Cold liquid water has a high density that increases upon warming
Water reaches its maximum density at 3.984 degrees Celsius, not at the freezing point.
Scientific Explanation
Most liquids become steadily denser as they cool — the colder, the denser. Water only partially follows this pattern. Below about 3.984 degrees Celsius, the trend reverses: further cooling actually makes water lighter instead of heavier. The density maximum therefore lies not at the freezing point, but about 4 degrees above it.
This behavior arises from the interplay of two competing effects. On one hand, cooling causes molecules to move closer together (as in other liquids). On the other hand, below about 4 degrees Celsius, water molecules increasingly form tetrahedral hydrogen bond arrangements that resemble the open structure of ice. These ice-like regions occupy more space and reduce density. Below 3.984 degrees Celsius, this structural effect outweighs the normal thermal contraction, and density decreases.
Step by Step
The Axes
We examine the density of water across a range of temperatures. The x-axis shows temperature from 0 to 50 degrees Celsius, and the y-axis shows density in grams per cubic centimeter.
The Density Curve
Starting from 0 degrees Celsius, the density rises until it reaches its peak near 4 degrees. Beyond that, it steadily decreases -- just like most other liquids.
The Maximum
At precisely 3.984 degrees Celsius, water reaches its highest density: 0.99997 g/cm3. This density maximum does not occur at the freezing point, but 4 degrees above it.
The Anomaly
While nearly all liquids grow denser as they cool, water reverses this trend at 4 degrees. Tetrahedral hydrogen bonds begin forming ice-like structures that occupy more space. This is why lakes freeze from the top down -- not from the bottom up.
Everyday Relevance
The density maximum at 4 degrees Celsius is one of the reasons deep lakes do not freeze solid in winter. As surface water cools in autumn, it sinks once it reaches 4 degrees — the temperature at which it is heaviest. Water that cools further becomes lighter and remains at the surface, where it eventually freezes to ice. Beneath the ice layer, water stays at about 4 degrees — cold, but liquid enough for fish and other organisms to survive.
This anomaly also matters for drinking water supply: deep water in lakes is often warmer than surface water in winter, which influences the stratification and seasonal mixing of lakes.