- Unlike most substances which become denser as they are cooled, water behaves unusually in the temperature range of 0°C to 4°C. - As liquid water cools from higher temperatures, its density increases until it reaches a maximum density at approximately 4°C (3.98°C to be precise). - Below 4°C, the density of liquid water starts to decrease as it approaches its freezing point (0°C). When it freezes to form ice at 0°C, its density decreases significantly, which is why ice floats on water. - This anomalous expansion of water is crucial for aquatic life in cold climates, as the densest water (at 4°C) sinks to the bottom of lakes, allowing life to survive below the surface ice.