice floats have profound implications for aquatic life. In deep bodies of water during winter, the 4°C water sinks to the bottom because it is the densest, while colder, less dense water remains near the surface. This stratification prevents the entire body of water from freezing solid from the bottom up. The surface ice acts as an insulating layer, protecting the warmer 4°C water and allowing aquatic organisms to survive the winter in the unfrozen depths. If water behaved like most other substances, lakes would freeze from the bottom, eliminating life.