10.4 Phase Diagrams
3 min read•june 25, 2024
Phase diagrams show how substances change states under different temperatures and pressures. They're like maps that tell us when a material will be , , or , and where these states meet at special points.
Supercritical fluids exist beyond a substance's , blending and properties. They're super useful for extracting things like caffeine from coffee beans or cleaning delicate electronics, thanks to their unique ability to dissolve and penetrate.
Phase Diagrams
Interpretation of phase diagrams
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- Graphical representations of the physical states of a substance under different conditions of temperature and pressure
- x-axis represents temperature (Kelvin or ℃)
- y-axis represents pressure (atmospheres or pascals)
- Distinct regions on the represent stable phases
- Solid phase exists in low temperature and high pressure region
- Liquid phase exists in moderate temperature and pressure region
- Gas phase exists in high temperature and low pressure region
- Lines separating different phase regions are called phase boundaries
- / curve separates solid and liquid regions
- / curve separates liquid and gas regions
- / curve separates solid and gas regions
- Unique point where all three phases (solid, liquid, and gas) coexist in is called the
- Highest temperature and pressure at which the liquid and gas phases can coexist is called the
- Above the critical point, the substance exists as a
- Phase transitions occur when crossing a by changing temperature or pressure
- is solid to liquid transition upon heating (ice melting)
- is liquid to solid transition upon cooling (water freezing)
- is liquid to gas transition upon heating or pressure reduction (boiling water)
- is gas to liquid transition upon cooling or pressure increase (steam condensing)
- is solid to gas transition upon heating or pressure reduction (dry ice)
- is gas to solid transition upon cooling or pressure increase (frost formation)
Components of phase diagrams
- Axes of a
- x-axis represents temperature, typically in Kelvin (K) or degrees Celsius (℃)
- y-axis represents pressure, usually in atmospheres (atm) or pascals (Pa)
- Phase regions in a phase diagram
- Solid region exists in low temperature and high pressure area
- Liquid region exists in moderate temperature and pressure area
- Gas region exists in high temperature and low pressure area
- Phase boundaries in a phase diagram
- Melting/freezing curve separates solid and liquid regions
- Vaporization/condensation curve separates liquid and gas regions
- Sublimation/deposition curve separates solid and gas regions
- Special points in a phase diagram
- is where all three phase boundaries intersect
- Critical point is the endpoint of the vaporization/condensation curve
- Phase boundaries represent equilibrium conditions between two phases
Supercritical Fluids
Properties of supercritical fluids
- Substances that exist above their critical temperature and pressure
- Exhibit properties intermediate between those of liquids and gases
- Have high density similar to liquids (water)
- Have low viscosity and high diffusivity similar to gases (helium)
- Unique properties of supercritical fluids
- Tunable density can be adjusted by changing temperature or pressure
- Enhanced solvent power can dissolve substances that are typically insoluble in either liquid or gas phase (oils)
- Low surface tension allows for better penetration and wetting of surfaces
- Applications of supercritical fluids
- extraction (SFE) used to extract valuable compounds from natural sources (caffeine from coffee beans, essential oils from plants)
- Supercritical fluid chromatography (SFC) combines the advantages of gas and liquid chromatography for separation
- Supercritical fluid cleaning removes contaminants from delicate materials (electronic components, historical artifacts)
- Supercritical fluid synthesis produces nanoparticles and other materials with unique properties
- Common supercritical fluids
- Carbon dioxide () has critical point at 31.1℃ and 73.8 atm, widely used due to its low cost, non-toxicity, and easy removal from products
- Water () has critical point at 374℃ and 218 atm, used in hydrothermal synthesis and oxidation processes
Thermodynamic Principles in Phase Diagrams
- Phase diagrams are based on thermodynamic principles governing the behavior of substances
- determines the stability of phases under different conditions
- The relates the number of components, phases, and degrees of freedom in a system
Key Terms to Review (34)
Amorphous solids: Amorphous solids are non-crystalline materials where the atoms and molecules are not arranged in a definite lattice pattern. They lack long-range order, distinguishing them from crystalline solids.
Chemical thermodynamics: Chemical thermodynamics studies the interrelation of heat and work with chemical reactions or physical changes. It applies principles of thermodynamics to predict the direction and extent of chemical processes.
Condensation: Condensation is the phase transition from gas to liquid. It occurs when a vapor cools and loses enough thermal energy to change state.
Condensation: Condensation is the process by which a gas or vapor transitions into a liquid state. It is a fundamental phase transition that occurs when a substance cools and the kinetic energy of its molecules decreases, allowing them to form a more condensed liquid phase.
Critical point: The critical point is the end point of a phase equilibrium curve, where the properties of gas and liquid phases become indistinguishable. It represents the highest temperature and pressure at which a substance can exist as a liquid and gas in equilibrium.
Critical Point: The critical point is a unique point on a phase diagram where the distinct liquid and gas phases of a substance merge into a single, homogeneous supercritical fluid phase. At the critical point, the properties of the liquid and gas phases become indistinguishable, marking the end of the phase transition between the two states.
Deposition: Deposition is the phase transition in which a gas turns directly into a solid, bypassing the liquid state. This process releases energy and occurs under specific temperature and pressure conditions.
Deposition: Deposition is the process by which a substance is deposited or laid down, typically from a state of suspension or vapor, onto a surface or into a solution. This term is particularly relevant in the context of phase transitions and phase diagrams, as it describes the direct transition from a gaseous state to a solid state without an intermediate liquid phase.
