Lesson 9 of 19
Two questions, not one
Show on a big screenA rule on the wall says, "Physical means we can undo it." A broken pot sits below that rule.
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The idea
A rechargeable cell can be charged again after use.
Step 1 of 5
- Two questions make four boxes.
- No new substance, and we can undo it: ice melting.
- No new substance, and we cannot undo it: a smashed pot.
- A new substance, and we can undo it: a cell that is charged, then used.
- A new substance, and we cannot undo it: rust.
The rule
Ask two questions. Is something new made? Can we undo it?
- Question one. New substance or not? This alone decides physical or chemical.
- Question two. Can we undo it? This is about going back, not about the substance.
A smashed pot makes nothing new. Glue leaves joins, so it is not the same pot. That one pot shows the wall rule is wrong.
Charging a rechargeable cell undoes its chemical change. So a chemical change is not always one-way.
Try the lab to finish this lesson.