chemistry : class11 Melting Point Definition : Melting point : States of Matter: Gases and Liquids MCQ Questions and Answers
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Melting Point Definition
MCQ questions and answers.
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MCQ Questions
1. Which statement most precisely defines the melting point of a pure crystalline substance?
A.
The temperature at which the substance changes completely from solid to liquid at any pressure
B.
The temperature at which solid and liquid phases coexist in equilibrium at a specified pressure
C.
The temperature at which the vapour pressure of the solid becomes equal to atmospheric pressure
D.
The temperature at which the average kinetic energy of the particles becomes maximum
Correct Answer:
The temperature at which solid and liquid phases coexist in equilibrium at a specified pressure
Explanation:
Melting point is defined as the temperature at which the solid and liquid phases of a substance coexist in equilibrium at a specified pressure.
2. Why must the pressure be stated when specifying the melting point of a substance?
A.
Because melting point is defined only for gases under pressure
B.
Because the temperature of solid–liquid equilibrium can vary with pressure
C.
Because pressure determines the molar mass of the solid
D.
Because pressure changes the chemical identity of the liquid phase
Correct Answer:
Because the temperature of solid–liquid equilibrium can vary with pressure
Explanation:
The melting point is an equilibrium temperature, and the solid–liquid equilibrium temperature may change when the applied pressure changes.
3. At the melting point of a pure crystalline substance under constant pressure, which condition is necessarily satisfied?
A.
The solid and liquid phases are simultaneously present and in equilibrium
B.
Only the liquid phase is present because melting is complete
C.
The solid phase has a higher temperature than the liquid phase
D.
The vapour phase must be present in greater amount than the solid phase
Correct Answer:
The solid and liquid phases are simultaneously present and in equilibrium
Explanation:
At the melting point, the defining condition is coexistence of solid and liquid phases in equilibrium at the stated pressure.
4. A temperature is reported as the melting point of a pure solid at 1 atm. What does this statement mean most accurately?
A.
The solid begins to evaporate at 1 atm
B.
The solid and liquid have equal stability at 1 atm at that temperature
C.
The liquid has its maximum vapour pressure at 1 atm
D.
All particles in the solid acquire identical kinetic energy at 1 atm
Correct Answer:
The solid and liquid have equal stability at 1 atm at that temperature
Explanation:
At the melting point for a specified pressure, the solid and liquid phases are in equilibrium, meaning neither phase is thermodynamically favoured over the other.
5. Which observation most directly indicates that a pure crystalline sample has reached its melting point during heating at constant pressure?
A.
Its temperature remains nearly constant while solid and liquid coexist
B.
Its temperature rises rapidly while only the solid is present
C.
Its temperature falls because the solid begins to sublime
D.
Its volume becomes zero as the solid loses rigidity
Correct Answer:
Its temperature remains nearly constant while solid and liquid coexist
Explanation:
During melting at constant pressure, added heat converts solid into liquid while the two phases coexist, so the temperature remains at the melting point until the phase change is complete.
6. Which distinction correctly separates melting point from the temperature at which a solid merely starts softening?
A.
Melting point refers to a definite solid–liquid equilibrium temperature for a pure crystalline substance
B.
Melting point is the temperature at which the solid first becomes warm to touch
C.
Melting point is the highest temperature reached by the solid before decomposition
D.
Melting point is the temperature at which the solid begins to emit vapour
Correct Answer:
Melting point refers to a definite solid–liquid equilibrium temperature for a pure crystalline substance
Explanation:
The melting point is not simply the onset of softening; it is the definite temperature corresponding to equilibrium between solid and liquid phases under specified pressure.
7. For a pure crystalline substance, what does the phrase “melting point at 1 atm” explicitly specify?
A.
The pressure at which the substance is heated until it decomposes
B.
The pressure condition for defining the solid–liquid equilibrium temperature
C.
The pressure exerted only by the vapour above the liquid
D.
The pressure required to prevent the solid from subliming
Correct Answer:
The pressure condition for defining the solid–liquid equilibrium temperature
Explanation:
The stated pressure identifies the condition under which the temperature of solid–liquid equilibrium, called the melting point, is being defined.
8. Which statement about the melting point of a pure crystalline substance is most scientifically precise?
A.
It is a phase-transition temperature rather than simply a measure of how hot the solid becomes
B.
It is the temperature at which all molecular motion in the solid stops
C.
It is the temperature at which the solid has the greatest possible density
D.
It is the temperature at which the substance must exist entirely as a liquid
Correct Answer:
It is a phase-transition temperature rather than simply a measure of how hot the solid becomes
Explanation:
Melting point identifies the temperature of the solid-to-liquid phase transition, specifically the condition where solid and liquid coexist in equilibrium.
9. A pure solid is held at a fixed pressure exactly at its melting point. If solid and liquid are both present, which statement follows from the definition?
A.
The solid must spontaneously disappear without any energy exchange
B.
The liquid must spontaneously freeze completely
C.
The two phases can persist together in dynamic equilibrium
D.
The solid and liquid must have different chemical compositions
Correct Answer:
The two phases can persist together in dynamic equilibrium
Explanation:
At the melting point, solid and liquid can coexist in dynamic equilibrium: melting and freezing occur at equal rates under the specified conditions.
10. Which temperature would correctly be called the melting point of a pure crystalline substance at a specified pressure?
A.
The temperature at which the first liquid appears while solid and liquid are in equilibrium
B.
The temperature at which the last vapour disappears from the sample
C.
The temperature at which the solid reaches its highest heat capacity
D.
The temperature at which the substance changes directly from solid to vapour
Correct Answer:
The temperature at which the first liquid appears while solid and liquid are in equilibrium
Explanation:
The appearance of liquid at the solid–liquid equilibrium temperature marks the melting point, provided the pressure is specified and the substance is pure.
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