What Form Do Things Take When They Freeze? The Science of Solids

When things freeze, they become solids. Most form crystalline solids, in which particles are arranged in a repeating pattern. Some form amorphous solids, whose particles lack long-range repeating order. Which form results depends on the substance and the conditions under which it cools.

What freezing means

Freezing is the change from a liquid to a solid as a substance loses thermal energy. For a pure substance at a given pressure, its freezing point is the same temperature as its melting point. At that temperature, solid and liquid can coexist; further cooling favors the solid.

The substance’s chemical identity does not change during freezing. Water, for example, remains H₂O. Its molecules become less free to move past one another and settle into positions in a solid structure. They still vibrate around those positions.

As a liquid cools, its particles generally move more slowly. When the solid phase is favored, particles form or join a solid structure. The forces that hold them in place vary by substance and may include attractions between molecules or electrical forces between ions.

Crystalline and amorphous solids

In a crystalline solid, particles follow an ordered pattern that repeats through the material. A crystal’s outer shape may not look geometric, but its internal structure is ordered.

Many crystalline solids melt over a relatively narrow temperature range. Amorphous solids, by contrast, lack long-range repeating order and may soften over a range of temperatures rather than melt at one sharp point. Glass is a familiar example.

The distinction is about internal structure, not simply hardness or appearance. Crystalline solids can be hard or soft, and amorphous solids can also have a rigid shape.

Cooling conditions can affect whether a material crystallizes. Slower cooling may give particles more time to organize; rapid cooling may limit that opportunity. The outcome also depends on the substance’s composition and surrounding conditions.

Freezing a liquid is different from forming crystals from a solution. During freezing, the liquid itself becomes solid. During crystallization from a solution, dissolved particles form crystals as they leave the liquid mixture. Both processes can produce ordered solids, but they are not the same process. See Encyclopaedia Britannica’s entry on freezing.

One example: water freezing into ice

At standard atmospheric pressure, pure water freezes at 0 °C. As it freezes, its molecules form a crystalline structure held together by hydrogen bonds. In ordinary ice, this arrangement leaves more space between molecules than in liquid water. Ice is therefore less dense than liquid water and floats, as the USGS Water Science School explains.

Water illustrates how a substance’s particles and their interactions affect the solid it forms. Other materials can have different structures and behave differently. For an introduction to crystalline ordering in phase changes, see Chemistry LibreTexts.

Key Takeaways

• Things become solids when they freeze. Most form crystalline solids with repeating particle patterns.
• Some form amorphous solids, which lack long-range repeating order.
• Freezing reduces particles’ freedom to move, but they continue to vibrate in a solid.
• Cooling conditions can affect crystallization, along with the substance’s composition.
• Ice is crystalline and less dense than liquid water.