Phase Changes

• Solid to Liquid: Melting

  • Example: Ice melting into water.

• Liquid to Solid: Freezing

  • Example: Water turning into ice.

• Liquid to Gas: Vaporization

• Gas to Liquid: Condensation

  • Example: Water droplets forming on a cold glass in humid air.

• Solid to Gas (skipping liquid): Sublimation

  • Example: Dry ice (solid CO2) turning directly into gas.

• Gas to Solid: Deposition

Phase Diagrams

CO2 Phase Diagram

• Axes: Temperature (x-axis), Pressure (y-axis)

• Regions: Solid, Liquid, Gas phases

• Triple Point: Point where solid, liquid, and gas phases coexist.

• Melting Point Line: Transition from solid to liquid (melting) and liquid to solid (freezing).

• Boiling Point Curve: Transition from liquid to gas (vaporization) and gas to liquid (condensation).

• At 1 atm, CO2 sublimes from solid to gas due to pressure being below the triple point.

• Density Comparison:

  • Increasing pressure leads to increased density.

  • In CO2, solid has greater density than liquid at higher pressures due to the melting point curve having a positive slope.

• Critical Point: Beyond this, substances exist as a supercritical fluid, having properties of both gas and liquid.

  • A gas above the critical temperature cannot be liquefied, only transitions to a supercritical state.

Water Phase Diagram

• Similar axes as CO2, but the melting point line has a negative slope meaning:

  • At 1 atm, ice melts into water as temperature increases.

• Triple Point: Standard pressure is above the triple point.

  • All three phases of H2O can be accessed by increasing temperature at 1 atm.

• Sublimation of ice to gas possible if below the triple point.

• Density Comparison:

  • Liquid water has a higher density than ice, causing ice to float.

• Normal Boiling Point vs Boiling Point:

  • Normal boiling point occurs at 1 atm; varies along the boiling point curve.

• Normal Melting Point: Occurs at 1 atm on the melting point curve.