Reversible changes: getting the material back

Science · Matter & Materials · ages 9-10

Name ______________________ Date ____________

Which sentence finishes this idea? A reversible change is a change that ...
- makes a brand new material
- can be undone
- can never be changed back
How can you reverse melted chocolate back to solid chocolate?
- Put it in the freezer
- Stir in some warm milk
- Leave it out in the sun
What happens to the material in a reversible change?
- A new material forms
- The material disappears forever
- No new material forms
A cup of salty water sits in a warm spot until all the water evaporates. What is left in the cup?
- Nothing at all
- The salt
- A brand new material
Which of these changes is NOT reversible?
- Melting chocolate
- Freezing water
- Toasting bread
Priya dissolves salt in water to make salty water. How can she get the solid salt back?
- Let the water evaporate
- Pour in more water
- Pour it through a paper filter
Freezing water to make ice is a reversible change.
Circle one: True False
Water in the freezer turned into ice. Which change undoes the freezing?
- Keeping the ice in the freezer longer
- Adding more cold water to the ice
- Melting the ice back into water
Nia says: 'Melting wax makes a new material, so it is not reversible.' What is wrong with Nia's idea?
- Wax can never melt
- Melting wax makes nothing new, and the same wax can cool back to solid
- Melting always makes something new
Sam dissolves sugar in water, then pours the sugary water down the sink. Is dissolving still a reversible change?
- No, because the sugar went down the sink
- Yes, because evaporating the water could bring the sugar back
- No, because dissolving always makes a new material

Answer key

For grown-ups. Fold this page away before handing over the rest.

Reversible changes: getting the material back W1-mt_UKmtuAsSLN-s1

can be undone · A reversible change can be undone: you get back the exact same material, with nothing new formed.
Put it in the freezer · Cooling melted chocolate makes it freeze back into solid chocolate, the same material as before.
No new material forms · In a reversible change the material only changes state or spreads out. Nothing new is made.
The salt · Evaporation removes the water, and the salt stays behind in the cup.
Toasting bread · Toasting makes the bread into something new that cannot change back. Melting and freezing can both be undone.
Let the water evaporate · Evaporating the water leaves the salt crystals behind, the same salt she started with.
True · Ice can melt back into water, so freezing and melting are both reversible changes.
Melting the ice back into water · Melting is the partner of freezing: it turns the ice back into the same water.
Melting wax makes nothing new, and the same wax can cool back to solid · Melting wax forms nothing new. The same wax cools back to solid, so the change is reversible.
Yes, because evaporating the water could bring the sugar back · A change is reversible if it can be undone. Sam did not undo it, but evaporating would have brought the sugar back.