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Contributed by: Connecticut Academic Performance Test (CAPT)
Students in a science class were conducting experiments to explore
the use of various substances in removing soap from the water. One
way to remove soap from water is to have it react with other substances.
When the reactions occur, a solid called a precipitate is sometimes
formed. A precipitate can be filtered out of the water.
Group A carried out the following experiment.
- We put soapy water into three separate plastic cups.
- A different substance was added to each of the cups.
- After waiting five minutes, the mixture in each cup
was filtered.
- We examined the precipitate (which remained in the
filter paper) and the filtrate (which was in the test
tube) for each mixture.
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The table below shows our results.
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1 |
2 |
3 |
| Substance added to Soapy Water |
Epsom Salt |
Table Salt |
Sugar |
| Precipitate |
White, milky |
White, milky |
None |
| Filtrate |
Clear |
Slightly cloudy |
Cloudy |
Question 1 - Event Score = 2
Based on their results, Group A concluded that Epsom salt removed
the soap from the water. Is this a valid conclusion? Explain your
answer fully.
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Component
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Score
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Commentary
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Question 1
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(2)
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The response correctly questions the validity
of the conclusion. It explains that, "Just because the
water looks clear it doesn't mean that it has no soap in it."
It explains that a shake test should have been performed. No
other considerations are discussed, and it is therefore less
complete than the score 3 responses. |
Question 2 - Event Score = 2
Do you have enough information to replicate Group A's experiment?
If you think so, explain what information you have. If you think not,
explain what other information you would need.
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Component
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Score
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Commentary
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Question 2
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(2)
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The response correctly states that there is not
enough information to replicate the experiment. It explains
that the amounts of materials added to the soapy water are needed
and should be controlled. Other missing information is not mentioned,
and so the response is less complete than the score 3 responses.
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Group B carried out the following experiment.
- We placed 50 mL of soapy water into three plastic cups.
- We added 10 grams of sugar to cup 1, 10 grams of table
salt to cup 2, and 10 grams of Epsom salt to cup 3. We
mixed each with a plastic spoon.
- We filtered all three mixtures.
- We then poured the filtrate (remaining liquid) into
3 separate test tubes, shook them and measured the height
of the soap suds.
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The table below shows our results.
| Cup |
Substance |
Final Soap Suds Height |
| 1 |
Sugar |
9.1 cm |
| 2 |
Table Salt |
1.2 cm |
| 3 |
Epsom Salt |
0.2 cm |
Question 3 - Event Score = 2
Group B did not include a control in their experiment. What would
be an appropriate control? Explain your answer fully, including how
the control might improve the experiment.
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Component
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Score
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Commentary
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Question 3
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(2)
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The response is correct but less elaborated than
the score 3 responses. There is no mention of the fact that
the control should be shaken after filtering and the suds measured
and compared to the others. |
Question 4 - Event Score = 2
Compare Group A's and Group B's experiments. Which experiment, if
either, is better designed? Explain your answer fully.
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Component
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Score
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Commentary
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Question 4
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(2)
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The response discusses one reason why Group B's
experiment was better (use of shake test) and explains that
neither group included a control. It is less complete than
the score 3 responses. |
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