1 of 10

Necessary Prior Knowledge

  • Matter has Mass
  • Particle model
  • Chemical reactions:
    • Synthesis, decomposition, oxidation, combustion
  • Conservation of mass

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2 of 10

Watch steel wool burning

3 of 10

Steel wool before and after being burned…

Before

After

4 of 10

Predict what will happen to the mass of the steel wool as it burns.

The mass will increase

The mass will decrease

The mass will stay the same

A

B

C

5 of 10

Predict what will happen to the mass of the steel wool as it burns.

6 of 10

What is the best way to fully explain the change in mass?

Before

After

7 of 10

Possible criteria

Select:

    • Scientific Accuracy (i.e. what known concepts or ideas ought to be used to explain the phenomenon correctly)

Organize:

    • Simplicity (i.e. the number of concepts and ideas used to explain is minimized)
    • Mode or Format of Presentation (i.e. an analogy, picture or written explanation is used that improves the communication of the concepts or ideas in the explanation)

Integrate:

    • Consistency with Established Knowledge (i.e. the explanation does not violate other known science rules or facts)

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8 of 10

Possible explanation meeting above criteria

  1. The “extra” mass comes from oxygen in the air combining chemically with the iron in the steel wool to form iron oxide – a synthesis chemical reaction.
  2. The overall mass goes up because the iron is now chemically bound to additional atoms of oxygen, so their mass is added to the steel wool’s.
  3. Although the burning process sometimes makes it look like material is being “lost,” in this case it is actually gaining mass thanks to the absorbed oxygen, illustrating the law of conservation of mass when the atmosphere is included in the system.
  4. Sketch:

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1.33 g

1.60 g

Before

After

Bonded

9 of 10

Use professional judgement to decide

  • When the students have sufficient prior knowledge to engage in this activity meaningfully
  • Where this line of questioning best fits into your lesson on chemical reactions
  • If this can be done in small groups (no more than 3)
  • What questions you might ask students who are stuck to help them get unstuck (best prepared ahead of time)
  • How much time you ought to spend in group discussion sharing the explanations so students can learn from their peers

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10 of 10

If all the products of the combustion paper are collected and weighed, what might be the mass after the paper is burned?

Before

After

A

B

C

200 g

230 g

130 g