Phase Diagrams: Quiz

Phase Diagrams: Quiz

7 questions

1. Question 1EasyOn a phase diagram, what is found at the triple point?

   AOnly the gasBSolid, liquid and gas in equilibriumCA supercritical fluidDThe normal boiling point
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   Answer: Solid, liquid and gas in equilibrium

   The triple point is where the three lines meet: all three phases coexist.

2. Question 2EasyWhat lies beyond the critical point?

   AA solidBA supercritical fluidCA mixture of solid and gasDNothing: the substance decomposes
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   Answer: A supercritical fluid

   Above the critical temperature and pressure, liquid and gas can no longer be distinguished: the substance is a supercritical fluid.

3. Question 3EasyWhat state is water in at 1 atm and 50 °C?

   ASolidBLiquidCGasDSupercritical fluid
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   Answer: Liquid

   At 1 atm water melts at 0 °C and boils at 100 °C, so at 50 °C it is liquid.

4. Question 4MediumWhy does solid CO₂ sublime at atmospheric pressure?

   AIt is very coldBIts triple-point pressure (5.11 atm) is above 1 atmCIt has no intermolecular forcesDIts critical point is below 0 °C
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   Answer: Its triple-point pressure (5.11 atm) is above 1 atm

   Liquid CO₂ only exists above 5.11 atm. At 1 atm the solid region borders the gas region directly.

5. Question 5MediumWater's solid–liquid line slopes to the left. What does this mean?

   AIncreasing pressure raises the melting pointBIncreasing pressure lowers the melting pointCIce is denser than liquid waterDWater has no triple point
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   Answer: Increasing pressure lowers the melting point

   Because ice is less dense than liquid water, squeezing ice favours the liquid, so the melting point falls slightly as pressure rises.

6. Question 6HardIn which state is CO₂ at 60 atm and 0 °C? (Triple point −56.6 °C, 5.11 atm; vapour pressure of liquid CO₂ at 0 °C ≈ 34 atm)

   ASolidBLiquidCGasDSupercritical fluid
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   Answer: Liquid

   The pressure is above the vapour pressure at 0 °C, and both T and P are below the critical point (31.0 °C, 72.8 atm), so CO₂ is liquid, as in a fire extinguisher.

7. Question 7MediumMoving straight up on a phase diagram (at constant temperature) means:

   Aheating at constant pressureBincreasing the pressure at constant temperatureCcooling at constant pressureDlowering the pressure at constant temperature
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   Answer: increasing the pressure at constant temperature

   Pressure is on the y-axis, so a vertical move upward is compression at constant temperature.