Last lesson, every change could be undone. Today is the opposite: changes that make a brand-new substance — and once they happen, there's no going back.
Quick recall before we meet the opposite kind of change.
How do we know a brand-new substance has formed? Scientists look for four telltale signs.
Let's test three everyday changes against our signs. Watch how each one makes something new.
| Change | Signs of new substance | Can you undo it? |
|---|---|---|
| Burning toast | colour change (black), smoke (gas), heat | No — you can't un-burn toast |
| Rusting iron | new material (orange rust), colour change | No — rust is a new substance (iron oxide) |
| Cooking an egg | colour change (clear→white), new texture | No — you can't un-cook an egg |
Goggles on! We'll mix bicarbonate soda with vinegar. Watch for the signs of a chemical change.
On your own, sort these four changes into the right column — and give a reason for each.
One last idea to lock in…