NDA Chemistry · Teaching notes

Chemical Reactions — NDA Chemistry

Chemical Reactions is the most reasoning-heavy chapter in NDA Chemistry — 30 PYQs across 2017–2026 with the highest share of HARD questions, almost all of them carried by redox. The bank rarely asks you to balance an equation; it asks you to CLASSIFY a reaction (combination / decomposition / displacement), to track oxidation numbers up and down, or to spot the one statement that is false. The chapter teaches in six movements, building from what a reaction even is up to the redox reasoning that earns the hard marks: (1) Physical vs chemical changes — the line between melting ice and burning magnesium, the test the bank uses; (2) Types of reactions — combination, decomposition, displacement and double displacement, with the match-list questions the bank loves; (3) Thermal and photochemical decomposition — which oxides break on heating, which salts break in sunlight, and the states of the products; (4) Redox — oxidation numbers, oxidising and reducing agents, the activity series, and the 'which is NOT a redox reaction' trap (this is the HARD pocket — read it twice); (5) Specific reactions of daily life — lime water, tarnishing silver, electrolytic refining, hydrogen evolution; (6) Endothermic and exothermic reactions — which way the heat flows and how to tell from the equation. 16 concepts, every PYQ tagged. The win is reasoning, not memorisation: learn to assign an oxidation number and the redox marks fall out.

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Subtopic notes

PYQ weightage by concept

13 concepts · 30 PYQs — where the marks actually sit, so you know what to drill first

Physical vs Chemical Changes2 PYQs · 7%

Concept	PYQs	Share
Telling a physical change from a chemical change	2	7%

Types of Reactions — Combination, Decomposition, Displacement7 PYQs · 23%

Concept	PYQs	Share
The four reaction shapes	3	10%
Displacement and metal reactivity	3	10%
Addition reactions and hydrogenation of oils	1	3%

Thermal and Photochemical Decomposition3 PYQs · 10%

Concept	PYQs	Share
Heat-driven vs light-driven decomposition	3	10%

Redox — Oxidation, Reduction and Reducing Agents10 PYQs · 33%

Concept	PYQs	Share
Spotting a redox reaction and oxidation in daily life	5	17%
Assigning oxidation numbers	3	10%
Defining oxidation and reduction	1	3%
Oxidising and reducing agents	1	3%

Specific Reactions — Precipitation, Electrolysis and Daily Life5 PYQs · 17%

Concept	PYQs	Share
The lime-water test for carbon dioxide	2	7%
Electrolytic refining and hydrogen evolution	2	7%
Tarnishing of silver and surface reactions	1	3%

Endothermic and Exothermic Reactions3 PYQs · 10%

Concept	PYQs	Share
Which way does the heat flow?	3	10%

Formula & revision sheet

4 formulas · 8 reference tables · 22 gotchas across all subtopics — the exam-eve cheat-sheet

Physical vs Chemical Changes

Reference tables (1)

Telling a physical change from a chemical change6 rows

Process	Change type	Why
Melting of ice	Physical	Still water, only state changes — reversible
Boiling / evaporation of water	Physical	Water vapour is still water
Dissolving sugar in water	Physical	Sugar can be recovered by evaporation
Mixing NaOH and HCl	Chemical	Neutralisation — new salt (NaCl) + water form Mixing an acid and a base is a chemical change, not just mixing — a new substance (the salt) is made.
Burning of magnesium ribbon	Chemical	New substance MgO forms with light and heat Burning is always a chemical change — a new oxide forms.
Rusting of iron	Chemical	New substance (hydrated iron oxide) forms

Watch out for (3)

Dissolving and boiling are physical, not chemical
→ Telling a physical change from a chemical change
Mixing an acid and a base is a chemical change
→ Telling a physical change from a chemical change
Burning is always chemical
→ Telling a physical change from a chemical change

Types of Reactions — Combination, Decomposition, Displacement

Formulas (1)

Displacement and metal reactivity · Reactivity (activity) series — selected metals $\text{K} > \text{Na} > \text{Ca} > \text{Mg} > \text{Al} > \text{Zn} > \text{Fe} > \text{Pb} > \text{Cu} > \text{Ag}$

Reference tables (1)

The four reaction shapes4 rows

Type	Pattern	Example
Combination	A + B → AB	C + O₂ → CO₂ (burning coal)
Decomposition	AB → A + B	2H₂O → 2H₂ + O₂ (electrolysis of water)
Displacement	A + BC → AC + B	Fe + CuSO₄ → FeSO₄ + Cu
Double displacement	AB + CD → AD + CB	BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl Double displacement = ions swap partners; a precipitate or water often forms.

