physics : class11 Angle of Contact and Wetting : Surface Tension : Properties of Bulk Matter MCQ Questions and Answers
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Angle of Contact and Wetting
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MCQ Questions
1. A liquid makes an angle of contact of 120° with a solid surface. Which statement correctly describes the relative strengths of the adhesive and cohesive forces at the interface?
A.
Adhesive forces are stronger than cohesive forces, so the liquid spreads completely
B.
Cohesive forces are stronger than adhesive forces, so the liquid does not wet the solid appreciably
C.
Adhesive and cohesive forces are equal, so the angle of contact must be 90°
D.
The angle of contact is independent of adhesive and cohesive forces
Correct Answer:
Cohesive forces are stronger than adhesive forces, so the liquid does not wet the solid appreciably
Explanation:
An obtuse angle of contact indicates that cohesive attraction within the liquid is stronger than adhesive attraction between the liquid and solid. Hence, the liquid tends not to wet the surface.
2. For a liquid in contact with a solid and air, Young's equation is γSA = γSL + γLA cos θ. If γSA = 0.080 N m⁻¹, γSL = 0.050 N m⁻¹, and γLA = 0.060 N m⁻¹, the angle of contact is closest to:
A.
30°
B.
60°
C.
90°
D.
120°
Correct Answer:
60°
Explanation:
Using Young's equation, cos θ = (γSA − γSL)/γLA = (0.080 − 0.050)/0.060 = 0.5. Therefore, θ = 60°.
3. A liquid completely wets a clean solid surface under equilibrium conditions. Which conclusion about its contact angle is correct?
A.
The contact angle is exactly 90°
B.
The contact angle is greater than 90°
C.
The contact angle approaches 0°
D.
The contact angle approaches 180°
Correct Answer:
The contact angle approaches 0°
Explanation:
Complete wetting occurs when the liquid spreads over the solid surface. In the ideal limiting case, the angle between the liquid surface and solid surface approaches 0°.
4. A liquid drop is placed on a solid surface. Its equilibrium contact angle changes from 70° to 110° after the surface is chemically treated, while the liquid surface tension remains unchanged. What has necessarily happened?
A.
The adhesive interaction between the liquid and solid has become relatively weaker
B.
The cohesive interaction within the liquid has become zero
C.
The liquid has become completely wetting
D.
The solid surface tension has become equal to the liquid surface tension
Correct Answer:
The adhesive interaction between the liquid and solid has become relatively weaker
Explanation:
An increase from an acute to an obtuse contact angle indicates reduced wettability. This corresponds to adhesive attraction between the liquid and solid becoming relatively weaker compared with cohesion within the liquid.
5. For a liquid-solid-air system, γLA = 0.050 N m⁻¹ and γSA − γSL = −0.025 N m⁻¹. Which statement about the contact angle is correct?
A.
The contact angle is 30°, indicating strong wetting
B.
The contact angle is 60°, indicating partial wetting
C.
The contact angle is 120°, indicating poor wetting
D.
The contact angle is 180°, indicating complete spreading
Correct Answer:
The contact angle is 120°, indicating poor wetting
Explanation:
Young's equation gives cos θ = (γSA − γSL)/γLA = −0.025/0.050 = −0.5. Thus θ = 120°, an obtuse angle associated with poor wetting.
6. At equilibrium, the tangent to the liquid surface at the point of contact with a solid makes an angle θ with the solid surface measured through the liquid. If θ = 0°, which condition is represented?
A.
The liquid forms a spherical cap and does not wet the surface
B.
The liquid exhibits complete wetting of the solid
C.
The liquid is in neutral wetting with the solid
D.
The liquid-solid adhesive force is necessarily zero
Correct Answer:
The liquid exhibits complete wetting of the solid
Explanation:
A zero contact angle means that the liquid surface becomes tangent to the solid surface and spreads over it. This is the ideal condition of complete wetting.
7. Two liquids have contact angles 30° and 150° on the same solid surface. Assuming their liquid-air surface tensions are equal, which comparison follows from Young's equation?
A.
The liquid with 150° contact angle has the greater value of γSA − γSL
B.
The liquid with 30° contact angle has the greater value of γSA − γSL
C.
Both liquids have the same value of γSA − γSL
D.
The values cannot be compared because contact angle is unrelated to surface energies
Correct Answer:
The liquid with 30° contact angle has the greater value of γSA − γSL
Explanation:
Young's equation gives γSA − γSL = γLA cos θ. With equal γLA, the value is proportional to cos θ. Since cos 30° is positive and greater than cos 150°, the 30° liquid has the greater difference.
8. A liquid drop on a solid has contact angle θ satisfying cos θ = 0.8. Which change would most directly make the liquid less wetting on the same surface, assuming γLA remains unchanged?
A.
Increase γSA − γSL from 0.8γLA to 0.9γLA
B.
Decrease γSA − γSL from 0.8γLA to 0.3γLA
C.
Change θ from about 37° to about 17°
D.
Increase the value of cos θ while keeping the liquid surface tension fixed
Correct Answer:
Decrease γSA − γSL from 0.8γLA to 0.3γLA
Explanation:
Since cos θ = (γSA − γSL)/γLA, decreasing γSA − γSL decreases cos θ and therefore increases θ. A larger contact angle means poorer wetting.
9. A liquid has an obtuse contact angle on a solid. Which alteration would convert the system toward better wetting, provided the liquid-air surface tension is unchanged?
A.
Increase γSL relative to γSA
B.
Decrease γSL relative to γSA
C.
Increase the liquid's cohesive force while leaving all interfacial energies unchanged
D.
Make cos θ more negative
Correct Answer:
Decrease γSL relative to γSA
Explanation:
From cos θ = (γSA − γSL)/γLA, decreasing γSL relative to γSA increases cos θ and reduces θ. A smaller contact angle corresponds to improved wetting.
10. A small liquid drop rests on a solid surface with an equilibrium contact angle of 90°. Which statement is most appropriate?
A.
The liquid completely wets the solid
B.
The liquid does not wet the solid at all
C.
The system represents the boundary between preferential wetting and non-wetting behavior
D.
The liquid must have zero surface tension
Correct Answer:
The system represents the boundary between preferential wetting and non-wetting behavior
Explanation:
At θ = 90°, cos θ = 0, so Young's equation gives γSA = γSL. This is the neutral-contact condition separating acute-angle wetting from obtuse-angle poor wetting.
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