Four test tubes contain solutions of CuSO4, MgSO4, ZnSO4, and AgNO3. A student adds zinc metal to each test tube. In which test tube(s) will a reaction occur?

Zinc will react only with CuSO4 and AgNO3 because zinc is more reactive than copper and silver, allowing it to displace them from their salt solutions: Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s); Zn(s) + 2AgNO3(aq) → Zn(NO3)2(aq) + 2Ag(s). In the reactivity series, zinc is placed above copper and silver, but it is below magnesium; therefore it cannot displace magnesium. Zinc cannot react with its own salt solution as there is no difference in chemical potential to trigger a displacement. In the CuSO4 tube, the blue colour will fade and reddish-brown copper will deposit, while in the AgNO3 tube, shiny silver crystals will form on the zinc surface.