1. What happens to the surface tension of a liquid when its temperature is increased?
A.
It generally increases
B.
It generally decreases
C.
It remains exactly constant
D.
It first becomes zero and then increases
Correct Answer:
It generally decreases
Explanation:
Surface tension generally decreases as temperature increases because the cohesive forces between liquid molecules become less effective.
2. At which temperature does the surface tension of a liquid become nearly zero?
A.
At its melting point
B.
At room temperature
C.
At its critical temperature
D.
At its boiling point only
Correct Answer:
At its critical temperature
Explanation:
At the critical temperature, the distinction between liquid and vapour disappears, so the surface tension becomes nearly zero.
3. If water is cooled, how does its surface tension generally change?
A.
It increases
B.
It decreases
C.
It becomes zero
D.
It remains unchanged
Correct Answer:
It increases
Explanation:
Cooling generally increases the cohesive attraction between molecules, causing the surface tension of water to increase.
4. Which statement correctly describes the temperature dependence of surface tension?
A.
Surface tension increases continuously with temperature
B.
Surface tension decreases as temperature rises
C.
Surface tension is independent of temperature
D.
Surface tension becomes maximum at the critical temperature
Correct Answer:
Surface tension decreases as temperature rises
Explanation:
An increase in temperature reduces the effective intermolecular cohesive forces at the liquid surface, so surface tension decreases.
5. A liquid has greater surface tension at 20°C than at 50°C. Which conclusion is correct?
A.
Surface tension increases with temperature
B.
Surface tension decreases with temperature
C.
Temperature has no effect on surface tension
D.
The liquid has reached its critical temperature at 50°C
Correct Answer:
Surface tension decreases with temperature
Explanation:
The lower surface tension at 50°C shows that surface tension decreases when the temperature of the liquid is increased.
6. Why does surface tension decrease when a liquid is heated?
A.
Molecular motion increases and cohesive effects become less effective
B.
The mass of each molecule increases
C.
The liquid changes directly into a solid
D.
The number of molecules in the liquid becomes zero
Correct Answer:
Molecular motion increases and cohesive effects become less effective
Explanation:
Heating increases molecular motion, which reduces the relative effect of cohesive forces responsible for surface tension.
7. Which liquid condition generally produces the highest surface tension for the same liquid?
A.
A higher temperature
B.
A lower temperature
C.
The critical temperature
D.
A temperature at which the liquid has no molecules
Correct Answer:
A lower temperature
Explanation:
For the same liquid, surface tension is generally higher at lower temperatures and decreases as the temperature rises.
8. What is the approximate value of surface tension at the critical temperature?
A.
Very large
B.
Nearly zero
C.
Exactly equal to the density
D.
Independent of the liquid
Correct Answer:
Nearly zero
Explanation:
At the critical temperature, the liquid and vapour phases become indistinguishable, so the surface tension approaches zero.
9. A liquid is heated from 25°C to 75°C without changing its composition. What will most likely happen to its surface tension?
A.
It will increase
B.
It will decrease
C.
It will become infinite
D.
It will remain exactly the same
Correct Answer:
It will decrease
Explanation:
Surface tension decreases when the temperature of a liquid is increased, provided the liquid remains in the liquid state.
10. Which graph best represents the usual relation between surface tension and temperature for a liquid?
A.
A graph showing surface tension increasing with temperature
B.
A graph showing surface tension decreasing with temperature
C.
A horizontal graph showing no temperature dependence
D.
A graph showing surface tension becoming infinite at the critical temperature
Correct Answer:
A graph showing surface tension decreasing with temperature
Explanation:
The usual trend is a decrease in surface tension with increasing temperature, approaching zero near the critical temperature.
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