Question

You are provided with three metals: sodium, magnesium and copper, using only water as the reactant, how will you identify each of them.

Answer 1

Let the Sodium, Magnesium and Copper be sample A, B and C.
When, we put the Sample A in water, we would observe that, the sample would react vigorously with water and catches fire instantly. We would find that the sample burning on the top of the water. With this observation, we can say that, the sample is Sodium (Na). This happens due to the highly re-activeness of Sodium and the huge amount of heat produced makes the metal catch fire immediately.
2Na(s) + 2H2O(l) -> 2NaOH(aq) + H2(g) + heat

When, we put the Sample B into water (cold), nothing could be observed but, if we heat the water, then, we would observe bubbles of hydrogen gas sticking to its surface and making it float to the surface and when, we take a burning matchstick in front of it, it would create a pop sound and the magnesium becomes a white solid. This observation, shows that the sample here is, Magnesium.
Mg(s) + H2O(l) --------(HEAT) -----> Mg(OH)2 (s) + H2(g)

When, we put the sample c into water there is no observation. If we heat the water, then also, there is no observation. If we let it react with steam, still there is no observation. This proves that the sample is Copper. This happens as because Copper is less reactive than Hydrogen. So, it cannot replace it and make its oxide.
Cu(s) + H2O(l) -> NO REACTION
Cu(s) + H2O(l) -----(HEAT)---> NO REACTION
Cu(s) + H2O(g) -> NO REACTION

Regards