Preparation of Dihydrogen

Amphoteric metals (those whose oxide/hydroxide reacts with both acids and bases) — zinc, aluminium, and beryllium among them — react with hot concentrated NaOH solution to liberate hydrogen gas, forming the corresponding soluble oxo-anion (Zn + 2NaOH → Na₂ZnO₂ + H₂;

Magnesium, by contrast, is NOT amphoteric — Mg(OH)₂ is a simple base with no acidic character, so magnesium metal simply does not react with NaOH at all, making Mg + NaOH the one reaction in a mixed list that is NOT a valid method for preparing H₂ this way.

Water gas (a mixture of CO and H₂, made by passing steam over red-hot coke) already contains hydrogen, but passing it together with additional steam over a catalyst converts more of the carbon monoxide into extra hydrogen: CO + H₂O → CO₂ + H₂ — this is specifically called the water-gas shift reaction, and it is the standard industrial route for boosting the proportion of H₂ obtainable from water gas (the resulting CO₂ is then removed, typically by scrubbing with a base, leaving purer H₂).

When the water-gas shift reaction (CO + H₂O → CO₂ + H₂) is carried out at an industrial scale using a specific catalyst combination — a mixture of Fe₂O₃ and Cr₂O₃, at around 673 K — this named, catalysed industrial process is called the Bosch process. Recognise this specific temperature-plus-catalyst combination as the identifying signature of the Bosch process, distinct from other named metallurgical or industrial processes that sound superficially similar (Nelson process, Serpeck process, Parkes process, none of which relate to hydrogen production this way).