4.2: Phases of Water

Overview 

Water consists of molecules made from two hydrogen atoms and one oxygen atom. Because of how these atoms are positioned, the molecule often resembles the silhouette of a famous cartoon mouse, where the larger oxygen atom serves as the head and the smaller hydrogen atoms act as the ears. These molecules stay together by forming bonds with the hydrogen atoms of the molecules next to them. Depending on how much energy is present, water can exist as a solid, a liquid, or a gas.

In its solid phase, such as ice, water molecules are organized into a specific, rigid structure. While they do not move around one another, they are not completely still; instead, they vibrate around a fixed position. This state represents the lowest amount of kinetic energy. When water is a liquid, the kinetic energy increases, allowing molecules to move more freely in clusters. While they can circulate throughout the fluid, they remain held together by a defined surface. In the gaseous state, known as water vapor, the molecules reach their highest level of kinetic energy. At this stage, the molecules move rapidly and independently, filling whatever space is available without being restricted by a surface or bound to each other.

Figure \(\PageIndex{1}\): Shows how phase changes of water involve energy transfer, explaining how heat is absorbed or released as matter changes state in natural systems. (CC BY-SA 4.0; Sohini Dutt via AI Image Generated by Gemini).

 Figure \(\PageIndex{1}\) shows the three states of water—solid, liquid, and gas—and the phase changes that occur between them. It also indicates whether latent heat is absorbed or released during each change.

The solid state, represented by ice, is shown as tightly packed particles. The liquid state, represented by water, is shown as more loosely arranged particles in a container. The gas state, represented by water vapor, is shown as widely spaced particles moving freely. These particle diagrams illustrate that water molecules are packed most closely in the solid state and are most spread out in the gas state.

An arrow from liquid to gas is labeled evaporation and indicates that latent heat is absorbed. This means energy is taken in as liquid water changes to water vapor.

An arrow from gas to liquid is labeled condensation and indicates that latent heat is released. This means energy is given off as water vapor changes back to liquid water.

An arrow from solid to liquid is labeled melting and indicates that latent heat is absorbed. Energy is required for ice to become liquid water.

An arrow from liquid to solid is labeled freezing and indicates that latent heat is released. Energy is released as liquid water becomes ice.

An arrow from solid directly to gas is labeled sublimation and indicates that latent heat is absorbed. This process skips the liquid phase.

An arrow from gas directly to solid is labeled deposition and indicates that latent heat is released. This process also skips the liquid phase.

For water to transition from a solid state into a liquid and eventually a gas, it must absorb external energy. This thermal energy is used to disrupt the hydrogen bonds that hold the water molecules in a structured network. When these bonds are sufficiently weakened in ice, the solid structure collapses, allowing the molecules to move past one another as a liquid. The specific amount of heat needed to trigger this phase change from ice to liquid water—without increasing the temperature—is known as the latent heat of fusion. For every gram of ice being converted into water, approximately 80 calories of energy must be added.