Equilibrium: Equilibrium is a state of balance or stability in a system, where the opposing forces or processes are in a state of dynamic balance. It is a fundamental concept that underpins various aspects of chemistry, including phase changes, chemical reactions, and thermodynamic processes.
Freezing: Freezing is the phase transition from the liquid state to the solid state. It occurs when a liquid's temperature is lowered below its freezing point.
Freezing: Freezing is the physical process by which a liquid transitions into a solid state due to the removal of thermal energy. It is a crucial phase transition that is central to understanding the behavior of substances in various contexts, particularly in the study of phase diagrams.
Gas: Gas is a state of matter characterized by having neither a fixed shape nor a fixed volume. Gas particles are in constant random motion and are widely spaced compared to liquids and solids.
Gas: A gas is one of the four fundamental states of matter, characterized by its ability to expand and fill any container, having no fixed shape or volume, and being easily compressible. Gases are essential in the study of chemistry, particularly in the context of phases and phase diagrams.
Gibbs Free Energy: Gibbs free energy is a thermodynamic property that combines the concepts of enthalpy and entropy to determine the spontaneity and feasibility of a chemical process. It is a crucial factor in understanding the driving forces behind chemical reactions and phase changes.
Gibbs free energy (G): Gibbs free energy (G) is a thermodynamic potential that measures the maximum reversible work obtainable from a system at constant temperature and pressure. It is used to predict the direction of chemical reactions.
Liquid: A liquid is a state of matter characterized by a definite volume but no fixed shape, conforming to the shape of its container. It has particles that are close together but can move past each other, allowing for fluidity.
Liquid: A liquid is a state of matter characterized by the ability to flow and take the shape of the container it occupies, while maintaining a fixed volume. Liquids have a high density compared to gases, and they are generally incompressible.
Melting: Melting is the phase transition from a solid to a liquid. It occurs when a substance absorbs sufficient heat to overcome its molecular forces.
Melting: Melting is the phase transition that occurs when a solid substance is heated, causing it to transform from a rigid, crystalline state into a liquid state. This change in phase is driven by the increase in kinetic energy of the atoms or molecules within the material, overcoming the intermolecular forces that hold the solid structure together.
Phase Boundary: A phase boundary is the interface or dividing line between two distinct phases of a substance, such as solid, liquid, and gas. It represents the conditions, like temperature and pressure, at which a phase transition occurs, and the phases coexist in equilibrium.
Phase diagram: A phase diagram is a graphical representation that shows the conditions of temperature and pressure under which distinct phases (solid, liquid, gas) of a substance exist. It illustrates the equilibrium between different states of matter.
Phase Diagram: A phase diagram is a graphical representation that shows the relationship between the physical states or phases of a substance, such as solid, liquid, and gas, as a function of variables like temperature and pressure. It provides a comprehensive overview of the conditions under which a substance can exist in different phases and the boundaries between these phases.
Phase Rule: The phase rule is a fundamental principle in physical chemistry that describes the relationship between the number of phases, components, and degrees of freedom in a thermodynamically stable system at equilibrium. It provides a way to predict the number of independent variables that can be manipulated without changing the number of phases present.
Phase Transition: A phase transition is a transformation of a substance from one physical state or phase to another, such as the transition from a solid to a liquid or from a liquid to a gas. These changes in phase are driven by changes in temperature, pressure, or other external conditions and involve the rearrangement of the molecular structure of the substance.
Solid: A solid is one of the fundamental states of matter, characterized by structural rigidity and resistance to changes in shape or volume. Solids have a defined shape and volume, and their particles are closely packed together with minimal movement.
Sublimation: Sublimation is the phase transition in which a substance changes directly from a solid to a gas without passing through the liquid state. This process occurs under specific conditions of temperature and pressure.
Sublimation: Sublimation is the direct transition of a substance from the solid phase to the gas phase without passing through the liquid phase. This phase change occurs when the vapor pressure of the solid is equal to the surrounding pressure, allowing the solid to transform directly into a gas.
Supercritical fluid: A supercritical fluid is a state of matter that occurs when a substance is above its critical temperature and pressure, where distinct liquid and gas phases do not exist. It exhibits unique properties that are intermediate between those of gases and liquids.
Supercritical Fluid: A supercritical fluid is a state of matter that occurs when a substance is heated and pressurized beyond its critical point, where the distinction between the liquid and gas phases disappears. In this state, the fluid exhibits properties of both liquids and gases, making it a versatile medium for various applications.
Thermodynamics: Thermodynamics is the branch of physics that deals with the relationships between heat, work, temperature, and energy. It describes the fundamental physical laws governing the transformation of energy and the flow of heat, which are essential to understanding the behavior of chemical systems and processes.
Triple point: The triple point is the unique set of conditions at which all three phases (solid, liquid, and gas) of a substance coexist in thermodynamic equilibrium. It is specific to each substance and occurs at a precise temperature and pressure.
Triple Point: The triple point is the unique temperature and pressure at which the solid, liquid, and gaseous phases of a substance can coexist in equilibrium. It is a critical point on a substance's phase diagram, where the three phases converge.
Vaporization: Vaporization is the phase transition from the liquid phase to the gas phase. It occurs when molecules in a liquid gain enough energy to overcome intermolecular forces and enter the vapor phase.
Vaporization: Vaporization is the process by which a substance in the liquid or solid state transitions to the gaseous state. It is a fundamental phase transition that occurs when the vapor pressure of a substance exceeds the surrounding atmospheric pressure, allowing the molecules to escape the liquid or solid and enter the gas phase.