Phase diagrams show how substances change states under different temperatures and pressures. They're like maps that tell us when a material will be solid, liquid, or gas, and where these states meet at special points.

Supercritical fluids exist beyond a substance's critical point, blending liquid and gas 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

• 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 phase diagram 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
  • Melting/freezing curve separates solid and liquid regions
  • Vaporization/condensation curve separates liquid and gas regions
  • Sublimation/deposition curve separates solid and gas regions
• Unique point where all three phases (solid, liquid, and gas) coexist in equilibrium is called the triple point
• Highest temperature and pressure at which the liquid and gas phases can coexist is called the critical point
  • Above the critical point, the substance exists as a supercritical fluid
• Phase transitions occur when crossing a phase boundary by changing temperature or pressure
  • Melting is solid to liquid transition upon heating (ice melting)
  • Freezing is liquid to solid transition upon cooling (water freezing)
  • Vaporization is liquid to gas transition upon heating or pressure reduction (boiling water)
  • Condensation is gas to liquid transition upon cooling or pressure increase (steam condensing)
  • Sublimation is solid to gas transition upon heating or pressure reduction (dry ice)
  • Deposition is gas to solid transition upon cooling or pressure increase (frost formation)

Components of phase diagrams

• Axes of a phase diagram
  • 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
  • Triple point 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
  1. Supercritical fluid extraction (SFE) used to extract valuable compounds from natural sources (caffeine from coffee beans, essential oils from plants)
  2. Supercritical fluid chromatography (SFC) combines the advantages of gas and liquid chromatography for separation
  3. Supercritical fluid cleaning removes contaminants from delicate materials (electronic components, historical artifacts)
  4. Supercritical fluid synthesis produces nanoparticles and other materials with unique properties
• Common supercritical fluids
  • Carbon dioxide ($CO_2$) 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 ($H_2O$) 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
• Gibbs free energy determines the stability of phases under different conditions
• The phase rule relates the number of components, phases, and degrees of freedom in a system