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THSB
    Substances, Chemical Reactions, and Conservation of Matter

Item SC075004: When mold grows on a piece of bread in a sealed container, the bag and its contents weigh the same before and after the mold start growing.

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A student placed a piece of bread in a plastic bag. He sealed the bag so that nothing could get in or get out. After two weeks, he noticed mold growing on the bread. (Note that the mold’s weight increases as it grows.)

The student weighed the bag and its contents before and after the mold started growing. After two weeks, did the bag and its contents weigh the same, more, or less than it did before the mold started growing?

  1. The bag and its contents weighed the same.
  2. The bag and its contents weighed more.
  3. The bag and its contents weighed less.
  4. More information is needed to tell if the weight of the bag and its contents changed.

Explain why you selected the answer choice you did. Be as complete as you can and include science ideas in your explanation.*

Topic
Correct Answer
A
Explanation Scoring Rubric

Reasoning – States the relevant general principles and links them to the claim (5 points)

  • States or uses the general principle that the bag was sealed, so matter can’t get in or out. (Closed system)
  • States the general principle that matter is not created or destroyed during chemical reactions. (Conservation)
  • States or uses the general principles of conservation and closed system in terms of atoms.
  • States or uses the general principle that the mass of an atom does not change.
  • Provides a mechanism that links the general principles to the answer choice selected (claim): Although new substances are produced as the mold grows, the new substances are produced by rearranging atoms that were already in the sealed bag. Hence, the bag and its contents weighed the same.


*The prompt shown for the explanation tier reflects revision following its use in the Year 3 study for which student results are shown.

 

Misconceptions
Answer Choice: B
SCM042: If something grows in a closed system, the total mass of the system increases. For example, when a tree grows in a closed system, the total mass of the system increases (Mitchell et al., 1984) or when mold grows in a closed system, the total mass of the system increases (DeBoer et al., 2009).
Distribution of Multiple Choice Responses
Chart showing distrubtion of responses for Item 075004
Multiple Choice Points Earned
Group Correct Total Percent
Overall 66 376 18%
Grades
  6–8 66 376 18%
  9–12 N/A N/A N/A
Gender
  Male4320621%
  Female2317014%
Primary Language
  English6534819%
  Other1284%
Explanation Points Earned
Group Earned Possible Percent
Overall 74 1890 4%
Grades
  6–8 74 1890 4%
  9–12 N/A N/A N/A
Gender
  Male 56 1035 5%
  Female 18 855 2%
Primary Language
  English 73 1750 4%
  Other 1 140 1%

View data table

Distribution of Multiple Choice Responses
Chart showing distrubtion of responses for Item 075004
Multiple Choice Points Earned
Group Correct Total Percent
Overall 217 378 57%
Grades
  6–8 217 378 57%
  9–12 N/A N/A N/A
Gender
  Male13220764%
  Female8517150%
Primary Language
  English20935060%
  Other82829%
Explanation Points Earned
Group Earned Possible Percent
Overall 376 1890 20%
Grades
  6–8 376 1890 20%
  9–12 N/A N/A N/A
Gender
  Male 223 1035 22%
  Female 153 855 18%
Primary Language
  English 364 1750 21%
  Other 12 140 9%

View data table