The density of water varies with temperature which helps the aquatic animals to live in cold water. At what temperature is the density of water maximum?

Water exhibits an anomalous expansion property where its density does not change linearly with temperature. While most substances become denser as they cool, water reaches its maximum density at approximately 4°C [t1][t2]. Between 0°C and 4°C, water actually contracts as it warms and expands as it cools [t2]. This unique behavior is critical for aquatic life; as surface water in lakes cools to 4°C, it becomes denser and sinks to the bottom, displacing lighter water [t7][t10]. This process ensures that the bottom layer of deep water bodies remains at a relatively stable 4°C even when the surface freezes [t4]. Because ice (at 0°C) is less dense than the 4°C water below it, it floats on the surface, providing an insulating barrier that prevents the entire water body from freezing solid, thus allowing aquatic organisms to survive in the liquid water beneath [t4][t6].

Sources

1. [1] https://pmc.ncbi.nlm.nih.gov/articles/PMC4686860/