The gas generated on reacting zinc with dilute sulphuric acid is:

Q: What is the answer to CAPF 2023 Paper I Q54?

C. Correct — C, Hydrogen. Zinc sits above hydrogen in the reactivity series, so it displaces hydrogen from a dilute acid: Zn + H2SO4 gives ZnSO4 and hydrogen gas, which bubbles off while zinc sulphate stays in solution. Even without the series, the atoms available settle it — the reactants supply only zinc, hydrogen, sulphur and oxygen, so no gas made of argon, helium or nitrogen can possibly come out of the flask. The gas is identified in the laboratory by bringing a burning splint to the mouth of the tube, where it burns with a squeaky pop. Two limits are worth carrying: dilute nitric acid does not give hydrogen with most metals because it oxidises the gas as it forms, and hot concentrated sulphuric acid gives sulphur dioxide with zinc instead.

(a)Argon
(b)Helium
(c)Hydrogen
(d)Nitrogen

Answer

Why

Correct — C, Hydrogen. Zinc sits above hydrogen in the reactivity series, so it displaces hydrogen from a dilute acid: Zn + H2SO4 gives ZnSO4 and hydrogen gas, which bubbles off while zinc sulphate stays in solution. Even without the series, the atoms available settle it — the reactants supply only zinc, hydrogen, sulphur and oxygen, so no gas made of argon, helium or nitrogen can possibly come out of the flask. The gas is identified in the laboratory by bringing a burning splint to the mouth of the tube, where it burns with a squeaky pop. Two limits are worth carrying: dilute nitric acid does not give hydrogen with most metals because it oxidises the gas as it forms, and hot concentrated sulphuric acid gives sulphur dioxide with zinc instead.

Why the others are wrong

(a)Argon — Argon is a noble gas, chemically inert, and is obtained from liquid air rather than from any acid-metal reaction. No argon atom is present in either reactant.
(b)Helium — Helium is also a noble gas and is extracted commercially from natural gas deposits. Nothing in zinc or in sulphuric acid could supply it.
(d)Nitrogen — There is no nitrogen in zinc or in sulphuric acid. Nitrogen appears in this family of reactions only when the acid used is nitric acid, and even then as an oxide of nitrogen rather than as nitrogen gas.

Concept

A metal placed in a dilute acid pushes hydrogen out of the acid and takes its place, forming a salt. Whether it does so depends on where the metal stands in the reactivity series: those above hydrogen — potassium, sodium, calcium, magnesium, aluminium, zinc, iron, lead — liberate the gas, while copper, silver and gold, standing below hydrogen, do not react with dilute acids at all.

This is the standard laboratory preparation of hydrogen, done with granulated zinc and dilute sulphuric acid, and it is asked in one form or another in nearly every general-science paper. The distractors here are all gases that cannot be produced by any reaction, which turns the question into a check on whether you can account for the atoms in front of you. That habit is worth building, since it also settles harder items — a reaction with no nitrogen among its reactants cannot yield ammonia, and one with no carbon cannot yield carbon dioxide.

Key facts

Zinc and dilute sulphuric acid give zinc sulphate in solution and hydrogen gas.
Metals above hydrogen in the reactivity series displace hydrogen from dilute acids; copper, silver and gold do not.
Hydrogen is tested by a burning splint, which produces a characteristic pop.
Dilute nitric acid usually gives an oxide of nitrogen rather than hydrogen with metals, because it is an oxidising acid; magnesium and manganese with very dilute nitric acid are the exceptions.
Zinc also releases hydrogen with a hot sodium hydroxide solution, forming sodium zincate — a reaction that shows zinc to be amphoteric.
Argon and helium are noble gases obtained by physical separation, from liquid air and from natural gas respectively, never from a chemical reaction of this kind.

Following the atoms through the reaction

Reactants→Zn + dilute H2SO4 — the only elements present are zinc, hydrogen, sulphur and oxygen

What zinc does→Standing above hydrogen in the reactivity series, it displaces hydrogen from the acid

Products→ZnSO4 in solution and H2 gas off the top

Confirming test→A burning splint at the mouth of the tube gives a squeaky pop

Why the other options fail→Argon, helium and nitrogen contain atoms that never entered the flask

Balance the atoms first and three of the four options disappear before any chemistry is needed.

Study next

The reactivity series of metals and what it predicts about reactions with water, acids and metal saltsLaboratory tests for the common gases — hydrogen, oxygen, carbon dioxide, ammonia and sulphur dioxideWhy nitric acid behaves differently from other dilute acids towards metalsAmphoteric metals such as zinc and aluminium, which react with both acids and alkalisIndustrial and laboratory routes to hydrogen, including the electrolysis of water and steam reforming

Common traps

Expecting hydrogen from dilute nitric acid, which oxidises it as it forms.
Expecting hydrogen from copper and dilute acid, when copper stands below hydrogen in the series.
Forgetting that hot concentrated sulphuric acid takes the reaction somewhere else entirely and gives sulphur dioxide.

The simplest possible form — one reaction, one gas. Harder versions of the same idea ask which of four listed reactions fails to give hydrogen.

Related PYQs

NDA_GAT_2021_I_Q842021Open & attempt →

Which one of the following reactions does not result in the evolution of hydrogen gas?

(a) Reaction of zinc metal with dilute sulphuric acid solution
(b) Mixing water to Plaster of Paris
(c) Heating zinc metal with sodium hydroxide solution
(d) Reaction of potassium metal with water

Answer(b) Mixing water to Plaster of Paris

This CAPF item is option (a) of that question, asked on its own. The NDA version adds two more routes to hydrogen — zinc with hot alkali and potassium with water — and one that gives none, since plaster of Paris merely takes up water to set as gypsum.

Practice

practice — not a real PYQ
Which one of the following metals will not liberate hydrogen from dilute hydrochloric acid?
- (a)Zinc
- (b)Iron
- (c)Magnesium
- (d)Copper
Answer(d) Copper — it lies below hydrogen in the reactivity series and so cannot displace hydrogen from a dilute acid.
practice — not a real PYQ
The gas that burns with a squeaky pop when a burning splint is brought near it is
- (a)carbon dioxide
- (b)hydrogen
- (c)oxygen
- (d)nitrogen
Answer(b) hydrogen — carbon dioxide and nitrogen put the splint out, and oxygen makes a glowing splint relight without any pop.