chemistry : class11 Depression of Melting Point : Melting point : States of Matter: Gases and Liquids MCQ Questions and Answers
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Depression of Melting Point
MCQ questions and answers.
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MCQ Questions
1. What is meant by depression of melting point?
A.
Increase in the melting point of a substance due to pressure
B.
Decrease in the melting point of a pure substance when an impurity is added
C.
Conversion of a liquid into a gas on heating
D.
Increase in the boiling point of a liquid
Correct Answer:
Decrease in the melting point of a pure substance when an impurity is added
Explanation:
Depression of melting point is the lowering of the melting point of a pure substance when a suitable impurity is dissolved in it.
2. What generally happens to the melting point of ice when salt is added to it?
A.
It increases
B.
It remains unchanged
C.
It decreases
D.
It becomes equal to the boiling point of water
Correct Answer:
It decreases
Explanation:
Salt lowers the melting point of ice, causing depression of the melting point.
3. Which substance is commonly added to ice to produce depression of its melting point?
A.
Sodium chloride
B.
Sand only
C.
Copper wire
D.
Glass powder only
Correct Answer:
Sodium chloride
Explanation:
Sodium chloride dissolves in the thin layer of water on ice and lowers the melting point of the ice.
4. The melting point of an impure solid is generally compared with that of the pure solid as:
A.
Higher
B.
Lower
C.
Always equal
D.
Unrelated
Correct Answer:
Lower
Explanation:
An impurity generally causes depression of the melting point, so the impure solid melts at a lower temperature than the pure solid.
5. What is the usual effect of increasing the amount of a dissolved impurity on the melting point of a solvent?
A.
The melting point generally decreases further
B.
The melting point always increases sharply
C.
The melting point becomes exactly zero degrees Celsius
D.
The melting point is unaffected in every case
Correct Answer:
The melting point generally decreases further
Explanation:
A greater amount of dissolved impurity generally produces a greater depression of the solvent's melting point.
6. Why is salt spread on icy roads in cold regions?
A.
To increase the melting point of ice
B.
To lower the melting point of ice and help it melt
C.
To convert ice directly into water vapour
D.
To make ice harder by removing impurities
Correct Answer:
To lower the melting point of ice and help it melt
Explanation:
Salt causes depression of the melting point of ice, allowing the ice to melt at temperatures below its normal melting point.
7. A pure solid melts at 80°C. After adding a suitable impurity, it melts at 75°C. What is observed?
A.
A depression of 5°C in melting point
B.
An elevation of 5°C in melting point
C.
No change in melting point
D.
A depression of 155°C in melting point
Correct Answer:
A depression of 5°C in melting point
Explanation:
The melting point changes from 80°C to 75°C, so the decrease is 80 - 75 = 5°C.
8. Which comparison correctly describes the effect of adding a suitable impurity to a pure solid?
A.
The impure solid usually melts at a lower temperature
B.
The impure solid always melts at a higher temperature
C.
The impurity changes the solid into a gas immediately
D.
The pure solid always melts below the impure solid
Correct Answer:
The impure solid usually melts at a lower temperature
Explanation:
A suitable impurity lowers the melting point of the pure solid, producing depression of melting point.
9. The lowering of the melting point of ice by adding salt is also commonly called:
A.
Freezing point depression
B.
Boiling point elevation
C.
Vapour pressure increase
D.
Condensation point elevation
Correct Answer:
Freezing point depression
Explanation:
For a liquid such as water, the temperature at which it freezes is the same equilibrium temperature at which ice melts. Lowering this temperature is called freezing point depression.
10. Which statement about depression of melting point is correct?
A.
It occurs when a suitable impurity lowers the melting temperature of a pure solid
B.
It occurs only when the pressure is increased greatly
C.
It means that the boiling point of a liquid becomes lower
D.
It requires the solid to be completely pure
Correct Answer:
It occurs when a suitable impurity lowers the melting temperature of a pure solid
Explanation:
Depression of melting point specifically refers to the lowering of the melting temperature caused by the addition of an impurity.
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