CDS Chemistry · Teaching notes

Chemical Reactions — CDS Chemistry

Chemical Reactions has 19 past-year questions in CDS General Knowledge, from 2016 (II) to 2026 (II), and 2023 alone set seven of them. Most are about a few reactions CDS returns to again and again: lead nitrate on heating, whitewashing with slaked lime, the thermite weld, and which everyday changes are oxidation. Three HARD questions reach into Class 11 (Gibbs energy, state functions, the hydrogen electrode); they are worth a read but are not where the marks usually are.

Every subtopic, worked example, formula and trap in one printable document — answers shown, ready to share.

All 19 past questions CDS home

Subtopic notes

Formula & revision sheet

1 formulas · 9 reference tables · 12 gotchas across all subtopics — the exam-eve cheat-sheet

Types of Reactions and Energy Changes

Formulas (1)

State functions and Gibbs energy · Gibbs energy $\Delta G = \Delta H - T\Delta S \qquad T_{\text{switch}} = \frac{\Delta H}{\Delta S}$

Reference tables (3)

Physical and chemical changes4 rows

Event	Kind of change
Freezing of water	Physical
Rusting of iron	Chemical
Cooking food	Chemical
Burning candle	Both physical and chemicalQ

Combination, decomposition, displacement and double displacement4 rows

Type	Pattern	Example
Combination	A + B → AB	2Mg + O₂ → 2MgO (white powder, heat and light)Q
Decomposition	AB → A + B	CaCO₃ → CaO + CO₂ (on heating)
Displacement	A + BC → AC + B	Zn + CuSO₄ → ZnSO₄ + Cu
Double displacement	AB + CD → AD + CB	K₂SO₄ + BaCl₂ → 2KCl + BaSO₄↓Q

Heating lead nitrate3 rows

Product	Formula	Seen as
Lead monoxide	PbO	Yellow solid left in the tubeQ
Nitrogen dioxide	NO₂	Brown fumesQ
Oxygen	O₂	Colourless gasQ CDS 2023 (I): the correct statements were 'colourless O₂ is released' and 'thermal decomposition producing NO₂'.

Watch out for (5)

The candle is the 'both' answer
→ Physical and chemical changes
Swapping partners is double displacement
→ Combination, decomposition, displacement and double displacement
Lead nitrate gives PbO, not PbO₂
→ Heating lead nitrate
q and w are path functions, their sum is not
→ State functions and Gibbs energy
Both negative means 'below', not 'above'
→ State functions and Gibbs energy

Oxidation and Reduction

Reference tables (3)

Oxidation and reduction in everyday changes6 rows

Change	Oxidation?
Rusting of iron	Yes
Butter turning rancid	Yes
Wine turning sour	Yes
Combustion	Yes
White BaSO₄ forming from BaCl₂ and Na₂SO₄	No — double displacementQ
Opening a soda bottle	No — physical release of CO₂Q

The thermite reaction4 rows

Statement	True or false
Iron oxide reacts with aluminium to join railway tracks	TrueQ
The heat given out is used for welding	True
Aluminium acts as an oxidising agent	False — it is the reducing agent
Molten iron and molten aluminium are formed	False — molten iron and aluminium oxide

The standard hydrogen electrode3 rows

Statement about the SHE	True or false
Its standard potential is taken as 0 V by convention	True
Its absolute electrode potential is not zero	TrueQ
Its potential is zero only at 25 °C	False

Watch out for (4)

A precipitate is not an oxidation
→ Oxidation and reduction in everyday changes
The reducing agent is oxidised
→ Oxidation and reduction in everyday changes
Aluminium reduces, it does not oxidise
→ The thermite reaction
The hydrogen electrode's zero is agreed, not measured
→ The standard hydrogen electrode

Reactions in Daily Life

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

PYQ weightage by concept

10 concepts · 19 PYQs — where the marks actually sit, so you know what to drill first

Types of Reactions and Energy Changes8 PYQs · 42%

Concept	PYQs	Share
Heating lead nitrate	3	16%
Combination, decomposition, displacement and double displacement	2	11%
State functions and Gibbs energy	2	11%
Physical and chemical changes	1	5%

Oxidation and Reduction5 PYQs · 26%

Concept	PYQs	Share
Oxidation and reduction in everyday changes	2	11%
The thermite reaction	2	11%
The standard hydrogen electrode	1	5%

Reactions in Daily Life6 PYQs · 32%

Concept	PYQs	Share
Quicklime, slaked lime and whitewashing	3	16%
Tests for common gases	2	11%
Electrolysis of brine	1	5%

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Types of Reactions and Energy Changes

Oxidation and Reduction

Reactions in Daily Life

Formula & revision sheet

PYQ weightage by concept

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