Which of the following reactions produces hydrogen?

M g + H_{2} O

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H_{2} S_{2} O_{8} + H_{2} O

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N a_{2} O_{2} + 2 H C l

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B a O_{2} + H C l

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Solution

The correct option is A $M g + H_{2} O$
Alkali and alkaline earth metals react with water to produce hydrogen gas and metal hydroxides. This is due to their high electropositive character.
$M g + 2 H_{2} O ⟶ M g (O H)_{2} + H_{2}$

$B a O_{2} + 2 H C l \to B a C l_{2} + H_{2} O_{2}$
$N a_{2} O_{2} + 2 H C l \to 2 N a C l + H_{2} O_{2}$
$H_{2} S_{2} O_{8} + 2 H_{2} O \to H_{2} O_{2} + 2 H_{2} S O_{4}$

Hence, only reaction (a) liberates hydrogen gas.

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