CDS Chemistry · Chemical Reactions

Reactions in Daily Life

The chemistry of whitewashing, the limewater test for carbon dioxide, and what electrolysis of brine gives.

Why this matters

Six CDS questions. Whitewashing alone came three times (2021, and twice in 2023): quicklime becomes slaked lime, which takes up carbon dioxide to leave a shiny layer of calcium carbonate.

Concept 1 of 3: Quicklime, slaked lime and whitewashing

Quicklime (calcium oxide) reacts with water so fiercely that it hisses and gets hot; the product is slaked lime. Whitewash is a suspension of slaked lime. On the wall it slowly takes carbon dioxide from the air and turns into a thin, shiny coat of calcium carbonate.

Definition

The two steps:

CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat: quicklime → slaked lime; strongly exothermic.
Walls are whitewashed with a suspension of slaked lime, Ca(OH)₂, not quicklime.
On the wall: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l). The thin calcium carbonate layer gives the shine, and it forms over two or three days.
Calcium hydroxide is an inorganic compound.

Name	Formula	Role
Quicklime	CaO	Reacts with water to make slaked lime
Slaked lime	Ca(OH)₂	Applied to the wall as whitewashQ CDS 2023 (I): it is slaked lime, not quicklime, that is used for whitewashing.
Calcium carbonate	CaCO₃	Shiny layer formed with CO₂ from airQ

Practice this conceptself-check · 3 quick reps

The same idea in a real exam question:

CDS · 2023 · CDS (II) 2023 — General Knowledge · Q9Moderate

Example 1 · Chemical Reactions · Reactions in Daily Life

During white-washing of walls, slaked lime reacts slowly with carbon dioxide in air to form a thin layer of calcium carbonate on the walls. Which of the following reactions represents this correctly ?

Whitewash is slaked lime, not quicklime

Walls are coated with Ca(OH)₂. Quicklime (CaO) is what you start from, and it is the slaked lime that reacts with CO₂ on the wall.

The shine is calcium carbonate

The shiny finish is CaCO₃, formed from Ca(OH)₂ and CO₂. Oxygen plays no part.

Drill 2 more on quicklime, slaked lime and whitewashing

Concept 2 of 3: Tests for common gases

Each common gas has a quick test. Carbon dioxide turns limewater milky, because it makes insoluble chalk. Hydrogen burns with a pop. Oxygen relights a glowing splint.

Definition

The tests:

Carbon dioxide: turns limewater milky (Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O). Exhaled air does this.
CO₂ comes from a carbonate + dilute acid, at room temperature.
Hydrogen: burns with a pop (a metal such as zinc + dilute acid makes it).
Oxygen: relights a glowing splint.

Gas	Test	Made by
Carbon dioxide	Limewater turns milky	Carbonate + dilute acid; breathing outQ
Hydrogen	Burns with a pop	Zinc or magnesium + dilute acid
Oxygen	Relights a glowing splint	Heating potassium permanganate

Practice this concept3 quick reps

The same idea in a real exam question:

CDS · 2018 · CDS (II) 2018 — General Knowledge · Q1Moderate

Example 2 · Chemical Reactions · Reactions in Daily Life

Two reactants in a flask at room temperature are producing bubbles of a gas that turn limewater milky. The reactants could be

Zinc and acid give hydrogen, not CO₂

Only a carbonate (or hydrogen carbonate) with acid gives CO₂, which turns limewater milky. A metal with acid gives hydrogen, which does not.

Drill 1 more on tests for common gases

Concept 3 of 3: Electrolysis of brine

Passing electricity through salt water splits it three ways: chlorine at one electrode, hydrogen at the other, and sodium hydroxide left in the solution. Because it makes chlorine and an alkali, it is called the chlor-alkali process.

Definition

The process:

2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂.
Chlorine at the anode, hydrogen at the cathode, sodium hydroxide in the solution.
Chlorine and slaked lime then give bleaching powder.

Product	Where it forms
Chlorine gas	Anode
Hydrogen gas	Cathode
Sodium hydroxide	Left in the solutionQ

Practice this concept2 quick reps

The same idea in a real exam question:

CDS · 2026 · CDS (II) 2026 — General Knowledge · Q72Moderate

Example 3 · Chemical Reactions · Reactions in Daily Life

When electricity is passed through an aqueous solution of sodium chloride, it decomposes to form

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.

Reference tables (3)

Quicklime, slaked lime and whitewashing3 rows

Name	Formula	Role
Quicklime	CaO	Reacts with water to make slaked lime
Slaked lime	Ca(OH)₂	Applied to the wall as whitewashQ CDS 2023 (I): it is slaked lime, not quicklime, that is used for whitewashing.
Calcium carbonate	CaCO₃	Shiny layer formed with CO₂ from airQ

Tests for common gases3 rows

Gas	Test	Made by
Carbon dioxide	Limewater turns milky	Carbonate + dilute acid; breathing outQ
Hydrogen	Burns with a pop	Zinc or magnesium + dilute acid
Oxygen	Relights a glowing splint	Heating potassium permanganate

Electrolysis of brine3 rows

Product	Where it forms
Chlorine gas	Anode
Hydrogen gas	Cathode
Sodium hydroxide	Left in the solutionQ

Watch out for (3)

Whitewash is slaked lime, not quicklime
→ Quicklime, slaked lime and whitewashing
The shine is calcium carbonate
→ Quicklime, slaked lime and whitewashing
Zinc and acid give hydrogen, not CO₂
→ Tests for common gases

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