Freshwater achieves its greatest density at

Freshwater reaches its maximum density at 4 degrees Celsius, when its molecules pack most tightly.

A reading of minus 4 degrees is below freezing, where water has already turned to less dense ice.

At 0 degrees water freezes into ice, which floats because it is lighter than liquid water.

Minus 2.5 degrees again lies below the freezing point and does not give peak density.

Below 4 degrees water expands again rather than contracting further.

This anomaly lets ice form on top while deeper water stays liquid for aquatic life.