chemistry : class11 Cohesive and Adhesive Forces : Surface tension : States of Matter: Gases and Liquids MCQ Questions and Answers
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Cohesive and Adhesive Forces
MCQ questions and answers.
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MCQ Questions
1. A liquid rises in a narrow glass capillary and forms a concave meniscus. Which relationship between intermolecular forces best explains both observations?
A.
Cohesive forces within the liquid are weaker than adhesive forces between the liquid and glass
B.
Cohesive forces within the liquid are stronger than adhesive forces between the liquid and glass
C.
Adhesive forces are absent, while cohesive forces act only at the surface
D.
Cohesive and adhesive forces are equal, but the liquid has unusually high density
Correct Answer:
Cohesive forces within the liquid are weaker than adhesive forces between the liquid and glass
Explanation:
When adhesion between the liquid and glass exceeds cohesion within the liquid, the liquid spreads along the glass, producing a concave meniscus and capillary rise.
2. Mercury forms a convex meniscus and is depressed in a glass capillary. What does this indicate about the relative strengths of the forces involved?
A.
Adhesion between mercury and glass is greater than cohesion within mercury
B.
Cohesion within mercury is greater than adhesion between mercury and glass
C.
Both cohesion and adhesion are negligible because mercury is a liquid
D.
Adhesion and cohesion are equal, but mercury has a high surface temperature
Correct Answer:
Cohesion within mercury is greater than adhesion between mercury and glass
Explanation:
Strong mercury-mercury cohesive forces compared with mercury-glass adhesive forces pull the surface inward, forming a convex meniscus and causing capillary depression.
3. A liquid completely wets a solid surface and spreads into a thin film. Which condition is most consistent with this behavior?
A.
The liquid-solid adhesive attraction is stronger than the liquid-liquid cohesive attraction
B.
The liquid-liquid cohesive attraction is stronger than the liquid-solid adhesive attraction
C.
The liquid has no intermolecular attractions but the solid has strong cohesion
D.
The liquid-solid and liquid-liquid attractions are both zero
Correct Answer:
The liquid-solid adhesive attraction is stronger than the liquid-liquid cohesive attraction
Explanation:
Spreading occurs when attraction between liquid molecules and the solid surface is sufficiently stronger than attraction among the liquid molecules, allowing adhesion to overcome cohesion.
4. Two liquids have similar densities and are placed separately in identical glass capillaries. Liquid X rises, whereas liquid Y is depressed. Which conclusion is most justified?
A.
Adhesion to glass dominates cohesion for X, while cohesion dominates adhesion for Y
B.
Cohesion dominates adhesion for X, while adhesion dominates cohesion for Y
C.
Both liquids have identical adhesive forces, but X has a lower boiling point
D.
The opposite capillary behaviors must result only from differences in density
Correct Answer:
Adhesion to glass dominates cohesion for X, while cohesion dominates adhesion for Y
Explanation:
A rising liquid has stronger liquid-glass adhesion relative to its internal cohesion, whereas a depressed liquid has stronger internal cohesion relative to adhesion with glass.
5. A small oil drop placed on a clean glass surface spreads widely, but the same oil placed on a wax-coated surface forms an almost spherical drop. Which explanation is correct?
A.
Oil-glass adhesion is stronger than oil-oil cohesion, while oil-wax adhesion is weaker than oil-oil cohesion
B.
Oil-oil cohesion is stronger than both oil-glass and oil-wax adhesion
C.
Oil-wax adhesion is stronger than oil-glass adhesion, causing the wax surface to repel gravity
D.
The oil becomes chemically different when it contacts the glass surface
Correct Answer:
Oil-glass adhesion is stronger than oil-oil cohesion, while oil-wax adhesion is weaker than oil-oil cohesion
Explanation:
Strong adhesion to glass allows the oil to spread, while weak adhesion to wax leaves oil-oil cohesion dominant, causing the drop to minimize its exposed surface and remain nearly spherical.
6. Water rises in a clean glass capillary, but after the glass is coated with a hydrophobic material, the water level is depressed. What change in intermolecular interaction caused this result?
A.
The coating increased water-water cohesion relative to water-surface adhesion
B.
The coating eliminated cohesion between water molecules
C.
The coating increased water-surface adhesion until it exceeded water-water cohesion
D.
The coating converted water into a liquid with zero surface tension
Correct Answer:
The coating increased water-water cohesion relative to water-surface adhesion
Explanation:
A hydrophobic coating weakens the attraction between water and the surface. Consequently, water-water cohesion becomes dominant, producing a convex meniscus and capillary depression.
7. A liquid has a contact angle greater than 90 degrees on a solid surface. Which interpretation is most appropriate in terms of cohesive and adhesive forces?
A.
Adhesive forces between the liquid and solid are weaker than cohesive forces within the liquid
B.
Adhesive forces between the liquid and solid are stronger than cohesive forces within the liquid
C.
Cohesive forces are absent, so the contact angle is determined only by gravity
D.
Cohesive and adhesive forces must be exactly equal at the three-phase boundary
Correct Answer:
Adhesive forces between the liquid and solid are weaker than cohesive forces within the liquid
Explanation:
A contact angle above 90 degrees indicates poor wetting. The liquid molecules prefer attracting one another over adhering strongly to the solid surface, so cohesion exceeds adhesion.
8. A detergent is added to water on a greasy solid surface, causing the water to spread more effectively. Which force-related change most directly explains this observation?
A.
Detergent increases the adhesive interaction between the aqueous liquid and the surface relative to water-water cohesion
B.
Detergent eliminates all cohesive forces among water molecules
C.
Detergent strengthens grease-grease cohesion, forcing water to spread
D.
Detergent makes the solid surface chemically identical to water
Correct Answer:
Detergent increases the adhesive interaction between the aqueous liquid and the surface relative to water-water cohesion
Explanation:
Detergent molecules can improve interaction between the liquid and the greasy surface, allowing adhesion to compete more effectively with cohesion and thereby promoting wetting.
9. A liquid forms a concave meniscus in a glass tube but does not rise appreciably in the tube. Which inference is most reasonable?
A.
Adhesion exceeds cohesion, but the net capillary effect is small because the relevant force balance is weak
B.
Cohesion greatly exceeds adhesion, which necessarily produces a concave meniscus
C.
Adhesion is absent, so the meniscus shape must result only from gravity
D.
The liquid must have no surface tension because it shows a curved surface
Correct Answer:
Adhesion exceeds cohesion, but the net capillary effect is small because the relevant force balance is weak
Explanation:
A concave meniscus indicates that adhesion to glass is stronger than cohesion within the liquid. However, if their difference is small, the resulting capillary rise may be negligible.
10. A liquid is placed in two capillaries made of different solids. It rises in capillary P but is depressed in capillary Q. What can be concluded without knowing the liquid's density?
A.
The liquid-solid adhesive force is relatively stronger than liquid cohesion for P and relatively weaker for Q
B.
The liquid has stronger cohesion in P than in Q because its composition changes inside the tubes
C.
The liquid-solid adhesive force is weaker than cohesion for P and stronger for Q
D.
The observations cannot provide any information about adhesive and cohesive forces
Correct Answer:
The liquid-solid adhesive force is relatively stronger than liquid cohesion for P and relatively weaker for Q
Explanation:
Capillary rise signifies that adhesion to the solid surface overcomes cohesion relatively, whereas capillary depression signifies that cohesion dominates adhesion. Changing the solid changes the adhesive interaction.
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