Lecture Notes: Phase Diagrams for CO2 and Water

Introduction to Phase Changes

• Melting: Solid to liquid (e.g., ice to water)
• Freezing: Liquid to solid (e.g., water to ice)
• Vaporization: Liquid to gas
• Condensation: Gas to liquid (e.g., water vapor condensing on a cold glass)
• Sublimation: Solid directly to gas (e.g., dry ice or solid CO2)
• Deposition: Gas directly to solid (reverse of sublimation)

Phase Diagram for CO2

• Axes:
  • X-axis: Temperature
  • Y-axis: Pressure
• Phases:
  • Solid
  • Liquid
  • Gas
• Key Points:
  • Triple Point: Where solid, liquid, and gas phases coexist
  • Melting Point Line: Between solid and liquid (melting/freezing)
  • Boiling Point Curve: Between liquid and gas (vaporization/condensation)
  • At 1 atm, CO2 sublimes directly from solid to gas since the pressure is below the triple point.

Density Considerations for CO2

• Solid CO2 is denser than liquid CO2 at higher pressures (positive slope of melting point curve).

Critical Point

• Beyond this point, CO2 becomes a supercritical fluid with properties of both gas and liquid.
• A gas cannot be liquefied by pressure alone if temperature is above the critical temperature.

Phase Diagram for Water (H2O)

• Differences Compared to CO2:
  • Melting point line has a negative slope.
  • Standard pressure (1 atm) is above the triple point, allowing all three phases to be achieved by increasing temperature.
• Sublimation: Occurs if pressure is below the triple point.

Density Considerations for Water

• Liquid water is denser than ice (negative slope of melting point line).
• This explains why ice floats on water.

Critical Point for Water

• Supercritical fluid exists above the critical temperature and pressure.

Boiling and Melting Points

• Boiling Point: Varies along the curve; normal boiling point is at 1 atm.
• Normal Boiling Point: Specific point where pressure is 1 atm.
• Normal Melting Point: Occurs at 1 atm; part of the melting point curve.
• Terminology:
  • "Normal" refers to conditions at 1 atm pressure.