physics : class11 Effect of Impurities on Surface Energy : Surface Energy : Properties of Bulk Matter MCQ Questions and Answers
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Effect of Impurities on Surface Energy
MCQ questions and answers.
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MCQ Questions
1. What is the usual effect of adding soap to water on its surface energy per unit area?
A.
It decreases
B.
It increases greatly
C.
It becomes zero
D.
It remains exactly unchanged
Correct Answer:
It decreases
Explanation:
Soap is an impurity that generally lowers the surface tension of water. Since surface energy per unit area is equal to surface tension, the surface energy per unit area also decreases.
2. Which of the following is an example of an impurity that lowers the surface energy per unit area of water?
A.
Detergent
B.
Sand settled at the bottom
C.
A glass container
D.
A metal spoon placed outside the water
Correct Answer:
Detergent
Explanation:
Detergent molecules collect at the water surface and reduce its surface tension, thereby lowering the surface energy per unit area.
3. The surface energy of a liquid surface is affected by impurities because impurities can change its:
A.
Surface tension
B.
Mass only
C.
Volume of the container only
D.
Gravitational acceleration
Correct Answer:
Surface tension
Explanation:
Surface energy per unit area is numerically equal to surface tension. Therefore, an impurity changes surface energy by changing the surface tension of the liquid.
4. If an impurity decreases the surface tension of a liquid, what happens to the surface energy required to create a fixed new area of its surface?
A.
It decreases
B.
It increases
C.
It becomes independent of surface area
D.
It becomes infinite
Correct Answer:
It decreases
Explanation:
The work needed to create a surface of area A is W = T A, where T is surface tension. If an impurity lowers T, the required surface energy for the same area decreases.
5. Why does soap help water spread more easily over a surface?
A.
Soap lowers the surface energy per unit area of water
B.
Soap makes the surface energy infinitely large
C.
Soap removes all molecules from water
D.
Soap changes water into a solid
Correct Answer:
Soap lowers the surface energy per unit area of water
Explanation:
Soap lowers the surface tension and hence the surface energy per unit area of water. This makes the water surface spread more easily.
6. For the same increase in surface area, which liquid requires less surface energy after a surface-active impurity is added?
A.
The liquid containing the surface-active impurity
B.
The pure liquid in every case
C.
Both liquids always require zero energy
D.
Neither liquid has a surface
Correct Answer:
The liquid containing the surface-active impurity
Explanation:
A surface-active impurity such as soap usually lowers surface tension. Since surface energy for a fixed area is proportional to surface tension, less energy is required after its addition.
7. The effect of an impurity on the surface energy of a liquid is generally:
A.
Dependent on the nature of the impurity
B.
Always exactly the same for every impurity
C.
Always zero
D.
Independent of the liquid
Correct Answer:
Dependent on the nature of the impurity
Explanation:
Different impurities interact differently with liquid molecules at the surface. Therefore, an impurity may increase or decrease surface tension and hence may increase or decrease surface energy per unit area.
8. A detergent is added to water in a vessel. Compared with pure water, the surface tension of the detergent solution is usually:
A.
Lower
B.
Higher in all cases
C.
Exactly zero
D.
Unrelated to surface energy
Correct Answer:
Lower
Explanation:
Detergent is a surface-active impurity. It usually lowers the surface tension of water, which also lowers the surface energy per unit area.
9. If the surface tension of a liquid is reduced by an impurity, the energy needed to increase its surface area by the same amount will be:
A.
Smaller
B.
Larger
C.
Unchanged in all cases
D.
Equal to the mass of the impurity
Correct Answer:
Smaller
Explanation:
The surface energy change is given by the product of surface tension and increase in area. A reduction in surface tension therefore reduces the energy needed for the same increase in area.
10. Which statement about impurities and surface energy is correct?
A.
Impurities can alter the surface energy of a liquid by changing its surface tension
B.
Impurities can never affect surface energy
C.
Only the volume of a liquid determines its surface energy
D.
Adding any impurity always makes surface energy zero
Correct Answer:
Impurities can alter the surface energy of a liquid by changing its surface tension
Explanation:
Surface tension determines surface energy per unit area. Since impurities can modify surface tension, they can also modify the surface energy of the liquid surface.
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Effect of Impurities on Surface Energy
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