chemistry : class11 Impurity Effects : Melting point : States of Matter: Gases and Liquids MCQ Questions and Answers
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Impurity Effects
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MCQ Questions
1. A pure crystalline organic compound melts sharply at 122.0 °C. When 5% of a chemically different, completely miscible solid impurity is added, the sample melts over the range 108–116 °C. Which conclusion is most appropriate?
A.
The impurity has increased the lattice energy of the compound
B.
The impurity has lowered and broadened the melting range by disrupting crystal packing
C.
The compound has become chemically purer because its melting range is lower
D.
The impurity has converted the crystalline solid into a liquid without changing its melting behavior
Correct Answer:
The impurity has lowered and broadened the melting range by disrupting crystal packing
Explanation:
A soluble impurity disrupts the regular crystal lattice, decreases the temperature at which the solid and liquid phases are in equilibrium, and usually causes melting to occur over a broader range rather than sharply.
2. Two samples of the same compound contain equal masses of impurity. Sample X contains an impurity that is highly soluble in the liquid phase but nearly insoluble in the solid crystal. Sample Y contains an impurity that forms a mixed crystal readily with the compound. Which sample is expected to show the greater depression of melting point?
A.
Sample X, because the impurity preferentially stabilizes the liquid phase
B.
Sample Y, because mixed-crystal formation always produces the largest depression
C.
Both samples, because the nature of the impurity does not affect melting point
D.
Sample Y, because a solid solution always has a lower melting point than a liquid solution
Correct Answer:
Sample X, because the impurity preferentially stabilizes the liquid phase
Explanation:
An impurity that remains mainly in the liquid phase lowers the chemical potential of the liquid more effectively than that of the pure solid, producing a stronger melting-point depression. Mixed-crystal formation can reduce this effect.
3. A binary mixture of solids A and B exhibits a minimum melting temperature at a particular composition. At this composition, the mixture melts at a constant temperature until the entire solid disappears. This behavior is best described as:
A.
Formation of a congruent compound with a maximum melting point
B.
Eutectic behavior caused by impurity-induced lowering of the melting temperature
C.
Superheating of pure A above its equilibrium melting point
D.
Elevation of the melting point due to complete lattice stabilization
Correct Answer:
Eutectic behavior caused by impurity-induced lowering of the melting temperature
Explanation:
A eutectic composition has the lowest melting temperature in a binary system. The components crystallize together at a fixed temperature, and the eutectic mixture behaves as an impurity-containing system with a markedly depressed melting point.
4. An impure solid begins melting at 74 °C and finishes melting at 81 °C, whereas the purified sample melts sharply at 82 °C. Which interpretation is most justified?
A.
The impurity raises the thermodynamic melting point but delays complete melting
B.
The impure sample contains a mixture whose melting begins below the melting point of the pure compound
C.
The purified sample is less crystalline and therefore melts at a higher temperature
D.
The impurity has no effect because the upper end of the range is close to 82 °C
Correct Answer:
The impure sample contains a mixture whose melting begins below the melting point of the pure compound
Explanation:
Impurities generally depress the onset of melting and broaden the melting interval. The pure compound melts sharply near its characteristic melting point, whereas the impure mixture begins melting at a lower temperature.
5. For a dilute solution of a nonvolatile impurity in a crystalline solvent, the melting-point depression is approximately proportional to the molality of the impurity. If the impurity concentration is doubled while the solvent and impurity remain the same, the melting-point depression will approximately:
A.
Be halved
B.
Remain unchanged
C.
Double
D.
Increase fourfold
Correct Answer:
Double
Explanation:
In the dilute-solution approximation, the depression in melting point follows ΔT_f = K_f m. Therefore, doubling the molality approximately doubles the melting-point depression, provided the solution remains sufficiently dilute.
6. A student mixes two solids in a 1:1 molar ratio and observes a melting point lower than that of either pure component. Which explanation most directly accounts for this observation?
A.
The mixture has a more ordered lattice than both pure solids
B.
The two components disrupt each other's crystal lattices and may approach a eutectic composition
C.
The mixture must contain a new pure substance with a higher lattice energy
D.
The lower melting point proves that both components have decomposed completely
Correct Answer:
The two components disrupt each other's crystal lattices and may approach a eutectic composition
Explanation:
Each component acts as an impurity in the crystal lattice of the other. Lattice disruption lowers the melting temperature, and a composition near the eutectic can produce a melting point lower than either pure component.
7. An impure sample of compound P melts from 96–103 °C. After repeated recrystallization, it melts from 118–119 °C. Which change most plausibly occurred?
A.
The impurity concentration decreased, reducing lattice disruption and narrowing the melting range
B.
The molecular mass of P increased during recrystallization
C.
The impurity became more soluble in the solid phase and therefore raised the melting point
D.
The sample gained a volatile impurity that increased its melting temperature
Correct Answer:
The impurity concentration decreased, reducing lattice disruption and narrowing the melting range
Explanation:
Recrystallization removes many soluble impurities. A purer crystal has a more uniform lattice, so its melting point rises toward the characteristic value of pure P and the melting interval becomes narrower.
8. A small amount of impurity is added separately to two portions of a pure solid. Impurity R is structurally similar to the solid and enters its lattice, whereas impurity S is structurally dissimilar and is excluded from the lattice. Which result is most likely?
A.
R necessarily causes a greater melting-point depression because it enters the lattice
B.
S can cause a pronounced depression because exclusion from the solid concentrates it in the liquid phase
C.
Neither impurity affects the melting point because both are present in small amounts
D.
R raises the melting point while S leaves it unchanged
Correct Answer:
S can cause a pronounced depression because exclusion from the solid concentrates it in the liquid phase
Explanation:
When an impurity is largely excluded from the solid, the liquid becomes relatively enriched in that impurity during melting. This lowers the liquid chemical potential and can produce a strong melting-point depression.
9. A sample containing compound A and impurity B starts melting at 65 °C. As heating continues, the remaining solid melts at 78 °C. Which statement best explains the progressive change in melting temperature?
A.
The composition of the remaining solid and liquid changes during partial melting
B.
The melting point of pure A continuously decreases with heating
C.
The impurity is destroyed at 65 °C, causing a random temperature increase
D.
The sample becomes more impure in the solid phase without any phase separation
Correct Answer:
The composition of the remaining solid and liquid changes during partial melting
Explanation:
An impure mixture does not generally melt at one fixed temperature. As melting proceeds, the compositions of the solid and liquid phases change, so the temperature required for further melting can rise toward the melting point of the purer remaining solid.
10. Which observation provides the strongest evidence that an unknown solid is impure rather than simply having an intrinsically low melting point?
A.
It melts sharply at a temperature lower than that of a related compound
B.
It melts over a broad range, and purification raises the melting point while narrowing the range
C.
It has a high density and forms transparent crystals
D.
It dissolves readily in a solvent at room temperature
Correct Answer:
It melts over a broad range, and purification raises the melting point while narrowing the range
Explanation:
A broad, depressed melting range is characteristic of impurity effects. If purification raises the melting point and produces a sharper transition, the original broad range was caused by impurities rather than by an intrinsically low melting point.
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