Question

Which metal easily gives ${\text{H}}_2$ gas when it reacts with NaOH?
(a). Mg
(b). Zn
(c). Cu
(d). Ag

Answer 1

Hint: While giving the answer, consider the amphoteric nature of metals. Amphoteric oxides are those oxides which can act as an acid or base in a reaction to produce salt and water.

Complete step by step answer:
We know the following basic concepts about acids and bases which we studied in higher secondary. Let us revisit those concepts again.
Acids react with most metals to form a salt and hydrogen gas. As discussed previously, metals that are more active than acids can undergo a single displacement reaction.
For e.g: Zinc metal reacts with hydrochloric acid producing zinc chloride and ${\text{H}}_2$ gas.

Bases also react with certain metals like zinc or aluminum to produce hydrogen gas.
For eg: Sodium hydroxide reacts with zinc and water to form sodium zincate and ${\text{H}}_2$gas.

Zinc and aluminium when treated with aqueous $\text{NaOH}$ solution yield ${\text{H}}_2$ gas and sodium zincate/aluminium respectively. These reactions are highly exothermic.$\text{Zn+NaOH}\to \text{N}{\text{a}}_2\text{Zn}{\text{O}}_2\text{ (Sodium Zincate)+}{\text{H}}_2$
$\text{Zn+NaOH}\to \text{N}{\text{a}}_2\text{Zn}{\text{O}}_2\text{ (Sodium Zincate)+}{\text{H}}_2$

This is because these metals are amphoteric in nature and can react with both bases and acids.

Additional information:
Amphoteric behaviour of Aluminium
Aluminium reacts with both acids and bases to show amphoteric properties.
Aluminium reacts with both acids and bases to show amphoteric properties. It reacts with HCl and gives aluminium chloride and hydrogen gas whereas it reacts with NaOH to sodium zincate which is an aqueous solution.
Therefore, option (b) Zinc metal is amphoteric in nature.

Note: There are five amphoteric elements (metals) in the periodic table. They are Be, Al, Zn, Sn, Pb. Also, those elements oxides and hydroxides are amphoteric compounds. Amphoteric elements and compounds react with both acids and bases and give relevant products.
All amphoteric elements are metals and they are located in s,p,d blocks.
Be: s block (IIA group)
Al: p block (IIIA group)
Zn: d block (IIB group)
Sn: p block (IVA group)
Pb: p block (IVA group)