NDA Chemistry · Chemical Reactions

Specific Reactions — Precipitation, Electrolysis and Daily Life

A handful of named everyday reactions the bank tests by name — the lime-water test, tarnishing of silver, electrolytic refining of copper, and which reactions give off hydrogen gas.

All Chemical Reactions notes NDA Chemistry strategy

Why this matters

Five PYQs of high-yield recall — each is a single named reaction with a single right answer. Know the products (silver sulphide for tarnish, calcium carbonate for the lime-water test) and the marks are quick.

Concept 1 of 3: The lime-water test for carbon dioxide

Bubbling carbon dioxide through lime water turns it milky white. The white solid is calcium carbonate (chalk) — the same reaction that confirms a gas is CO₂. It is a precipitation reaction.

Definition

The lime-water test:

Lime water is a solution of calcium hydroxide, Ca(OH)₂.
Passing CO₂ through it: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O.
The white precipitate is calcium carbonate (CaCO₃) — the milkiness confirms the gas is carbon dioxide.
With EXCESS CO₂ the milkiness disappears as soluble calcium bicarbonate forms (Ca(HCO₃)₂).

Lime-water test for CO₂

\text{Ca(OH)}_2 + \text{CO}_2 \to \text{CaCO}_3\!\downarrow + \text{H}_2\text{O}

Worked example

CO₂ gas is passed through lime water and a white precipitate forms. Write the reaction and name the precipitate and its colour.

Practice this concept4 quick reps

The same idea in a real exam question:

NDA · Sep 2025 · Q96Easy

Example 1 · Chemical Reactions · Specific Reactions: Precipitation, Electrolysis and Daily Life

The reaction that occurs on passing carbon dioxide gas through lime water is

The precipitate is calcium carbonate, white

Passing CO₂ through lime water gives a WHITE precipitate of CaCO₃ — not calcium hydroxide or calcium oxide. The white milkiness is the standard confirmatory test for carbon dioxide.

Drill 1 more on the lime-water test for carbon dioxide

Concept 2 of 3: Tarnishing of silver and surface reactions

Silver darkens over time because it reacts with traces of hydrogen sulphide in the air to form a black layer of silver sulphide. It is a slow surface reaction, like rusting is for iron.

Definition

Everyday surface reactions:

Tarnishing of silver — silver reacts with H₂S in air to form a black layer of silver sulphide (Ag₂S): 2Ag + H₂S → Ag₂S + H₂.
The tarnish is silver sulphide, NOT silver oxide, chloride or sulphate.
Rusting of iron — the analogous reaction for iron, forming hydrated iron(III) oxide (brown rust).

Metal	Reacts with	Product (the tarnish/coating)
Silver	H₂S in air	Silver sulphide, Ag₂S (black) Silver tarnish is silver SULPHIDE (Ag₂S) — not oxide, chloride or sulphate.
Iron	O₂ + moisture	Hydrated iron(III) oxide (brown rust)
Copper	Moist CO₂ / air	Green basic copper carbonate (verdigris)

Practice this concept4 quick reps

The same idea in a real exam question:

NDA · Sep 2022 · Q74Moderate

Example 2 · Chemical Reactions · Specific Reactions: Precipitation, Electrolysis and Daily Life

Silver artefacts get tarnished in air due to the formation of

Silver tarnish is the sulphide, not the oxide

Silver tarnishes to silver SULPHIDE (Ag₂S) by reaction with H₂S in the air. Silver oxide, silver chloride and silver sulphate are the tempting wrong answers — the black tarnish is the sulphide.

Concept 3 of 3: Electrolytic refining and hydrogen evolution

Electrolysis uses electricity to drive a reaction — copper is purified by electrolysing acidified copper sulphate. Separately, the bank asks which reactions give off hydrogen gas: reactive metals with acids, water or alkali do; setting plaster of Paris does not.

Definition

Two daily-life electro/gas facts:

Electrolytic refining of copper uses an electrolyte of acidified copper sulphate (CuSO₄) solution; impure copper is the anode, pure copper deposits on the cathode.
Hydrogen gas IS evolved when a reactive metal meets an acid, water or alkali: Zn + dilute H₂SO₄, K + H₂O, Zn + NaOH all release H₂.
No hydrogen is evolved when water is added to Plaster of Paris — it merely re-hydrates and SETS (CaSO₄·½H₂O + water → gypsum); this is not a hydrogen-releasing reaction.

Process	Hydrogen gas evolved?
Zinc + dilute H₂SO₄	Yes
Potassium + water	Yes
Zinc + sodium hydroxide solution	Yes
Water added to Plaster of Paris	No — it just sets (rehydrates) Setting of Plaster of Paris is rehydration to gypsum — NO hydrogen gas is released.

Electrolytic refining of copper uses an electrolyte of acidified copper sulphate solution.

Practice this concept4 quick reps

The same idea in a real exam question:

NDA · Apr 2021 · Q84Moderate

Example 3 · Chemical Reactions · Specific Reactions: Precipitation, Electrolysis and Daily Life

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

Setting Plaster of Paris releases no hydrogen

Adding water to Plaster of Paris makes it SET (rehydrate to gypsum) — no gas is produced. The hydrogen-evolving reactions are reactive metals with acid, water or alkali. The 'odd one out' that gives no H₂ is the Plaster of Paris setting.

Drill 1 more on electrolytic refining and hydrogen evolution

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

The lime-water test for carbon dioxide
Lime-water test for CO₂
$\text{Ca(OH)}_2 + \text{CO}_2 \to \text{CaCO}_3\!\downarrow + \text{H}_2\text{O}$

Reference tables (2)

Tarnishing of silver and surface reactions3 rows

Metal	Reacts with	Product (the tarnish/coating)
Silver	H₂S in air	Silver sulphide, Ag₂S (black) Silver tarnish is silver SULPHIDE (Ag₂S) — not oxide, chloride or sulphate.
Iron	O₂ + moisture	Hydrated iron(III) oxide (brown rust)
Copper	Moist CO₂ / air	Green basic copper carbonate (verdigris)

Electrolytic refining and hydrogen evolution4 rows

Process	Hydrogen gas evolved?
Zinc + dilute H₂SO₄	Yes
Potassium + water	Yes
Zinc + sodium hydroxide solution	Yes
Water added to Plaster of Paris	No — it just sets (rehydrates) Setting of Plaster of Paris is rehydration to gypsum — NO hydrogen gas is released.

Electrolytic refining of copper uses an electrolyte of acidified copper sulphate solution.

Watch out for (3)

The precipitate is calcium carbonate, white
→ The lime-water test for carbon dioxide
Silver tarnish is the sulphide, not the oxide
→ Tarnishing of silver and surface reactions
Setting Plaster of Paris releases no hydrogen
→ Electrolytic refining and hydrogen evolution

Test yourself on Chemical Reactions

15 past NDA questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

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