Final answer:
The maximum density of pure water is reached at approximately 4°C, a unique characteristic that enables ice to float on liquid water and allows aquatic life to survive under ice.
Step-by-step explanation:
The density of pure water reaches its maximum at an approximate temperature of 4°C. This unusual behavior is due to the fact that as water cools, it becomes denser up to 4°C. This is a unique characteristic of water, as most other substances continue to increase in density as they approach their solid state; however, water expands as it goes below 4°C towards freezing, becoming less dense. This phenomenon is important in the natural world, explaining why ice forms at the top of bodies of water, allowing aquatic life to survive beneath.