physics : class11 Surface Energy in Wetting : Surface Energy : Properties of Bulk Matter MCQ Questions and Answers
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Surface Energy in Wetting
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MCQ Questions
1. A liquid drop rests in equilibrium on a smooth solid surface with contact angle θ. If the solid–vapour, solid–liquid, and liquid–vapour surface energies per unit area are γSV, γSL, and γLV respectively, which expression gives the work of adhesion between the liquid and solid?
A.
γSV + γSL − γLV
B.
γSV + γLV − γSL
C.
γSL + γLV − γSV
D.
γLV(1 − cos θ)
Correct Answer:
γSV + γLV − γSL
Explanation:
The work of adhesion is the decrease in surface energy when separate solid and liquid surfaces form a solid–liquid interface: Wa = γSV + γLV − γSL. Using Young's equation, this is also γLV(1 + cos θ).
2. For a liquid drop on a solid surface, the spreading coefficient is defined as S = γSV − γSL − γLV. Which condition corresponds to complete wetting at equilibrium?
A.
S < 0 and θ = 180°
B.
S = 0 and θ = 90°
C.
S ≥ 0 and θ = 0°
D.
S < 0 and θ = 0°
Correct Answer:
S ≥ 0 and θ = 0°
Explanation:
Complete wetting occurs when spreading lowers or does not increase the total surface energy, so S is non-negative. The liquid spreads into a thin film and the equilibrium contact angle is zero.
3. A liquid has γLV = 0.050 N m−1 and makes a contact angle of 60° with a solid surface. What is the work of adhesion between the liquid and the solid?
A.
0.025 J m−2
B.
0.050 J m−2
C.
0.075 J m−2
D.
0.100 J m−2
Correct Answer:
0.075 J m−2
Explanation:
The work of adhesion is Wa = γLV(1 + cos θ). Since cos 60° = 0.5, Wa = 0.050(1 + 0.5) = 0.075 J m−2.
4. A liquid partially wets a solid with equilibrium contact angle θ. If γLV remains constant but θ decreases from 120° to 30°, what happens to the work of adhesion?
A.
It decreases because the liquid becomes more spread out
B.
It increases because 1 + cos θ increases
C.
It remains unchanged because γLV is constant
D.
It becomes zero because the contact angle decreases
Correct Answer:
It increases because 1 + cos θ increases
Explanation:
Wa = γLV(1 + cos θ). As θ decreases from 120° to 30°, cos θ increases from −0.5 to about 0.866, so the work of adhesion increases. Smaller contact angle indicates stronger wetting.
5. A liquid has surface energy γLV = 0.072 N m−1 and contact angle 90° on a solid. What is the difference γSV − γSL for this system?
A.
−0.072 J m−2
B.
0 J m−2
C.
0.036 J m−2
D.
0.144 J m−2
Correct Answer:
0 J m−2
Explanation:
Young's equation is γSV − γSL = γLV cos θ. At θ = 90°, cos θ = 0, hence γSV − γSL = 0.
6. A liquid drop spreads spontaneously over a solid surface when the surface-energy change per unit newly covered area is negative. Which expression represents this change when a liquid–vapour interface and a solid surface are replaced by a solid–liquid interface?
A.
γSL − γSV − γLV
B.
γSV + γSL − γLV
C.
γSV − γSL + γLV
D.
γSL + γLV − γSV
Correct Answer:
γSL − γSV − γLV
Explanation:
When unit area spreads, the solid–vapour and liquid–vapour surfaces disappear while a solid–liquid surface forms. Therefore ΔE = γSL − γSV − γLV. Spontaneous spreading requires ΔE < 0, equivalent to S > 0.
7. For a liquid on a solid, γSV = 0.080 N m−1, γSL = 0.020 N m−1, and γLV = 0.050 N m−1. What is the equilibrium wetting condition predicted from the spreading coefficient?
A.
Partial wetting with cos θ = 0.2
B.
Partial wetting with cos θ = −0.2
C.
Complete wetting because S = 0.010 N m−1
D.
No wetting because S = −0.010 N m−1
Correct Answer:
Complete wetting because S = 0.010 N m−1
Explanation:
S = γSV − γSL − γLV = 0.080 − 0.020 − 0.050 = 0.010 N m−1, which is positive. Hence spreading is energetically favorable and complete wetting occurs.
8. Two liquids have the same liquid–vapour surface energy. Liquid X has contact angle 20° and liquid Y has contact angle 100° on the same solid. Which statement is correct regarding their works of adhesion?
A.
Liquid X has a smaller work of adhesion because its contact angle is smaller
B.
Liquid Y has a larger work of adhesion because its contact angle is larger
C.
Both have equal work of adhesion because γLV is equal
D.
Liquid X has a larger work of adhesion because 1 + cos 20° > 1 + cos 100°
Correct Answer:
Liquid X has a larger work of adhesion because 1 + cos 20° > 1 + cos 100°
Explanation:
Wa = γLV(1 + cos θ). With equal γLV, the liquid having the smaller contact angle has the larger work of adhesion. Since cos 20° is greater than cos 100°, liquid X adheres more strongly.
9. A liquid has γLV = 0.040 N m−1 and γSV − γSL = 0.020 N m−1. Assuming partial wetting, what is the contact angle?
10. A liquid initially forms a contact angle of 150° on a solid. A surface treatment changes the solid surface energy while leaving γSL and γLV unchanged, and the contact angle becomes 30°. Which conclusion follows?
A.
The work of adhesion decreases and the solid becomes less wettable
B.
The work of adhesion increases and the solid becomes more wettable
C.
The liquid–vapour surface energy must have become zero
D.
The surface treatment cannot affect wetting because γSL is unchanged
Correct Answer:
The work of adhesion increases and the solid becomes more wettable
Explanation:
The work of adhesion is Wa = γLV(1 + cos θ). Changing the angle from 150° to 30° increases 1 + cos θ substantially, so adhesion increases. The much smaller contact angle signifies improved wetting.
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