Interpreting phase diagrams to understand the states of matter and phase transitions.

Let’s solve each question step by step using the phase diagram provided.

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Phase Diagram Interpretation (Questions 1–10)

The first diagram shows a typical phase diagram with pressure (y-axis) vs. temperature (x-axis). The three regions are labeled A, B, and C. There are also two lines meeting at point d (triple point), and one line ends at point e (critical point).

#### 1. Which section represents the solid phase?
> Answer: A

Explanation: In most phase diagrams, the solid phase is found at low temperatures and high pressures — typically the region to the left and above the fusion curve. Region A is bounded by the solid-liquid and solid-gas lines, so it represents the solid phase.

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#### 2. What section represents the liquid phase?
> Answer: C

Explanation: The liquid phase exists between the melting curve (solid-liquid boundary) and the vaporization curve (liquid-gas boundary). Region C is between those two curves, so it’s the liquid phase.

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#### 3. What section represents the gas phase?
> Answer: B

Explanation: The gas phase is found at low pressures and high temperatures — region B is below both the solid-gas and liquid-gas lines, so it’s the gas phase.

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#### 4. What letter represents the triple point? _______ In your own words, what is the definition of a triple point?

> Answer: d

> Definition: The triple point is the unique combination of temperature and pressure at which all three phases (solid, liquid, and gas) of a substance coexist in thermodynamic equilibrium.

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#### 5. What is this substance’s melting point at 1 atmosphere of pressure?

> Answer: 60°C

Explanation: To find the melting point at 1 atm, go to 1.0 atm on the y-axis, move right until you hit the solid-liquid boundary (the steep line from d to higher T), then drop down to the x-axis. That temperature is approximately 60°C.

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#### 6. What is this substance’s boiling point at 1 atmosphere of pressure?

> Answer: 110°C

Explanation: Boiling point is where the liquid-gas boundary meets 1 atm. From 1.0 atm, move right to point e (on the liquid-gas curve), then drop down to the x-axis → 110°C.

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#### 7. Above what temperature is it impossible to liquefy this substance, no matter what the pressure?

> Answer: 110°C

Explanation: This is the critical temperature. Beyond this temperature (at point e, the critical point), the substance cannot be liquefied no matter how much pressure is applied. So the answer is 110°C.

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#### 8. At what temperature and pressure do all three phases coexist?

> Answer: 45°C and 0.5 atm

Explanation: This is the triple point (point d). Read off the coordinates: Temperature = 45°C, Pressure = 0.5 atm.

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#### 9. At a constant temperature, what would you do to cause this substance to change from the liquid phase to the solid phase?

> Answer: Increase the pressure

Explanation: If you’re in the liquid region (C) and want to turn it into solid (A), you need to cross the solid-liquid boundary. At constant temperature (say, 50°C), increasing pressure will push you from region C into region A — causing freezing/solidification.

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#### 10. What does sublimation mean?

> Answer: Sublimation is the direct transition of a substance from the solid phase to the gas phase without passing through the liquid phase.

Example: Dry ice (solid CO₂) turning directly into CO₂ gas.

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Second Phase Diagram (Questions 11–13)

This diagram shows pressure vs. temperature for a pure substance. We must determine the phase change that occurs under given conditions.

Options:
(A) Sublimation
(B) Condensation
(C) Vaporization
(D) Melting
(E) Deposition

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#### 11. If the pressure decreases from 1.5 to 0.5 atmospheres at a constant temperature of 50°C, which of the processes occurs?

> Answer: (C) Vaporization

Explanation: At 50°C, if pressure drops from 1.5 atm to 0.5 atm, we’re moving horizontally left on the diagram. Assuming the substance was liquid at 1.5 atm (since 50°C is above the triple point temp ~30°C), dropping pressure may cross the liquid-gas boundary → vaporization (liquid turns to gas).

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#### 12. If the temperature increases from 10°C to 50°C at a constant pressure of 0.5 atmospheres, which of the processes occurs?

> Answer: (A) Sublimation

Explanation: At 0.5 atm, starting at 10°C — likely solid (below triple point temp ~30°C). As temperature increases to 50°C, you cross the solid-gas boundary directly (since pressure is below triple point pressure). So solid → gas = sublimation.

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#### 13. If the temperature decreases from 110°C to 40°C at a constant pressure of 1.1 atmospheres, which of the processes occurs?

> Answer: (B) Condensation

Explanation: At 1.1 atm (above 1.0 atm), starting at 110°C — likely gas (since boiling point at 1 atm is ~110°C, so at slightly higher pressure, boiling point is higher, meaning 110°C is still gas). Cooling to 40°C crosses the liquid-gas boundary → gas turns to liquid = condensation.

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## ✔ Final Answers:

1. A
2. C
3. B
4. d; The triple point is the unique temperature and pressure at which solid, liquid, and gas phases coexist in equilibrium.
5. 60°C
6. 110°C
7. 110°C
8. 45°C and 0.5 atm
9. Increase the pressure
10. Sublimation is the direct change from solid to gas without becoming liquid.
11. (C) Vaporization
12. (A) Sublimation
13. (B) Condensation

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