Phase Changes and Diagrams

Overview

This lecture covers phase changes and phase diagrams of CO₂ (carbon dioxide) and H₂O (water), highlighting key points about phase boundaries, critical points, and density relationships.

Phase Changes

• Solid to liquid is called melting.
• Liquid to solid is called freezing.
• Liquid to gas is vaporization.
• Gas to liquid is condensation.
• Solid to gas is sublimation.
• Gas to solid is deposition.

CO₂ Phase Diagram

• Phase diagrams show temperature (x-axis) and pressure (y-axis).
• Solid, liquid, and gas regions are separated by phase boundaries.
• The triple point is where all three phases coexist.
• The line between solid and liquid is the melting point curve.
• The line between liquid and gas is the boiling point curve.
• At 1 atm, CO₂ sublimes (solid to gas) because this pressure is below the triple point.
• The melting point curve for CO₂ has a positive slope, so solid CO₂ is denser than liquid CO₂.
• The critical point defines the start of the supercritical fluid, which has properties of both gas and liquid.
• Above the critical temperature, CO₂ cannot liquefy by pressure alone.

H₂O Phase Diagram

• Water's melting point curve has a negative slope, unlike CO₂'s positive slope.
• At 1 atm (above the triple point), heating ice leads to melting, then vaporization.
• Below the triple point, heating solid water leads to sublimation.
• For water, liquid is denser than solid, so ice floats on liquid water.
• The critical point marks the boundary for supercritical water.
• The boiling point curve shows various boiling points at different pressures; the normal boiling point is at 1 atm.
• The normal melting point also occurs at 1 atm.

Key Terms & Definitions

• Melting — solid to liquid phase change.
• Freezing — liquid to solid phase change.
• Vaporization — liquid to gas phase change.
• Condensation — gas to liquid phase change.
• Sublimation — solid directly to gas phase change.
• Deposition — gas directly to solid phase change.
• Triple Point — condition where solid, liquid, and gas coexist.
• Critical Point — temperature and pressure above which a substance forms a supercritical fluid.
• Supercritical Fluid — state with properties of both liquid and gas.
• Normal Boiling Point — boiling point at 1 atm pressure.
• Normal Melting Point — melting point at 1 atm pressure.

Action Items / Next Steps

• Review phase diagrams for CO₂ and H₂O.
• Practice identifying regions and boundary lines on phase diagrams.
• Know definitions and be ready to explain phase changes.