NDA Chemistry · Free notes
Chemical Reactions — NDA Chemistry
Chemical Reactions is the most reasoning-heavy chapter in NDA Chemistry — 32 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.
The handout has every subtopic, worked example, formula and trap with answers shown. The formula sheet is the chapter’s formulas and traps alone, on one page.
Subtopic notes
Physical vs Chemical Changes
2 PYQsA physical change alters only the form or state of a substance with no new substance formed; a chemical change rearranges atoms into a genuinely new substance — that is what a chemical reaction is.
Types of Reactions — Combination, Decomposition, Displacement
8 PYQsEvery reaction in the bank falls into one of four shapes — combination, decomposition, displacement or double displacement — and recognising the shape from the equation is most of the marks.
Thermal and Photochemical Decomposition
3 PYQsA decomposition reaction splits one compound into two or more products; the energy for the split comes from heat (thermal) or light (photochemical).
Redox — Oxidation, Reduction and Reducing Agents
11 PYQsOxidation is loss of electrons (oxidation number goes up); reduction is gain of electrons (oxidation number goes down); the two always happen together in a redox reaction.
Specific Reactions — Precipitation, Electrolysis and Daily Life
5 PYQsA 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.
Endothermic and Exothermic Reactions
3 PYQsAn exothermic reaction releases heat to the surroundings (gets warm); an endothermic reaction absorbs heat from the surroundings (needs heat in).
Formula & revision sheet
4 formulas · 8 reference tables · 22 gotchas across all subtopics — the exam-eve cheat-sheet
Formula & revision sheet
4 formulas · 8 reference tables · 22 gotchas across all subtopics — the exam-eve cheat-sheet
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
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
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
Formulas (2)
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
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. |
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
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?
PYQ weightage by concept
13 concepts · 32 PYQs — where the marks actually sit, so you know what to drill first
PYQ weightage by concept
13 concepts · 32 PYQs — where the marks actually sit, so you know what to drill first
| Concept | PYQs | Share |
|---|---|---|
| Telling a physical change from a chemical change | 2 | 6% |
| Concept | PYQs | Share |
|---|---|---|
| The four reaction shapes | 4 | 13% |
| Displacement and metal reactivity | 3 | 9% |
| Addition reactions and hydrogenation of oils | 1 | 3% |
| Concept | PYQs | Share |
|---|---|---|
| Heat-driven vs light-driven decomposition | 3 | 9% |
| Concept | PYQs | Share |
|---|---|---|
| Spotting a redox reaction and oxidation in daily life | 5 | 16% |
| Assigning oxidation numbers | 4 | 13% |
| Defining oxidation and reduction | 1 | 3% |
| Oxidising and reducing agents | 1 | 3% |
| Concept | PYQs | Share |
|---|---|---|
| The lime-water test for carbon dioxide | 2 | 6% |
| Electrolytic refining and hydrogen evolution | 2 | 6% |
| Tarnishing of silver and surface reactions | 1 | 3% |
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.