Drop a stone into a pond, and it sinks. Drop an ice cube into a glass of water, and it floats. That familiar difference reveals something unusual about how water behaves as its temperature changes.

Most substances become denser when they cool and solidify, but water does something different near its freezing point. This property affects everything from the ice in a drink to the conditions beneath a frozen lake.

Most Things Shrink as They Cool

Most substances contract as they cool because their particles generally move more slowly and can pack more closely together. Their mass stays the same, but their volume decreases, making them denser.

Water behaves this way over much of its liquid temperature range. However, at ordinary atmospheric pressure, it reaches its maximum density at approximately 4°C.

Below that temperature, liquid water begins to expand slightly as it cools further. When it freezes at about 0°C, its volume increases much more noticeably.

This unusual behavior is called the density anomaly of water. It explains why solid ice can float on liquid water rather than sinking to the bottom.

The Shape Water Molecules Prefer

Each water molecule contains one oxygen atom bonded to two hydrogen atoms in a bent arrangement. Because oxygen attracts shared electrons more strongly than hydrogen, the molecule has an uneven distribution of electrical charge.

This allows neighboring water molecules to form attractions called hydrogen bonds.

In liquid water, hydrogen bonds continually break and reform as molecules move around. The molecules can therefore occupy a relatively compact arrangement.

When ordinary water freezes, its molecules organize into a more regular crystal structure. Hydrogen bonds help hold them in an open, roughly tetrahedral network that produces the familiar hexagonal form of ordinary ice.

This arrangement leaves more space between molecules than exists, on average, in liquid water. The open structure is the main reason ice has a lower density.

Less Stuff in the Same Space

Density describes how much mass is contained within a particular volume. If the same amount of material takes up more space, its density decreases.

When water freezes under ordinary conditions, its volume increases by roughly 9%. As a result, ordinary ice is about 8% to 9% less dense than liquid water near the freezing point.

For comparison, ice has a density of approximately 0.917 grams per cubic centimeter at 0°C, while liquid water has a density of approximately 1.0 grams per cubic centimeter.

Because ice is less dense than the water surrounding it, it floats. A floating ice cube displaces enough liquid water to balance its own weight, in accordance with Archimedes' principle.

That is also why most of a floating iceberg remains below the water's surface.

Why This Matters in a Real Lake

As a freshwater lake cools in autumn, surface water becomes denser and tends to sink. This helps mix the lake as warmer water rises and cooler water moves downward.

When the surface temperature drops below approximately 4°C, the situation changes. That colder water becomes less dense and tends to remain near the surface, where it can eventually freeze.

Because ice floats, it forms a layer above the liquid water rather than collecting on the lakebed. The ice also slows heat loss from the water below, especially when snow covers its surface.

In sufficiently deep freshwater lakes, bottom water can remain near 4°C during winter, even when the surface is frozen. However, temperatures vary with lake depth, mixing, weather, and local conditions.

Lake Mendota in Wisconsin is one example of a lake that can develop winter ice cover while retaining liquid water beneath it.

This allows many fish and other aquatic organisms to survive the cold season, although oxygen shortages can still threaten aquatic life under prolonged ice cover.

The One You Can Test at Home

You can observe water's unusual behavior by placing an ice cube in a transparent glass of water. Most of the cube stays submerged, but a small portion rises above the surface because the ice is less dense than the liquid.

The expansion that occurs during freezing also explains why containers filled completely with water can break or split when placed in a freezer.

For a safer experiment, pour a small amount of water into a flexible, freezer-safe container, mark the water level, and leave plenty of empty space above it. After freezing, compare the volume of the ice with the original liquid level.

The same expansion can create problems in household plumbing. When water freezes inside a pipe, expanding ice can raise the pressure in the trapped liquid, sometimes causing the pipe to rupture.

Leaving room for expansion in suitable containers helps prevent damage when freezing liquids.

It's Not True for Everything

Water is unusual, but it is not the only substance that expands upon solidification.

Elements such as gallium, bismuth, and silicon can also occupy more volume in their solid forms than in their liquid forms under appropriate conditions.

Most familiar substances behave differently. Their particles generally pack more closely together when they solidify, so the solid is denser than the liquid.

For example, solid iron is denser than molten iron under ordinary casting conditions. This is one reason many metals contract as they cool and solidify.

Water's behavior is especially noticeable because it is so common in daily life. Floating ice cubes, frozen lakes, and expanding water in containers are all familiar consequences of the same molecular property.

Ice floats because freezing gives water molecules a more open arrangement, making the solid less dense than the liquid. It is an unusual property with consequences far beyond a glass of water.

By allowing lakes to freeze from the top down, floating ice helps preserve liquid water beneath the surface. The same expansion also explains why freezing water can damage containers and pipes.