A phase transition is a physical process where matter changes from one state, or phase, to another due to alterations in its surrounding conditions, such as temperature or pressure. This transformation involves a change in the physical properties of a substance, like its density, without changing its chemical identity. For example, liquid water, solid ice, and gaseous steam are all the same chemical substance, \(text{H}_2text{O}\), but they exist in different phases. Understanding the conditions at which these transformations occur helps define the boundaries between states on a phase diagram.
The Three Fundamental States of Matter
The most common states of matter—solid, liquid, and gas—are distinguished by the arrangement and energy of their constituent particles. In a solid, particles are tightly packed and held in fixed positions by strong attractive forces, though they still vibrate slightly. This highly ordered structure gives solids a definite shape and a fixed volume.
Liquids have particles that remain close together, but they possess enough energy to move around and slide past one another. The attractive forces are weaker than in a solid, allowing the substance to maintain a fixed volume while adopting the shape of its container. This ability to flow is what makes them fluids.
Gas particles have the highest amount of kinetic energy, causing them to be widely spaced and move rapidly and randomly. The intermolecular forces are extremely weak in this state. This allows the gas to expand to fill both the shape and volume of any container it occupies.
Energy and the Mechanism of Change
The transition between states is governed by the transfer of energy, typically heat, and the influence of external pressure. When energy is added to a substance, it first increases the kinetic energy of the particles, causing a rise in temperature. Once the substance reaches the temperature of a phase change, additional energy is used to break or form the intermolecular bonds holding the particles together.
This energy absorbed or released during a phase transition without causing a change in temperature is known as Latent Heat. For instance, when ice melts, the latent heat of fusion is used to overcome the forces locking the water molecules into the solid structure. The temperature remains constant at the melting point until all the ice has converted to liquid water.
Pressure is the second major factor affecting the temperatures at which these transitions happen. Increasing the external pressure on a liquid makes it more difficult for particles to escape into the gaseous phase, thereby increasing its boiling point. This principle is exploited in a pressure cooker, where increased pressure raises the water’s boiling temperature. Conversely, at high altitudes where atmospheric pressure is lower, water boils below \(100,^circtext{C}\).
Common Phase Transitions Explained
The six most common phase transitions describe the movement of a substance between the solid, liquid, and gaseous states. Transitions that require the absorption of heat, moving toward higher energy states, are called endothermic. Melting (fusion) is the transition from solid to liquid. Vaporization is the change from liquid to gas, which occurs as slow evaporation or rapid boiling.
The reverse transitions release heat energy and move toward lower energy states, known as exothermic. Freezing is the liquid-to-solid transition. Condensation is the change from gas to liquid, often seen when water vapor forms droplets on a cold surface.
Two transitions bypass the liquid phase entirely. Sublimation is the direct change from a solid to a gas, famously seen with dry ice (solid carbon dioxide). The reverse process is deposition, where a gas changes directly into a solid, such as when water vapor turns into frost.
Exotic Transitions and the Critical Point
Beyond the conventional solid, liquid, and gas phases, matter can exist in more exotic states that also undergo phase transitions. One phenomenon occurs at the critical point, a specific temperature and pressure above which a substance cannot exist as a distinct liquid or gas. Above this point, the substance transitions into a single, uniform state called a supercritical fluid (SCF).
A supercritical fluid possesses a density similar to a liquid but flows with the viscosity of a gas. This allows it to dissolve materials effectively while penetrating them easily. Supercritical carbon dioxide is used in industrial processes, such as removing caffeine from coffee beans.
Other exotic phases include plasma, the most abundant state of matter in the universe, which is an ionized gas found in stars. Materials cooled to near absolute zero can undergo quantum phase transitions to become superfluids (zero viscosity flow) or superconductors (zero resistance conduction).