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.
This simulation visualises why ice floats, the consequence of water's anomalous density. A molecular view shows roughly 100 H₂O molecules arranging into an open hexagonal lattice as they cool, with hydrogen bonds drawn between neighbours, while a side chart plots density against temperature. The density curve uses the standard polynomial ρ = 999.842594 + 6.79×10⁻²T − 9.10×10⁻³T² + …, which peaks near 4°C.
The temperature slider runs from −20°C to 100°C, and preset buttons jump to ice (−10°C), maximum density (4°C), warm water (20°C) and boiling. As you cool below 4°C, molecules lock into a spacious crystal and the density falls again. This single property keeps lakes liquid beneath winter ice, allowing aquatic life to survive cold seasons.
Why does ice float on water?
Below 0°C, hydrogen bonds force water molecules into an open hexagonal lattice that takes up more space than the jostling molecules of liquid water. Solid ice is therefore about 9% less dense (roughly 917 kg/m³ versus 998 kg/m³), so it floats. The simulation shows this lattice forming as you lower the temperature.
What is the density anomaly of water?
Most liquids grow denser as they cool, but water reaches its maximum density at about 4°C and then becomes less dense again on the way to freezing. The density chart in this simulation marks that peak near 999.97 kg/m³ with an orange dot, then shows the curve dropping toward the ice value below 0°C.
What do the temperature controls do?
The slider sets the temperature from −20°C to 100°C, redrawing both the molecular view and the density marker. Four preset buttons jump to ice at −10°C, maximum density at 4°C, warm water at 20°C and boiling near 99°C, so you can quickly compare each regime.
Each water molecule has a bent shape with a bond angle near 104.5°, letting it form up to four hydrogen bonds. In ice these bonds arrange molecules into a roomy hexagonal cage. Warming breaks some bonds, allowing tighter packing until thermal motion takes over above 4°C and the liquid expands again.
The liquid branch uses a published polynomial fit that reproduces the real density of water to several decimal places between 0°C and 100°C, including the 4°C peak near 999.97 kg/m³. The ice branch is a simplified linear approximation around 917 kg/m³, and the steam value is a rough estimate, so the qualitative shape is faithful while the solid and vapour ranges are demonstrative.
The lines linking molecules represent hydrogen bonds, and the code fades them out as temperature rises above 5°C. This reflects reality: bonds are continually breaking and reforming in warm water and survive only fleetingly, whereas in ice they form a stable, long-range network that the lattice drawing makes visible.
At about 3.98°C liquid water is at its densest, so the simulation labels this state as maximum density. Hydrogen bonds have largely rebuilt while molecules still pack tightly, giving the best balance. Above this point thermal expansion dominates and density falls; below it the open lattice begins forming and density also falls.
Because water is densest at 4°C, the coldest near-freezing water rises rather than sinks, so ice forms at the surface first. This insulating ice layer keeps deeper water liquid through winter. The simulation illustrates the underlying cause by showing how sub-4°C water becomes less dense as it approaches freezing.
It shows about 100 simplified water molecules: a glowing oxygen core with two small hydrogen dots. They spring toward target positions that are hexagonal when cold and increasingly random when warm, with added jitter that grows with temperature. It is a schematic 2D illustration of ordering, not a full molecular dynamics calculation.
If ice sank, lakes and oceans would freeze solid from the bottom up, destroying aquatic ecosystems. Instead, floating ice insulates the water below and allows fish and other organisms to survive cold periods. Many scientists regard this anomaly as one of the conditions that made complex life on Earth possible.