chemistry : class11 Triple Point : Melting point : States of Matter: Gases and Liquids MCQ Questions and Answers
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Triple Point
MCQ questions and answers.
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MCQ Questions
1. What is the defining condition of the triple point of a pure substance?
A.
Solid and liquid phases coexist in equilibrium at the normal melting point
B.
Solid, liquid, and vapour phases coexist in dynamic equilibrium at a unique temperature and pressure
C.
The liquid phase changes completely into vapour at its critical temperature
D.
The solid phase becomes unstable at a pressure of exactly one atmosphere
Correct Answer:
Solid, liquid, and vapour phases coexist in dynamic equilibrium at a unique temperature and pressure
Explanation:
The triple point is the unique temperature and pressure at which the solid, liquid, and vapour phases of a pure substance coexist in equilibrium.
2. At the triple point of a pure substance, how many independent intensive variables can be changed while maintaining simultaneous equilibrium among all three phases?
A.
Two
B.
Three
C.
One
D.
Zero
Correct Answer:
Zero
Explanation:
Using the phase rule F = C - P + 2, for a one-component system with three phases, F = 1 - 3 + 2 = 0. Thus, the triple-point temperature and pressure are fixed.
3. The triple point of water is approximately 0.01 °C and 611 Pa. What happens if the pressure is reduced below the triple-point pressure while the temperature is kept near 0.01 °C?
A.
Liquid water remains stable and freezes at a lower temperature
B.
Ice and water coexist indefinitely at the same temperature
C.
The liquid phase cannot exist stably, so ice tends to sublime into vapour
D.
Water changes directly from vapour into liquid without heat transfer
Correct Answer:
The liquid phase cannot exist stably, so ice tends to sublime into vapour
Explanation:
Below the triple-point pressure, the liquid region disappears from the phase diagram. Therefore, ice can change directly into vapour by sublimation, but stable liquid water cannot exist.
4. Why is the melting point of a pure substance at one atmosphere generally different from its triple-point temperature?
A.
The triple point occurs only when the substance is impure
B.
The melting point at one atmosphere corresponds to solid-liquid equilibrium at a pressure that may differ from the triple-point pressure
C.
The triple point is always above the critical temperature
D.
At the triple point, only the solid and liquid phases are present
Correct Answer:
The melting point at one atmosphere corresponds to solid-liquid equilibrium at a pressure that may differ from the triple-point pressure
Explanation:
The triple-point temperature is defined at the unique triple-point pressure, whereas the ordinary melting point is measured at a specified pressure, commonly one atmosphere. These temperatures need not be identical.
5. On a pressure-temperature phase diagram for a pure substance, the triple point is located at the intersection of which three equilibrium curves?
A.
The isotherm, adiabat, and critical isochore
B.
The fusion, vaporisation, and sublimation curves
C.
The boiling, condensation, and Joule-Thomson curves
D.
The melting, critical, and Boyle-temperature curves
Correct Answer:
The fusion, vaporisation, and sublimation curves
Explanation:
At the triple point, solid-liquid, liquid-vapour, and solid-vapour equilibria all occur simultaneously. Their corresponding curves are the fusion, vaporisation, and sublimation curves.
6. A sealed vessel contains a pure substance at its triple-point temperature and pressure. If a small amount of heat is supplied without changing the external pressure, what is the most appropriate description initially?
A.
The temperature must rise immediately because no phase change is possible
B.
The system can redistribute the three phases, with supplied energy contributing to phase conversion while the triple-point temperature remains fixed
C.
All three phases disappear simultaneously and form a supercritical fluid
D.
Only the solid phase can increase because vapour cannot coexist with liquid
Correct Answer:
The system can redistribute the three phases, with supplied energy contributing to phase conversion while the triple-point temperature remains fixed
Explanation:
At the triple point, phase changes can occur at fixed temperature and pressure because the phases coexist in equilibrium. Heat supplied initially changes the relative amounts of the phases rather than raising the temperature.
7. Which statement correctly distinguishes the triple point from the critical point of a pure substance?
A.
At the triple point, three phases coexist; at the critical point, liquid and vapour become indistinguishable
B.
At the triple point, only vapour exists; at the critical point, three phases coexist
C.
Both points represent coexistence of solid, liquid, and vapour phases
D.
The triple point is defined only by temperature, whereas the critical point is defined only by pressure
Correct Answer:
At the triple point, three phases coexist; at the critical point, liquid and vapour become indistinguishable
Explanation:
The triple point involves simultaneous equilibrium of solid, liquid, and vapour. The critical point is the endpoint of the liquid-vapour coexistence curve, where the two phases lose their distinct identities.
8. For a one-component system at its triple point, which change would destroy the three-phase equilibrium unless another variable is adjusted appropriately?
A.
Changing the pressure while keeping the temperature fixed
B.
Allowing the three phases to remain in contact
C.
Maintaining the system as a pure substance
D.
Keeping the temperature and pressure at their triple-point values
Correct Answer:
Changing the pressure while keeping the temperature fixed
Explanation:
The triple point has zero degrees of freedom. Its temperature and pressure are uniquely fixed, so changing either variable alone moves the system away from simultaneous three-phase equilibrium.
9. A pure substance has a triple-point pressure greater than one atmosphere. At one atmosphere, which statement about its solid-liquid equilibrium is necessarily valid?
A.
Its solid-liquid equilibrium must occur at the triple-point temperature
B.
Its solid-liquid equilibrium cannot occur at any temperature
C.
Its solid-liquid equilibrium, if present, occurs on the fusion curve at a pressure different from the triple-point pressure
D.
It must exist simultaneously as solid, liquid, and vapour
Correct Answer:
Its solid-liquid equilibrium, if present, occurs on the fusion curve at a pressure different from the triple-point pressure
Explanation:
The solid-liquid boundary is the fusion curve and extends away from the triple point. At one atmosphere, equilibrium melting can occur at the temperature corresponding to that pressure, which need not equal the triple-point temperature.
10. Why can the triple-point temperature serve as a highly reproducible reference temperature for a pure substance?
A.
It depends only on the volume of the sample
B.
It is fixed by the simultaneous equilibrium of three phases at a unique temperature and pressure
C.
It is always equal to the substance's normal boiling point
D.
It changes significantly with the quantity of substance present
Correct Answer:
It is fixed by the simultaneous equilibrium of three phases at a unique temperature and pressure
Explanation:
For a pure substance, the triple point has zero degrees of freedom. Therefore, its temperature is uniquely determined when the three phases coexist, making it a reproducible reference.
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