Extreme Drawing

Reversible and irreversible changes: freezer vs oven

Lesson plan

Can you melt it back?

A reversible change can be undone: melted chocolate sets again and frozen paint thaws. An irreversible change makes something new that won't go back, like a cooked egg, caramel or burnt paper. In 40 minutes pupils predict, test it with the freezer and the oven on the whiteboard, then melt chocolate for real.

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  • 40 minutes
  • Year 5 · Grade 2
  • Free tools only
Painted illustration of two sheets of paper, one covered in frost ferns from the freezer and one baked golden from the oven, the painter holding an ice cube and an oven glove.

At a glance

Age
7 to 10 (England Year 5; US grade 2; NL groep 5 to 7)
Fits
KS2 Year 5 properties and changes of materials · NGSS 2-PS1-4 · kerndoel 42
Time
40 minutes
In the app (all free)
Ice cream, Chocolate, Egg, Ice cube, Iron, Bake in the oven, Put in the freezer, Undo
For real
Chocolate, small zip bags, bowls of warm water, ice cubes, bread and a slice of toast, a raw and a hard-boiled egg
On screen only
Burning (the Fire tool), as an example of an irreversible change

Learning objective and standards

Objective: pupils can sort everyday changes into ones that can be reversed and ones that can't, and give evidence for their choice.

Success criteria

  • I can predict whether a change can be undone.

  • I can test my prediction and say what I saw.

  • I can explain that some changes make a new material that won't turn back.

NGSS 2-PS1-4England science programmes of study

Where it fits

  • England, Year 5, Properties and changes of materials: changes of state are reversible; some changes form new materials and are not usually reversible, including burning.

  • NGSS 2-PS1-4: construct an argument with evidence that some changes caused by heating or cooling can be reversed and some cannot.

  • Netherlands, kerndoel 42: investigating materials and physical phenomena such as temperature.

  • Flanders: the minimumdoelen wetenschap en techniek on materials and their changes.

Painted illustration of two sheets of paper, one covered in frost ferns from the freezer and one baked golden from the oven, the painter holding an ice cube and an oven glove.

What the app shows

Everything for this lesson is in the free part of the app.

  • Put in the freezer gives the page three hours in a freezer. Everything with water in it freezes solid: puddles, paint, sauces, and ice cream goes rock hard again. Wax, eggshell and caramel don't freeze, because there's no water in them. Then it thaws back within a minute or so: thin films first, thick lumps last.

  • Chocolate melts glossy and runny when it's warmed (hold the Iron over it) and sets hard again about 20 seconds after the heat goes.

  • Bake in the oven gives the page twenty minutes at 180 °C. Egg white sets white with crisp brown edges, honey and ice cream cook down to caramel, and the paper toasts brown at its edges. None of that comes back when it cools.

  • Fire burns the paper into holes. That one stays on the screen.

One thing to point out: the app has an Undo button, and it will put a baked page back. Real life has no undo button. That's the whole difference between the two kinds of change.

Play in your browser

The lesson, step by step

Set up before the lesson: the browser version open on the board, a fresh sheet of paper, and the real materials on a tray per group. Boil one egg beforehand.

  1. 5 minutes

    Hook: an ice cream on the board

    Drop an Ice cream scoop on the paper and let the class watch it slump into a puddle (a scoop takes about a minute and a half). Ask: can we get our ice cream back?

  2. 5 minutes

    Predict

    Show four things on the board: the melted ice cream, a bar of Chocolate, an Egg and the bare paper. For each, pupils predict in pairs: freezer or oven, and will it go back afterwards? Record the guesses in a two-column table: goes back / doesn't go back.

  3. 10 minutes

    Try it on the board

    Tap Put in the freezer: the puddle goes hard, and then thaws again. Melt the chocolate with the Iron, take the iron away and count to twenty: it sets. Then tap Bake in the oven: the egg white turns white, the ice cream becomes caramel, the paper browns. Wait for it to cool. Does anything turn back? Let a pupil press Undo and ask why real life can't.

  4. 15 minutes

    Do it for real

    In groups: put a few squares of chocolate in a zip bag and hold it in a bowl of warm water until it melts, then lay it flat on a cool surface and watch it set. Put an ice cube on a saucer and watch it melt. Then compare a slice of bread with a slice of toast, and a raw egg with the hard-boiled one: can anyone turn them back?

  5. 5 minutes

    Explain

    Back to the table of predictions. Which were right? Draw out the rule: melting and freezing change the state and can go back; cooking and burning make a new material that can't. Exit question: is a burnt match reversible?

Vocabulary and differentiation

Key words

  • reversible change: a change that can be undone

  • irreversible change: a change that can't be undone, usually because a new material forms

  • melt / freeze: solid to liquid, liquid to solid

  • set / solidify: a liquid turning solid as it cools

  • thaw: frozen things warming and melting

  • state: solid, liquid or gas

  • new material: something that wasn't there before, like caramel or ash

Support

Sort, don't write

Give picture cards (ice cube, chocolate, egg, toast, ash) to sort into two hoops. Younger pupils (grade 2) can stop here: that's the NGSS argument with evidence.

Core

Predict and record

Use the two-column table, with one sentence of evidence for each item: I saw…, so I think…

Stretch

Is it really the same?

Melted ice cream refrozen for real comes out icy, not creamy. Is that still a reversible change? And why can the oven at 180 °C toast paper without setting it alight, when the Fire tool burns it? (Paper lights at about 230 °C.)

What is the difference between a reversible and an irreversible change?

A reversible change can be undone, like melting chocolate and letting it set again. An irreversible change makes a new material that can't easily turn back, like cooking an egg or burning paper.

Is melting chocolate a reversible change?

Yes. Melting changes chocolate from solid to liquid, and it sets solid again as it cools. If it's heated too much it can burn, and burning isn't reversible.

Is cooking an egg reversible?

No. Heat changes the proteins in the egg white so it turns from clear to white and firm, and cooling it doesn't change it back.

Is freezing water reversible?

Yes. Ice melts back into water when it warms up, and the water can be frozen again.

Do pupils need accounts to use the app in class?

No. Open play.extremedrawing.com in the browser; nobody logs in and no data is collected.

Next lessons and experiments

Painted illustration of the painter pointing at a golden-brown heart and star that appeared on paper after ironing, with an iron, half a lemon and a cotton bud.

Invisible ink

Lemon juice writing that heat turns brown for good: an irreversible change.

Freeze it, bake it, then do it for real

Free in the browser, no pupil accounts. More lessons and the curriculum map are on the teachers' page.

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