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The Browning Apple Brain Teaser
An apple is cut in half and left on a plate. Within an hour the white flesh has turned brown, but the inside of a whole, uncut apple stays white. What causes the cut surface to turn brown?
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What's the puzzle?
An apple is cut in half and left on a plate. Within an hour the white flesh has turned brown, but the inside of a whole, uncut apple stays white. What causes the cut surface to turn brown?
What's the answer?
The flesh reacts with oxygen in the air (oxidation)
How do you work it out?
- Cutting the apple breaks open its cells and exposes the flesh to the air.
- Air contains oxygen. Chemicals in the apple react with that oxygen, helped along by an enzyme released from the broken cells.
- This reaction, called oxidation, produces brown substances on the cut surface.
- The cut apple turns brown because its flesh reacts with oxygen in the air.
Frequently asked questions
Why does lemon juice stop a cut apple going brown?
Lemon juice coats the cut surface and is acidic, which slows down the enzyme that speeds up the browning reaction. Covering the apple also helps, because it reduces contact with oxygen in the air.
Is the browning apple a chemical or a physical change?
The browning of a cut apple is a chemical change, because new brown substances are made when the flesh reacts with oxygen. LoveMaths Tutoring UK uses this puzzle to connect an everyday kitchen change with chemical reactions.
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