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Four colourless salt solutions are taken in separate test tubes and a copper strip is dipped in each. Which solution will turn blue?

1. KNO3
2. AgNO3
3. Zn(NO3)2
4. ZnSO4

Correct answer: AgNO3

Solution

Copper is more reactive than silver, so it displaces silver from AgNO3, forming blue Cu(2+) ions in solution.

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