Explore the anomalous density of water. Below 4°C, hydrogen bonds lock molecules into an open hexagonal lattice, making ice less dense than liquid water — one of nature's most important physical properties.
Water reaches maximum density at ~4°C. Below that, hydrogen bonds force molecules into a spacious hexagonal crystal structure. The result: ice is about 9% less dense than liquid water, which is why it floats and why lakes freeze from the top down.
Adjust the temperature slider from 20°C to −10°C. Watch molecular arrangement shift from random liquid to ordered crystal. The density curve shows the anomalous peak at 4°C.
If ice were denser than water, lakes would freeze from the bottom up, killing aquatic life. This single anomaly — caused by the 104.5° bond angle of H₂O — made complex life possible.