Watch out for (4)

Decomposition vs double displacement in match-lists
→ The four reaction shapes
Single vs double displacement
→ The four reaction shapes
'Copper is more reactive than iron' is always false
→ Displacement and metal reactivity
Hydrogenation is addition, not displacement
→ Addition reactions and hydrogenation of oils

Thermal and Photochemical Decomposition

Reference tables (1)

Heat-driven vs light-driven decomposition4 rows

Reaction	Trigger	Product states / note
2HgO → 2Hg + O₂	Heat	Solid → liquid Hg + gas O₂ Mercury is the metal that comes off as a LIQUID — states are solid, liquid, gas.
2Ag₂O → 4Ag + O₂	Heat	Silver oxide decomposes on heating
2AgCl → 2Ag + Cl₂	Sunlight	Photochemical — silver chloride darkens in light Silver halides (AgCl, AgBr) decompose in SUNLIGHT, not heat — the basis of photography.
ZnO, MgO	—	Thermally STABLE — do not decompose on heating

Watch out for (3)

Silver halides break in LIGHT, not heat
→ Heat-driven vs light-driven decomposition
HgO gives liquid mercury
→ Heat-driven vs light-driven decomposition
ZnO and MgO are thermally stable
→ Heat-driven vs light-driven decomposition

Redox — Oxidation, Reduction and Reducing Agents

Formulas (2)

Assigning oxidation numbers · Oxidation-number bookkeeping $\sum (\text{oxidation numbers}) = \text{net charge}; \qquad \text{H} = +1,\ \text{O} = -2,\ \text{element} = 0$
Oxidising and reducing agents · The agent does the opposite to itself $\text{Reducing agent} \Rightarrow \text{itself oxidised}; \qquad \text{Oxidising agent} \Rightarrow \text{itself reduced}$

Reference tables (2)

Defining oxidation and reduction6 rows

Change to a substance	Oxidation or reduction?
Loses electrons	Oxidation
Gains electrons	Reduction
Gains oxygen	Oxidation
Loses oxygen	Reduction
Loses hydrogen	Oxidation Losing hydrogen is OXIDATION, not reduction — this is the bank's favourite false statement.
Gains hydrogen	Reduction

Spotting a redox reaction and oxidation in daily life7 rows

Reaction or process	Redox? / Note
2Mg + O₂ → 2MgO	Redox — Mg oxidised (0 → +2), O reduced
Cu + Zn-salt displacement	Redox — electron transfer between metals
AlCl₃ + 3H₂O → Al(OH)₃ + 3HCl	NOT redox — hydrolysis, no oxidation-state change Hydrolysis/double displacement with no oxidation-number change is NOT a redox reaction.
Rusting of iron	Oxidation — Fe → hydrated Fe³⁺ oxide
Burning of fuel	Oxidation — carbon/hydrogen oxidised
Rancidity of oils and fats	Oxidation — fatty acids oxidise
Browning of cut fruit	Oxidation — chemical/enzymatic

Watch out for (7)

Fe₂O₃ is uniform; Fe₃O₄ is mixed
→ Assigning oxidation numbers
Unchanged oxidation number = neither oxidised nor reduced
→ Assigning oxidation numbers
'Loses hydrogen → reduced' is false
→ Defining oxidation and reduction
The reducing agent is the one OXIDISED
→ Oxidising and reducing agents
A halogen in a displacement is the OXIDISING agent
→ Oxidising and reducing agents
No oxidation-state change → not redox
→ Spotting a redox reaction and oxidation in daily life
Reducing power follows reactivity
→ Spotting a redox reaction and oxidation in daily life

Specific Reactions — Precipitation, Electrolysis and Daily Life

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

Endothermic and Exothermic Reactions

Reference tables (1)

Which way does the heat flow?5 rows

Reaction	Endothermic or exothermic?
CaO + H₂O → Ca(OH)₂ (slaking lime)	Exothermic — releases heat Adding water to quicklime gets HOT — it is strongly exothermic.
Combustion of CH₄ or glucose	Exothermic
Haber process N₂ + 3H₂ → 2NH₃	Exothermic
2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂	Endothermic — needs heat in Thermal decompositions absorb heat — they are endothermic.
N₂ + O₂ → 2NO	Endothermic — needs very high temperature Air's N₂ and O₂ do not react at ordinary temperatures because the reaction is endothermic and needs > 2000°C.

Watch out for (2)

Combustion and slaking are exothermic; decomposition is endothermic
→ Which way does the heat flow?
N₂ + O₂ needs huge energy
→ Which way does the heat flow?