Select from A to G the reactant added, to give the products 1 to 5, in the preparation of hydrogen gas.

A : dilute acid
B : dilute alkali
C : cold water
D : conc. alkali
E : boiling water
F : conc. acid
G : steam

1. Ca(OH)₂ + H₂
2. MgO + H₂
3. Fe₃O₄ + H₂
4. Al₂(SO₄)₃ + H₂
5. NaAlO₂ + H₂

1. C : cold water

$\text{Ca} + \underset{\text{cold water}}{2\text{H}$2\text{O}} \longrightarrow \text{Ca(OH)}2 + \text{H}_2

2. E : boiling water

$\text{Mg} + \underset{\text{boiling water}}{\text{H}$2\text{O}} \longrightarrow \text{MgO} + \text{H}2

3. G : steam

$3\text{Fe} + \underset{\text{steam}}{4\text{H}$2\text{O}} \rightleftharpoons \text{Fe}3\text{O}4 + 4\text{H}2

4. A : dilute acid

$2\text{Al} + \underset{\text{dil. acid}}{3\text{H}$2\text{SO}4} \longrightarrow \text{Al}2(\text{SO}4)3 + 3\text{H}2

5. D : conc. alkali

$2\text{Al} + \underset{\text{conc. alkali}}{2\text{NaOH}} + 2\text{H}$2\text{O} \longrightarrow 2\text{NaAlO}2 + 3\text{H}_2