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Each of the following pairs reacts with water. Which pair produces the same gas in both cases?

1. K and KO2
2. Na and Na2O2
3. Ca and CaH2
4. Ba and BaO2

Correct answer: Ca and CaH2

Solution

Ca + 2H2O -> Ca(OH)2 + H2 and CaH2 + 2H2O -> Ca(OH)2 + 2H2 both evolve hydrogen. The stored pair K/KO2 gives H2 and O2 respectively (different gases).

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