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Changes Around Us: Physical and Chemical

Two questions, not one

A 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.

Two questions make four boxes.
Step 1 of 5
  1. Two questions make four boxes.
  2. No new substance, and we can undo it: ice melting.
  3. No new substance, and we cannot undo it: a smashed pot.
  4. A new substance, and we can undo it: a cell that is charged, then used.
  5. 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.

Jump to lesson

This track

  1. 0 Start here
  2. 1 Fold it, cut it, crumple it
  3. 2 Ice, water and steam are one substance
  4. 3 Dissolving is not disappearing
  5. 4 Some of these will not go back
  6. 5 Checkpoint: changes that make nothing new
  7. 6 A test for something new
  8. 7 Five signs, and what each one is worth
  9. 8 One candle, two changes at once
  10. 9 Two questions, not one
  11. 10 Slow changes in rocks
  12. 11 Checkpoint: new substances and the two questions
  13. 12 Burning needs air
  14. 13 The fire triangle
  15. 14 A change written in words
  16. 15 Useful and harmful changes
  17. 16 Checkpoint: burning, equations and useful changes
  18. 17 The big picture
  19. 18 Track quiz

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