1. At the melting point of a pure substance, which two phases can exist in equilibrium?
A.
Solid and liquid
B.
Liquid and gas
C.
Solid and gas
D.
Only liquid
Correct Answer:
Solid and liquid
Explanation:
At the melting point, the solid and liquid phases coexist in equilibrium.
2. In solid–liquid equilibrium, the rates of melting and freezing are:
A.
Both zero
B.
Equal
C.
Melting is greater than freezing
D.
Freezing is greater than melting
Correct Answer:
Equal
Explanation:
Solid–liquid equilibrium is dynamic, so melting and freezing occur at equal rates.
3. What happens to a solid when it is heated to its melting point at constant pressure?
A.
It begins to melt
B.
It immediately changes into gas
C.
It becomes colder
D.
It changes into another solid only
Correct Answer:
It begins to melt
Explanation:
At its melting point, a solid starts changing into a liquid while the two phases may coexist.
4. The temperature at which a solid and its liquid coexist in equilibrium at a given pressure is called the:
A.
Boiling point
B.
Freezing point only
C.
Melting point
D.
Critical temperature
Correct Answer:
Melting point
Explanation:
The melting point is the temperature at which solid and liquid phases are in equilibrium at a specified pressure.
5. During the melting of a pure solid at constant pressure, its temperature generally:
A.
Remains constant until melting is complete
B.
Increases continuously
C.
Decreases continuously
D.
Becomes equal to the boiling point
Correct Answer:
Remains constant until melting is complete
Explanation:
At solid–liquid equilibrium, supplied heat is used for the phase change, so the temperature remains constant.
6. If a liquid is cooled to its freezing point, it can establish equilibrium with its:
A.
Gas phase
B.
Solid phase
C.
Plasma phase
D.
Vapour only
Correct Answer:
Solid phase
Explanation:
At the freezing point, the liquid and solid phases can coexist in solid–liquid equilibrium.
7. What is the main characteristic of dynamic solid–liquid equilibrium?
A.
Only melting occurs
B.
Only freezing occurs
C.
Melting and freezing continue at equal rates
D.
Neither melting nor freezing can occur
Correct Answer:
Melting and freezing continue at equal rates
Explanation:
The equilibrium is dynamic because both forward and reverse phase changes continue, but their rates are equal.
8. For a pure substance at a fixed pressure, the melting point is a:
A.
Fixed temperature
B.
Fixed volume
C.
Fixed mass
D.
Fixed pressure only
Correct Answer:
Fixed temperature
Explanation:
At a given pressure, a pure substance has a definite temperature at which its solid and liquid phases are in equilibrium.
9. At solid–liquid equilibrium, the amounts of solid and liquid remain constant because:
A.
No particles move between the phases
B.
The melting and freezing rates are equal
C.
The solid has no energy
D.
The liquid cannot freeze
Correct Answer:
The melting and freezing rates are equal
Explanation:
Equal rates of melting and freezing keep the macroscopic amounts of solid and liquid constant.
10. Which condition is required for solid–liquid equilibrium of a pure substance?
A.
The temperature must be at the melting point for the given pressure
B.
The temperature must always be above the boiling point
C.
Only the solid must be present
D.
Only the liquid must be present
Correct Answer:
The temperature must be at the melting point for the given pressure
Explanation:
Solid and liquid phases coexist in equilibrium only at the melting point corresponding to the applied pressure.
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