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Chemistry · Practice book · Senior 5 · Unit 10 of 15

Phase diagrams

Senior 5 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 10 Phase diagrams

Key unit competence: Interpret phase diagrams of different compounds to explain their relevance in real-life situations.

Key ideas

  • A phase is a part of a system that is uniform in composition and physical state (solid, liquid, gas). Phase equilibrium exists when two or more phases coexist with no net change (e.g. ice ⇌ water at 0 °C).
  • Homogeneous equilibrium: all species in one phase; heterogeneous: species in more than one phase (e.g. CaCO3(s) ⇌ CaO(s) + CO2(g)).
  • A phase diagram shows the phases present at different temperatures and pressures. Lines show conditions where two phases are in equilibrium (melting/fusion curve, boiling/vaporisation curve, sublimation curve).
  • The triple point is where solid, liquid and gas coexist (water: 0.01 °C, 0.006 atm; CO2: −56.6 °C, 5.1 atm). The critical point is the end of the liquid–gas line; above it there is a supercritical fluid (water: 374 °C, 218 atm; CO2: 31 °C, 73 atm).
  • Water is unusual: its solid–liquid line slopes slightly to the LEFT (negative slope) because ice is less dense than water, so increasing pressure lowers the melting point (ice skating, glacier flow). CO2 has a positive slope (solid denser than liquid) and its triple point is above 1 atm, so solid CO2 (dry ice) sublimes at 1 atm (−78 °C).
  • Applications: pressure cookers (higher pressure raises boiling point), freeze-drying of food and medicines (sublimation below the triple point), supercritical CO2 for decaffeinating coffee, dry ice for cooling, boiling water at lower temperature at high altitude (e.g. Musanze vs Kigali).

Common misconceptions

  • "Increasing pressure always raises the melting point." No: For water, higher pressure LOWERS the melting point because ice is less dense than liquid water.
  • "At the triple point only two phases exist." No: All three phases (solid, liquid, gas) coexist at the triple point.
  • "Dry ice melts into liquid CO2 at normal pressure." No: At 1 atm (below the triple-point pressure of 5.1 atm) dry ice sublimes directly to gas.

Worked examples

Example 1. Sketch (describe) the phase diagram of water and label its main features.

Solution

Axes: pressure (y) against temperature (x).

Three regions: solid (left), liquid (top middle), gas (bottom right).

Lines: sublimation curve (solid–gas), melting curve (solid–liquid, slightly negative slope), vaporisation curve (liquid–gas).

Triple point at 0.01 °C, 0.006 atm; critical point at 374 °C, 218 atm; normal m.p. 0 °C and b.p. 100 °C at 1 atm.

Example 2. Compare the phase diagrams of water and carbon dioxide.

Solution

Water: solid–liquid line slopes left (ice less dense than water); triple point below 1 atm, so ice melts and water boils at 1 atm.

CO2: solid–liquid line slopes right (solid denser); triple point (−56.6 °C, 5.1 atm) above 1 atm, so solid CO2 sublimes at 1 atm and liquid CO2 does not exist at normal pressure.

Critical points: water 374 °C/218 atm; CO2 31 °C/73 atm.

Practice questions

  1. Define the triple point.

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    The temperature and pressure at which solid, liquid and gas phases coexist in equilibrium.

    Common mistake: Saying 'the boiling point at high pressure'.

  2. What is a supercritical fluid?

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    A substance above its critical temperature and pressure, with properties between liquid and gas.

    Common mistake: Saying a very hot solid.

  3. Why does dry ice not melt at 1 atm?

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    1 atm is below its triple-point pressure (5.1 atm), so it sublimes.

    Common mistake: Saying it is too cold to melt.

  4. Give two applications of phase diagrams.

    Show answer

    Pressure cookers, freeze-drying, supercritical CO2 extraction, dry ice, understanding ice skating (any two).

    Common mistake: Saying titration.

  5. Explain the negative slope of the water melting line.

    Show answer

    Ice is less dense than water; increasing pressure favours the denser liquid, lowering the melting point.

    Common mistake: Saying ice is denser.

  6. Distinguish homogeneous and heterogeneous equilibria.

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    Homogeneous: all species in one phase; heterogeneous: more than one phase.

    Common mistake: Saying they differ in temperature.

  7. State the effect of high altitude on the boiling point of water.

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    It is lowered because atmospheric pressure is lower.

    Common mistake: Saying it is raised.

  8. What do the lines on a phase diagram represent?

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    Conditions where two phases are in equilibrium.

    Common mistake: Saying the substance is a gas.

  9. At the critical point, what happens to the liquid–gas boundary?

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    It disappears; liquid and gas become indistinguishable.

    Common mistake: Saying it becomes a solid.

  10. Why does a pressure cooker save fuel?

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    Food cooks faster at the higher boiling temperature, so less heating time is needed.

    Common mistake: Saying it needs no heat.

Matching and drag-and-drop

  1. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Dry ice is solid:a. higher pressure raises the boiling point of water
    2. Freeze-drying of food uses:b. solid, liquid and gas coexist in equilibrium
    3. The triple point is where:c. carbon dioxide
    4. A pressure cooker cooks food faster because:d. sublimation of ice at low pressure
    Show answer

    1 → c (carbon dioxide) · 2 → d (sublimation of ice at low pressure) · 3 → b (solid, liquid and gas coexist in equilibrium) · 4 → a (higher pressure raises the boiling point of water)

    • It sublimes at −78 °C.
    • Water removed without heat damage.
    • Unique T and P.
    • Food cooks at about 120 °C.

  2. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    ice is less dense than water  ·  atmospheric pressure is lower  ·  water is lighter there  ·  can lower the melting point slightly
    1. The solid–liquid line of water slopes to the left because .
    2. Water boils at a lower temperature on a high mountain because .
    3. Ice skating is easier because pressure under the blade .
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    1: ice is less dense than water · 2: atmospheric pressure is lower · 3: can lower the melting point slightly

    • Melting point falls with pressure.
    • Lower external pressure.
    • Thin liquid layer (with friction heating).

  3. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. The triple point of water is about:a. heterogeneous equilibrium
    2. The slope of the solid–liquid line for CO2 is:b. decaffeinate coffee
    3. Supercritical CO2 is used to:c. 0.01 °C and 0.006 atm
    4. CaCO3(s) ⇌ CaO(s) + CO2(g) is an example of:d. positive (to the right)
    Show answer

    1 → c (0.01 °C and 0.006 atm) · 2 → d (positive (to the right)) · 3 → b (decaffeinate coffee) · 4 → a (heterogeneous equilibrium)

    • Very low pressure.
    • Solid CO2 is denser than liquid.
    • Good, non-toxic solvent.
    • Two solids and a gas.

  4. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    sublimes  ·  31 °C  ·  uniform in composition and physical state  ·  a chemical reaction
    1. The critical temperature of CO2 is about .
    2. A phase is a part of a system that is .
    3. At 1 atm, dry ice .
    Show answer

    1: 31 °C · 2: uniform in composition and physical state · 3: sublimes

    • Low enough to use supercritical CO2 easily.
    • e.g. ice in water is a separate phase.
    • Triple point pressure is 5.1 atm.

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