Explanation:
By lowering surface tension, soap allows water to spread and wet the cloth more readily.
6. If a surface-active impurity is added to a liquid, the cohesive force at the liquid surface usually becomes:
A.
Smaller
B.
Larger
C.
Infinite
D.
Unrelated to the impurity
Correct Answer:
Smaller
Explanation:
Surface-active impurities weaken the effective cohesive attraction at the surface, causing a decrease in surface tension.
7. A student adds detergent to pure water. Which observation is most likely?
A.
The water forms drops more easily because surface tension increases
B.
The water spreads more easily because surface tension decreases
C.
The water immediately becomes solid
D.
The water loses all its molecules
Correct Answer:
The water spreads more easily because surface tension decreases
Explanation:
Detergent lowers the surface tension of water, so the water can spread over surfaces more easily.
8. Which impurity would most likely make water droplets less tightly held together?
A.
Soap
B.
Sodium chloride
C.
A dissolved salt that raises surface tension
D.
A substance that increases cohesive forces at the surface
Correct Answer:
Soap
Explanation:
Soap lowers the surface tension of water, reducing the tendency of water molecules at the surface to hold together.
9. The addition of a small quantity of camphor to water can cause the camphor to move on the surface because it:
A.
Increases surface tension uniformly
B.
Creates differences in surface tension
C.
Removes gravity from the water
D.
Changes water into ice
Correct Answer:
Creates differences in surface tension
Explanation:
Camphor dissolves unevenly and lowers surface tension differently at different places, producing surface-tension differences that move the camphor.
10. Which statement correctly compares soap and sodium chloride when added to water?
A.
Both always decrease surface tension
B.
Both always increase surface tension
C.
Soap generally decreases surface tension, while sodium chloride generally increases it
D.
Neither affects surface tension
Correct Answer:
Soap generally decreases surface tension, while sodium chloride generally increases it
Explanation:
Soap is a surface-active impurity that lowers surface tension, whereas dissolved sodium chloride generally raises the surface tension of water.
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Effect of Impurities
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