One-Component Systems & Phase Diagrams

• Chapter focuses on one-component systems and their phase diagrams.

One-Component Systems

• Only one component is present throughout all phases in a one-component system.
• Pressure is produced by that component's vapor or mechanical means if no vapor exists.
• Phase diagrams are constructed using vapor pressure curves for liquid and solid, melting point curves as a function of pressure, solid-liquid-vapor equilibrium data, and solid-phase transition data.
• In a one-component system in equilibrium, the number of phases can be one, two, or three.
• Using the Phase Rule (C=1):
  • If P=1 and F=2 the system is bivariant.
  • If P=2 and F=1 the system is univariant.
  • If P=3 and F=0 the system is invariant.

The Water System

• The water system diagram is divided into solid (ice), liquid (water), and vapor areas, each containing only one phase.
• Pressure and temperature can vary independently without a second phase appearing when only one phase is present.

Two Phases in Water System Equilibrium

• Areas in the phase diagram are bounded by AC, AD, and AB curves, where two phases are in equilibrium.
• On curve AC, saturated vapor pressure of water has a fixed value in equilibrium with water, making the system univariant.
• The extension of curve CA to point E shows metastable equilibrium between supercooled water and its vapor.
• Line AD indicates that increased pressure lowers ice's melting point with the true melting point of ice being 0.0023°C at 760 mm pressure.

Three Phases in Water System Equilibrium

• At Point A (the Triple Point), solid, liquid, and vapor are in equilibrium.
• The system at the triple point is invariant at 0.0098°C and 4.58 mm pressure.
• F=0, P=3 and C=1 at the triple point.
• The triple point is a fundamental constant for water and cannot be changed.

Carbon Dioxide System

• The pressure-temperature diagram for carbon dioxide is similar to water.
• The slope of line AD shows that increased pressure increases the melting point.
• Application of carbon dioxide in fire extinguishers represents non-equilibrium conditions.
• Cylinders are filled with liquefied carbon dioxide gas that solidifies rapidly upon release before subliming.

The Sulphur System

• Solid Sulphur exists in polymorphic or allotropic forms.
• Rhombic Sulphur is stable at room temperature.
• Slow heating to 95.5°C converts Rhombic Sulphur to monoclinic.
• Under suitable conditions, both forms of Sulphur can exist in equilibrium with Sulphur vapor (point B); these forms are enantiotropic.
• Upon cooling, monoclinic Sulphur converts to the rhombic form in 24 hours.
• The Sulphur system diagram is divided into rhombic, liquid, monoclinic, and vapor areas. Four phases cannot coexist in equilibrium (C=1, P=4, F=-1, which is impossible).
• Four triple points (A, B, C, D) represent phase combinations.
• At point A, liquid, rhombic Sulphur, and vapor may coexist.
• The metastable triple point A is observed when rapid heating does not allow the transition from rhombic to monoclinic Sulphur at normal temperature.
• Six two-phase equilibria include Rhombic/monoclinic (curve BD), Rhombic/vapor (curve GBA), Rhombic/liquid (curve ADE), Monoclinic/liquid (curve CD), Monoclinic/vapor (curve BC), and Liquid/vapor (curve ACF).