Matter exists in distinct forms called phases or states of matter. A phase is defined by the unique physical properties a substance exhibits under specific conditions of temperature and pressure. Water, a ubiquitous and fundamental substance, is singular among common chemical compounds because it naturally exists in multiple states across the normal range of temperatures and pressures found on Earth’s surface. Understanding these states and how water moves between them is fundamental to comprehending natural phenomena.
The Three Primary Phases of Water
Water is commonly found in three main phases: solid, liquid, and gas. Each phase possesses a unique set of macroscopic properties that define its behavior.
Solid water, known as ice, has a fixed volume and a definite shape, meaning it maintains its form without a container. The molecules in ice are held in place, allowing the solid to resist changes to its shape and making it virtually incompressible.
Liquid water, the most familiar state, maintains a fixed volume but has an indefinite shape, taking on the form of its container. The molecules are close together but can move past one another, allowing the liquid to flow. Liquid water is almost entirely incompressible because there is very little empty space between the molecules.
Gaseous water, or steam/water vapor, is characterized by both an indefinite volume and an indefinite shape. The gas expands to completely fill any container it occupies. Water vapor is highly compressible because its molecules are widely spaced with large amounts of empty space between them.
How Phase Transitions Occur
The change from one phase of water to another is called a phase transition, and it requires the absorption or release of energy, typically in the form of heat. There are six primary phase transitions, each named for the direction of the change.
Transitions that absorb energy from the surroundings are called endothermic processes, such as melting (solid to liquid), vaporization (liquid to gas), and sublimation (solid to gas). Conversely, transitions that release energy into the surroundings are exothermic processes, which include freezing (liquid to solid), condensation (gas to liquid), and deposition (gas to solid). Deposition is responsible for the direct formation of frost from water vapor without first becoming liquid water.
During a phase transition, the added or removed energy is known as latent heat, or “hidden heat,” because it does not cause a change in temperature. Instead, this energy is used entirely to break or form the attractive forces holding the molecules together. When ice melts at 0°C, the latent heat of fusion breaks the rigid molecular structure without raising the temperature above 0°C until all the ice has turned into liquid water.
Water’s Unique Molecular Behavior
The unusual properties of water are governed by the structure and behavior of its molecules, specifically the presence of hydrogen bonds. The \(\text{H}_2\text{O}\) molecule is highly polar, meaning the oxygen atom pulls electrons toward itself, creating a slight negative charge near the oxygen and slight positive charges near the two hydrogen atoms. These opposite partial charges allow neighboring water molecules to form relatively strong intermolecular attractions called hydrogen bonds.
In liquid water, these hydrogen bonds are constantly breaking and reforming, allowing the molecules to move freely but remain relatively close together. As liquid water cools, the molecules slow down, and the hydrogen bonds begin to lock into a more stable, ordered arrangement.
This ordered structure in the solid phase, ice, is an open, crystalline lattice that creates more empty space between the molecules than is present in the liquid state. Because the same mass of water occupies a larger volume when frozen, solid water is less dense than liquid water, a rare property among substances.
This density anomaly is why ice floats, which is a life-sustaining phenomenon in nature. Floating ice insulates the water below and prevents entire bodies of water from freezing solid. Liquid water reaches its maximum density at about \(4^\circ\text{C}\) before the formation of this open structure begins to reduce its density as it cools further toward the freezing point.