physics : class11 Surface Energy and Wetting : Surface Energy : Properties of Bulk Matter MCQ Questions and Answers
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Surface Energy and Wetting
MCQ questions and answers.
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MCQ Questions
1. What is meant by the surface energy of a liquid?
A.
Energy required to increase the surface area of the liquid
B.
Energy required to increase the volume of the liquid
C.
Energy stored due to the temperature of the liquid
D.
Energy required to decrease the density of the liquid
Correct Answer:
Energy required to increase the surface area of the liquid
Explanation:
Surface energy is the work or energy required to create a unit increase in the surface area of a liquid.
2. The SI unit of surface energy per unit area is:
A.
N m
B.
N/m
C.
J/m²
D.
J m²
Correct Answer:
J/m²
Explanation:
Surface energy per unit area is measured in joules per square metre, which is also equivalent to newtons per metre.
3. Why do small liquid drops tend to become spherical?
A.
A sphere has the minimum surface area for a given volume
B.
A sphere has the maximum surface area for a given volume
C.
A sphere has no surface energy
D.
A sphere always has the highest temperature
Correct Answer:
A sphere has the minimum surface area for a given volume
Explanation:
Surface energy is proportional to surface area. A liquid drop becomes spherical to minimize its surface area and hence its surface energy.
4. If the surface area of a liquid is increased, its surface energy generally:
A.
Decreases
B.
Increases
C.
Becomes zero
D.
Remains unchanged in every case
Correct Answer:
Increases
Explanation:
Creating more liquid surface requires work, so the surface energy increases when the surface area increases.
5. A liquid wets a solid surface when:
A.
The liquid spreads over the solid surface
B.
The liquid always forms a perfect sphere on the solid
C.
The liquid evaporates immediately
D.
The liquid has no surface energy
Correct Answer:
The liquid spreads over the solid surface
Explanation:
Wetting occurs when adhesive forces between the liquid and solid are sufficiently strong, causing the liquid to spread over the solid surface.
6. Which liquid is generally considered to wet clean glass?
A.
Mercury
B.
Water
C.
Kerosene only
D.
Molten wax only
Correct Answer:
Water
Explanation:
Water adheres strongly to clean glass and spreads over it, so water wets clean glass.
7. Mercury does not wet ordinary glass mainly because:
A.
Cohesive forces in mercury are stronger than its adhesive forces with glass
B.
Mercury has no surface energy
C.
Adhesive forces between mercury and glass are always zero
D.
Mercury has no particles at its surface
Correct Answer:
Cohesive forces in mercury are stronger than its adhesive forces with glass
Explanation:
Mercury forms a rounded shape on glass because the cohesion between mercury molecules is stronger than the adhesion between mercury and glass.
8. A liquid that spreads over a solid surface usually has a contact angle that is:
A.
Greater than 180 degrees
B.
Less than 90 degrees
C.
Exactly 180 degrees
D.
Always equal to 90 degrees
Correct Answer:
Less than 90 degrees
Explanation:
Good wetting is generally associated with a small contact angle, usually less than 90 degrees, indicating that the liquid spreads over the solid.
9. Why does a liquid surface behave like a stretched elastic membrane?
A.
Because molecules at the surface experience a net inward force
B.
Because surface molecules have no intermolecular forces
C.
Because the liquid surface has no energy
D.
Because the liquid surface is made of a solid material
Correct Answer:
Because molecules at the surface experience a net inward force
Explanation:
Surface molecules experience an unbalanced inward cohesive force, giving the liquid surface a tendency to contract and behave like a stretched membrane.
10. Adding soap to water generally helps the water to wet a surface because soap:
A.
Increases the surface tension of water greatly
B.
Reduces the surface tension of water
C.
Removes all surface energy from water
D.
Changes water into a solid
Correct Answer:
Reduces the surface tension of water
Explanation:
Soap lowers the surface tension of water, allowing it to spread more easily over surfaces and improving wetting.
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