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

Intuition

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

From the bank · past-year question

Example 1Chemical ReactionsEASY

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

[Q96 · Sep · 2025]

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

Intuition

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

From the bank · past-year question

Example 2Chemical ReactionsMODERATE

Silver artefacts get tarnished in air due to the formation of

[Q74 · Sep · 2022]

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

Intuition

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

From the bank · past-year question

Example 3Chemical ReactionsMODERATE

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

[Q84 · Apr · 2021]

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

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