How Does Water Behave When It Freezes: The Science of Ice

Water expands as it freezes. The resulting ice is less dense than the liquid water it came from, so it floats. This unusual behavior comes from the way water molecules arrange themselves in ice: they form an open structure with more space between them than in liquid water.

The change begins before water reaches its freezing point. As water cools from room temperature to about 4 °C, it contracts and becomes denser. At roughly 4 °C, it reaches its maximum density. Below that temperature, further cooling makes it expand. When it freezes under ordinary conditions, its volume increases by about 9% (USGS Water Science School).

The 9% figure describes the change in volume, not a loss of mass. The same amount of water takes up more room as ice, so its density decreases. This is why ice floats on liquid water.

Why ice takes up more space

A water molecule has one oxygen atom and two hydrogen atoms. The molecule has a bent shape, with uneven electrical charges that let it attract nearby water molecules. These attractions, called hydrogen bonds, are constantly forming and breaking in liquid water.

As water freezes, the molecules settle into a more ordered arrangement. In ordinary ice, each molecule bonds with four neighbors in an open, roughly hexagonal crystal structure. The molecules are held farther apart than they are, on average, in liquid water. That extra space makes ice less dense.

The molecular picture is more complex than a set of fixed groups that suddenly snap into place. Liquid water remains in motion, and its molecules form changing arrangements. Freezing occurs when a stable ice crystal begins to grow and the surrounding molecules join its ordered structure.

Ice crystals often have six-fold symmetry because of the way the molecules are arranged. Snowflakes can develop six arms, though their shapes vary with the conditions as they grow. The exact form of a snowflake is not a simple copy of a single ice crystal: temperature and moisture affect its growth.

What happens as water cools

Water’s maximum density near 4 °C affects how it moves as it cools. Water near that temperature is denser than colder water, so it tends to sink. Water cooled below about 4 °C becomes less dense and tends to remain above it. This process can help keep a body of water from freezing all the way through at once.

Example: a freshwater lake in winter. As the lake’s surface loses heat, water cools and moves downward until it nears 4 °C. Once surface water cools below that temperature, it becomes less dense and stays closer to the top. If the surface reaches its freezing point, ice forms there and floats. The ice can slow heat loss from the water below, while deeper water may remain liquid. The exact pattern depends on the lake and local conditions.

This surface-first freezing is one reason aquatic life can survive winter in some lakes. The ice cover does not guarantee that every part of a lake stays at the same temperature, or that every lake remains liquid below its surface. It describes a common pattern for freshwater under ordinary conditions.

Freezing point and other conditions

Pure water freezes at about 0 °C under ordinary atmospheric pressure. In practice, dissolved substances can change the freezing point. Salt lowers it, so seawater freezes at a lower temperature than pure water. Small droplets can also remain liquid below 0 °C if ice crystals have not begun to form. This state is called supercooling.

Pressure can affect the melting point of ice, too. For ordinary ice near the conditions found at Earth’s surface, greater pressure favors the denser liquid phase and can lower the melting point. The effect depends on the type of ice and the conditions, so it is not a general explanation for every case of melting.

The familiar 4 °C maximum-density point and roughly 9% expansion refer to water near ordinary surface pressure. Salts and other dissolved substances, as well as pressure and confinement in very small spaces, can change how water freezes. Those conditions do not change the basic reason ordinary ice floats: its crystal structure leaves more space between molecules.

Key Takeaways

• Water is densest at about 4 °C. Below that temperature, it expands as it cools.
• When water freezes under ordinary conditions, its volume increases by about 9%, while its mass stays the same.
• Ice floats because its open crystal structure makes it less dense than liquid water (USGS Water Science School).
• In a freshwater lake, colder water near the freezing point can remain at the surface, where ice forms first.
• Dissolved substances and pressure can affect freezing and melting.