NDA Chemistry · Formula sheet
Chemical Reactions formulas
4 formulas, 8 reference tables and 22 common traps for NDA Chemistry Chemical Reactions, grouped by subtopic.
Physical vs Chemical Changes
Learn this subtopic in the notesTelling a physical change from a chemical change
| 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 |
Common traps
Dissolving and boiling are physical, not chemical
Dissolving sugar, boiling water and melting ice make NO new substance — they are physical changes even though the substance looks different. The marker for a chemical change is a NEW substance, not just a new appearance.
Mixing an acid and a base is a chemical change
Mixing NaOH and HCl is not 'just mixing' — they react (neutralisation) to form a new salt and water, so it is a chemical change.
Burning is always chemical
Any burning or combustion (magnesium ribbon, fuel, a candle wick) is a chemical change — a new oxide or combustion product forms, releasing heat and often light.
Types of Reactions — Combination, Decomposition, Displacement
Learn this subtopic in the notesDisplacement and metal reactivity
Reactivity (activity) series — selected metals
The four reaction shapes
| 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. |
Common traps
Decomposition vs double displacement in match-lists
Electrolysis of water is DECOMPOSITION (one → many), while BaCl₂ + a sulphate is DOUBLE displacement (two compounds swap ions). Both feel like 'breaking up', but only one reactant breaks in decomposition.
Single vs double displacement
Single (simple) displacement has a free ELEMENT kicking another out (Fe + CuSO₄). Double displacement has TWO compounds swapping ions (Na₂SO₄ + BaCl₂) — no free element is involved.
'Copper is more reactive than iron' is always false
If iron displaces copper from CuSO₄, iron MUST be the more reactive metal. Any statement that copper is more reactive than iron is incorrect — it would mean copper could displace iron, which it cannot.
Hydrogenation is addition, not displacement
In hydrogenation the whole H₂ molecule ADDS across a double bond — nothing leaves the oil. So it is an addition reaction, not displacement (which requires one element to leave).
Thermal and Photochemical Decomposition
Learn this subtopic in the notesHeat-driven vs light-driven decomposition
| 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 |
Common traps
Silver halides break in LIGHT, not heat
2AgCl → 2Ag + Cl₂ is a PHOTOchemical decomposition — it happens in sunlight, not on heating. The light-sensitivity of silver salts is what made black-and-white photography work.
HgO gives liquid mercury
In 2HgO → 2Hg + O₂ the product states are solid (HgO), LIQUID (Hg) and gas (O₂). Mercury is a liquid metal at room temperature, so do not write it as a solid.
ZnO and MgO are thermally stable
Not every oxide decomposes on heating. ZnO and MgO are stable; silver oxide (Ag₂O) and mercury oxide (HgO) are the ones that break down.
Redox — Oxidation, Reduction and Reducing Agents
Learn this subtopic in the notesAssigning oxidation numbers
Oxidation-number bookkeeping
Oxidising and reducing agents
The agent does the opposite to itself
Defining oxidation and reduction
| 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 life
| 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 |
Common traps
Fe₂O₃ is uniform; Fe₃O₄ is mixed
Fe₂O₃ contains only Fe³⁺ (one oxidation state), but Fe₃O₄ contains BOTH Fe²⁺ and Fe³⁺. The same trap applies to Pb₃O₄ (Pb²⁺ + Pb⁴⁺) and Mn₃O₄ (Mn²⁺ + Mn³⁺).
Unchanged oxidation number = neither oxidised nor reduced
In CH₄ combustion, hydrogen stays +1 throughout, so it is NEITHER oxidised nor reduced. Only carbon (−4 → +4) changes. Do not assume every atom in a redox reaction is itself oxidised or reduced.
'Loses hydrogen → reduced' is false
Loss of hydrogen is OXIDATION, not reduction. The four markers of reduction are: gains electrons, loses oxygen, gains hydrogen, oxidation number falls. Any statement pairing 'loses hydrogen' with 'reduced' is incorrect.
The reducing agent is the one OXIDISED
The naming feels reversed: the REDUCING agent is the species that itself gets OXIDISED (it donates electrons). Mixing this up flips every answer. Remember: the agent does to itself the opposite of its name.
A halogen in a displacement is the OXIDISING agent
In Br₂ + 2I⁻ → 2Br⁻ + I₂, Br₂ gains electrons (0 → −1), so it is REDUCED and is the oxidising agent — not the reductant. Calling Br₂ the reducing agent is a classic false statement.
No oxidation-state change → not redox
AlCl₃ + 3H₂O → Al(OH)₃ + 3HCl LOOKS like a reaction, but every element keeps its oxidation number (Al stays +3, Cl stays −1). With no electron transfer it is hydrolysis, NOT a redox reaction.
Reducing power follows reactivity
Electron-releasing tendency (reducing power) is Zn > Cu > Ag, mirroring the activity series — not the reverse. A more reactive metal gives up electrons more readily.
Specific Reactions — Precipitation, Electrolysis and Daily Life
Learn this subtopic in the notesThe lime-water test for carbon dioxide
Lime-water test for CO₂
Tarnishing of silver and surface reactions
| 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 evolution
| 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.
Common traps
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.
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.
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.
Endothermic and Exothermic Reactions
Learn this subtopic in the notesWhich way does the heat flow?
| 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. |
Common traps
Combustion and slaking are exothermic; decomposition is endothermic
When asked 'which is endothermic?', rule out combustion (CH₄, glucose) and the Haber process — those RELEASE heat. The endothermic one is usually a thermal decomposition that needs heat in (e.g. lead nitrate).
N₂ + O₂ needs huge energy
Air's nitrogen and oxygen do not combine at room temperature because N₂ + O₂ → 2NO is ENDOTHERMIC and requires very high temperature (lightning or engine heat). It is not that the oxides are unstable or that a catalyst is missing.