Physical transformations of pure substances
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Physical Transformations of Pure Substances. Chapter 4. Stabilities of Phase. A phase of a substance is a form of matter that is uniform throughout in chemical composition and physical state. A phase transition is the spontaneous conversion of one phase into another.

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Stabilities of phase
Stabilities of Phase

  • A phase of a substance is a form of matter that is uniform throughout in chemical composition and physical state.

  • A phase transition is the spontaneous conversion of one phase into another.

  • Phase transitions occur at a characteristic temperature and pressure.


Stabilities of phase1
Stabilities of Phase

  • At 1 atm, < 0 °C, ice is the stable phase of H2O, but > 0 °C, liquid water is the stable phase.

  • The transition temperature, Ttrs, is the temperature at which two phases are in equilibrium.

  • So what happens to Gibbs energy?


Stabilities of phase2
Stabilities of Phase

  • At 1 atm, < 0 °C, ice is the stable phase of H2O, but > 0 °C, liquid water is the stable phase.

  • The transition temperature, Ttrs, is the temperature at which two phases are in equilibrium.

  • So what happens to Gibbs energy?

  • < 0 °C Gibbs energy decreases as liquid  solid.

  • > 0 °C Gibbs energy decreases as solid  liquid.


Stabilities of phase3
Stabilities of Phase

  • Thermodynamics does not provide information regarding the rate of phase change.

  • Diamond  graphite

  • Thermodynamically unstable phases that persist due to slow kinetics are called metastable phases.


Phase diagrams
Phase Diagrams

  • Phase boundaries show the values of p and T at which two phases coexist in equilibrium.


Vapor pressure
Vapor Pressure

  • The pressure of a vapor in equilibrium with a liquid is called the vapor pressure.

  • The pressure of a vapor in equilibrium with a solid is called the sublimation vapor pressure.


Boiling point
Boiling Point

  • Liquid can vaporize from a liquid surface below it’s boiling point – as we learnt from the Drinking Bird.

  • In an open vessel, the temperature at which the vapor pressure equals the external pressure, is called the boiling temperature.

  • At 1 atm, it’s called the normal boiling temperature, Tb.

  • At 1 bar, it’s called the standard boiling point.

  • Normal point of H2O is 100.0 °C; it’s standard boiling point is 99.6 °C.


Critical point
Critical Point

  • In a closed rigid vessel, boiling does not occur.

  • As the temperature is raised the density of vapor increases and the density of the liquid decreases.

  • When the density of the vapor and liquid phases are equal the surface between the two phases disappears.

  • The temperature at which this occurs is called the critical temperature, Tc.

  • The vapor pressure at the critical temperature is called the critical pressure, pc.



Melting and freezing
Melting and Freezing

  • The temperature at which, under a specified pressure, the liquid and solid phases of a substance coexist in equilibrium is called them melting temperature.

  • The freezing temperature is the same as the melting point.

  • At 1 atm, the freezing temperature is called the normal freezing point, Tf.

  • At 1 bar, it’s called the standard freezing point.

  • The difference is negligible in most cases.

  • The normal freezing point is also called the normal melting point.


Triple point
Triple Point

  • There is a set of conditions under which three different phases of a substance (typically solid, liquid and vapor) all simultaneously coexist in equilibrium.

  • This point is called the triple point.

  • For any pure substance the triple point occurs only at single definite pressure and temperature.

  • The triple point of water lies at 273.16 K and 611 Pa.


Triple point1
Triple Point

  • The triple point marks the lowest pressure at which a liquid phase can exist.





Thermodynamics of phase transitions
Thermodynamics of Phase Transitions

  • The molar Gibbs energy, Gm, is also called chemical potential, m. Phase transitions will be investigated primarily considering the change in m.

  • Thermodynamic definition of equilibrium: At equilibrium the chemical potential of a substance is the same throughout the sample, regardless of how many phases are present.



Thermodynamics of phase transitions2
Thermodynamics of Phase Transitions

  • At low temperatures, and provided the pressure is not too low, the solid phase of a substance has the lowest chemical potential and is therefore the most stable.

  • Chemical potentials change with temperature: this explains why different phases exist.


Temperature dependence of phase transitions
Temperature Dependence of Phase Transitions

  • As temperature increases, chemical potential decreases.


Melting and applied pressure
Melting and Applied Pressure

  • Molar volume of solid is smaller than that of the liquid.


Melting and applied pressure1
Melting and Applied Pressure

  • Molar volume of solid is greater than that of the liquid.



Melting and applied pressure3
Melting and Applied Pressure

  • Calculate the effect on the chemical potentials of ice and water of increasing pressure from 1.00 to 2.00 bar at 0 °C. The density of ice is 0.917 g cm-3 and that of liquid water is 0.999 g cm-3.


Melting and applied pressure4
Melting and Applied Pressure

  • Calculate the effect on the chemical potentials of ice and water of increasing pressure from 1.00 to 2.00 bar at 0 °C. The density of ice is 0.917 g cm-3 and that of liquid water is 0.999 g cm-3.


Vapor pressure and applied pressure
Vapor Pressure and Applied Pressure

  • When pressure is applied to a condensed phase, its vapor pressure rises.

  • This is interpreted as molecules get squeezed out of the condensed phase and escape as a gas.



Location of phase boundaries
Location of Phase Boundaries

  • Locations of phase boundaries – pressures and temperatures - can be located precisely by making use of the fact that at when two phases are in equilibrium, their chemical potentials must be equal














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