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Mystery #1Tier 1-2chemistry / reactions

Why does a sliced apple go brown?

Cut an apple and leave it on the side for a while, and the white flesh slowly turns an ugly brown, yet the skin-covered apple stayed perfectly fine for weeks. What changes the moment you slice it?

Chemical equation: a colourless phenol plus oxygen, sped up by the enzyme PPO, becomes a brown quinone and water, the reaction that browns a cut apple.
Photo: A2569875, Wikimedia Commons · Public domain · via Wikimedia Commons

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