Phase Diagrams, the Clapeyron Equation and the Phase Rule · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.
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Reading the map of phases

Science · Chemistry · ages 20-22
Name ______________________   Date ____________

  1. What is the critical point?

    • The end of the liquid-gas line, beyond which lies supercritical fluid
    • The point where ice melts at normal pressure
    • The point of lowest vapour pressure

  2. What is the triple point?

    • The end of the liquid-gas line
    • The single point where solid, liquid, and gas coexist
    • The lowest temperature on the diagram

  3. Ice is less dense than liquid water.

    Circle one:   True   False

  4. A system has 2 components and 3 phases. How many variables are free? Type the number.

    Answer: ______________

  5. Why does the melting line of water slope backwards?

    • Because water boils at a high temperature
    • Because the triple point sits at low pressure
    • Because ice is less dense than liquid water

  6. When the Clapeyron equation is turned into the Clausius-Clapeyron form, which assumption brings ln p into the equation?

    • That the liquid volume is exactly zero at all temperatures
    • That the vapour behaves as an ideal gas
    • That the enthalpy never changes with temperature

  7. Two labs report three coexisting phases of pure water at the same temperature but different pressures. What must be wrong?

    • At least one pressure reading is in error
    • Triple points always span a pressure range
    • Pure water cannot show three phases

  8. A log vapour pressure plot against inverse temperature is straight with a negative slope. What does the slope give?

    • The enthalpy of vaporisation, up to constants
    • The triple point pressure directly
    • The number of free variables
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Reading the map of phases W1-mt_U-zj6r4WFM-s1

  1. The end of the liquid-gas line, beyond which lies supercritical fluid · Past it, liquid and gas merge into one fluid.
  2. The single point where solid, liquid, and gas coexist · All three phases meet at exactly one temperature and pressure.
  3. True · That odd density flips the slope of the melting line.
  4. 1 · Freedom is components plus two minus phases: 2 plus 2 minus 3.
  5. Because ice is less dense than liquid water · Equal potentials favour the denser liquid when squeezed.
  6. That the vapour behaves as an ideal gas · The ideal gas law replaces the vapour volume with RT over p. Dividing through by that p gives d(ln p).
  7. At least one pressure reading is in error · Zero freedom fixes both temperature and pressure, so one report must err.
  8. The enthalpy of vaporisation, up to constants · The slope encodes the enthalpy through the simplified Clapeyron form.
Worksheet · LightMySky