Can it be undone?

The idea

Some changes can be reversed, giving back the original object or substance, while others cannot be reversed because the original cannot be got back.

Undo it?

Ice melts into water. A carrot is chopped into pieces. Can we get the ice back? Can we get the whole carrot back?

Which of these is true?

• Every change can be undone
• Some changes can be undone and some cannot
• No change can be undone

Do it, then undo it

Pick a change. Do it. Then try to undo it. Does the original come back?

Reversed or not

If we can get the original object or substance back, the change can be reversed. If we cannot, the change cannot be reversed.

So the changes around us can be grouped into those that can be reversed and those that cannot be reversed.

• Change | Original got back? | So the change
• Ice melts | yes, by freezing the water | can be reversed
• Water evaporates | yes, by condensing the vapour | can be reversed
• A vegetable is chopped | no | cannot be reversed
• Popcorn is made | no | cannot be reversed

Notes

Some changes can be reversed: we get the original back, as when ice melts and is refrozen. Some changes cannot be reversed, like chopping a vegetable or making popcorn.

Check yourself

Ice melts into water, and the water is frozen back into ice. What does this show?

Which of these changes cannot be reversed?

A chopped vegetable cannot be turned back into the whole vegetable. This change...

Ice melting, water evaporating, chopping a vegetable and making popcorn. How many of these four changes can be reversed?

Answer: 2

Two: melted ice can be refrozen, and water vapour can be condensed. Chopped vegetables and popcorn cannot go back.

• The change cannot be reversed. We got the ice back, so the change can be reversed.
• The change can be reversed — correct. Yes! We got the original back.
• No change happened. The ice did change into water. Then it changed back.
• Ice melting. Melted ice can be refrozen into ice.
• Water evaporating. Water vapour can be condensed back into liquid water.
• Making popcorn — correct. Yes! Popcorn cannot go back to its original form.
• cannot be reversed — correct. Yes! The original size and shape cannot be got back.
• does not change the vegetable. The vegetable is in pieces now. Its size and shape changed.
• can be reversed. We cannot get the original back, so it cannot be reversed.