G3 Pure / O-Level Chemistry Cheatsheet
Chemistry cheatsheet covering definitions, formulas, observations, tests and reaction conditions for focused revision.
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This cheatsheet covers the most critical information to memorise for the equivalent G3 Pure / O-Level Chemistry K324 / 6092 course.
Fast exam fixes (high impact)
If you keep losing marks for avoidable reasons (MCQ traps, missing “aqueous”, wrong units), start here:
1. Important Definitions
| Term | Definition |
|---|---|
| Element | A substance made of only one type of atom. |
| Compound | A substance formed when two or more different elements are chemically combined. |
| Mixture | Two or more substances mixed physically (can be separated by physical methods). |
| Isotopes | Atoms of the same element with the same number of protons but different number of neutrons. |
| Acid | A substance that produces hydrogen ions (H⁺) in aqueous solution. |
| Base | A substance that neutralises an acid to form a salt and water only. |
| Alkali | A soluble base that produces hydroxide ions (OH⁻) in aqueous solution. |
| Oxidation | Gain of Oxygen / Loss of Hydrogen / Loss of Electrons / Increase in Oxidation State. |
| Reduction | Loss of Oxygen / Gain of Hydrogen / Gain of Electrons / Decrease in Oxidation State. |
| Oxidising agent | A substance that oxidises another substance and is itself reduced. |
| Reducing agent | A substance that reduces another substance and is itself oxidised. |
| Catalyst | A substance that increases the speed of a chemical reaction and remains chemically unchanged at the end. |
| Rate of reaction | The change in amount of reactant used (or product formed) per unit time. |
| Electrolysis | The decomposition of an ionic compound (electrolyte) in molten or aqueous state by passing electricity through it. |
| Electrolyte | A substance that conducts electricity when molten or aqueous due to mobile ions. |
2. Key Chemistry Maths (Calculations)
| Quantity | Exam formula (common forms) |
|---|---|
| moles | n = m/M |
| mass | m = nM |
| concentration | C = n/V (use V in dm³) |
| moles in solution | n = CV |
| molar gas volume (r.t.p.) | 1 mol gas = 24 dm³ |
| percentage yield | % yield = actual/theoretical × 100 |
| percentage purity | % purity = (mass of pure substance)/(mass of sample) × 100 |
3. Solubility Rules (Salts)
| Type of Salt | Solubility Rule |
|---|---|
| Nitrates | ALL are soluble. |
| Sulfates | MOST are soluble. Exceptions: Lead(II) (PbSO₄), Barium (BaSO₄), Calcium (CaSO₄). |
| Chlorides | MOST are soluble. Exceptions: Lead(II) (PbCl₂), Silver (AgCl). |
| Carbonates | MOST are insoluble. Exceptions: Group 1 and ammonium carbonates. |
| Oxides/Hydroxides | MOST are insoluble. Exceptions: Group 1 hydroxides; calcium hydroxide is sparingly soluble. |
4. Colours to Memorise
Indicators
| Indicator | In Acid | In Alkali |
|---|---|---|
| Litmus | Red | Blue |
| Methyl Orange | Red | Yellow |
| Universal Indicator | Red (pH 1) - Orange/Yellow (pH 3-6) | Blue (pH 8-11) - Violet (pH 13-14) |
Anions (tests)
| Anion | Test | Result |
|---|---|---|
| carbonate (CO₃²⁻) | add dilute acid | effervescence; CO₂ turns limewater milky |
| chloride (Cl⁻) | acidify with dilute HNO₃, add aqueous AgNO₃ | white precipitate |
| iodide (I⁻) | acidify with dilute HNO₃, add aqueous AgNO₃ | yellow precipitate |
| nitrate (NO₃-) | add aqueous NaOH, then Al foil, warm | ammonia produced (turns damp red litmus paper blue) |
| sulfate (SO₄²⁻) | acidify with dilute HNO₃, add aqueous barium nitrate | white precipitate |
Precipitates (Cations)
| Ion | NaOH | Ammonia (NH₃) |
|---|---|---|
| Cu²⁺ | Light Blue ppt (insoluble in excess) | Light Blue ppt (soluble in excess → Dark Blue soln) |
| Fe²⁺ | Green ppt (insoluble) | Green ppt (insoluble) |
| Fe³⁺ | Red-Brown ppt (insoluble) | Red-Brown ppt (insoluble) |
| Zn²⁺ | White ppt (soluble in excess) | White ppt (soluble in excess) |
| Al³⁺ | White ppt (soluble in excess) | White ppt (insoluble in excess) |
| Pb²⁺ | White ppt (soluble in excess) | White ppt (insoluble in excess) |
| Ca²⁺ | White ppt (insoluble in excess) | No ppt |
| NH₄ + | Warm with NaOH: ammonia gas produced | — |
Other Key Colours
- Copper(II) Carbonate: Green
- Copper(II) Oxide: Black
- Chlorine Gas (Cl₂): Pale Green-Yellow
- Bromine (Br₂): Red-Brown
- Iodine (I₂): Black solid / Brown solution / Purple vapour
- Fluorine (F₂): Pale Yellow
5. Key Reaction Conditions
| Process | Reactants | Conditions | Product |
|---|---|---|---|
| Haber Process | N₂ + 3H₂ | about 450 °C, about 200 atm, iron catalyst | ammonia (NH₃) |
| Contact Process | 2SO₂ + O₂ | about 450 °C, about 1 atm, V₂O₅ catalyst | sulfur trioxide (SO₃) |
| Cracking | Long alkane | 600 °C, Al₂O₃ / SiO₂ catalyst | Alkenes + H₂ |
| Hydration | Ethene + Steam | 300 °C, 60 atm, H₃PO₄ catalyst | Ethanol |
| Esterification | Acid + Alcohol | Reflux, conc. H₂SO₄ catalyst | Ester + Water |
| Hydrogenation | Alkene + H₂ | 200 °C, Nickel catalyst | Alkane |
6. Organic Chemistry General Formulas
| Series | General Formula | Functional Group | Suffix |
|---|---|---|---|
| Alkanes | CₙH₂ₙ₊₂ | None | -ane |
| Alkenes | CₙH₂ₙ | C = C | -ene |
| Alcohols | CₙH₂ₙ₊₁OH | -OH | -anol |
| Carboxylic Acids | CₙH₂ₙ₊₁COOH | -COOH | -anoic acid |
7. Gas Tests
| Gas | Test | Observation |
|---|---|---|
| Hydrogen (H₂) | Lighted splint | ’Pop’ sound |
| Oxygen (O₂) | Glowing splint | Relights |
| Carbon Dioxide (CO₂) | Bubble through limewater | White precipitate formed |
| Chlorine (Cl₂) | Damp blue litmus paper | Turns red then bleaches white |
| Ammonia (NH₃) | Damp red litmus paper | Turns blue |
| Sulfur Dioxide (SO₂) | Aqueous acidified KMnO₄ | Purple to colourless |
8. Electrolysis + Redox (Core Keywords)
- In electrolysis: anode is positive (oxidation happens here).
- In electrolysis: cathode is negative (reduction happens here).
- PANIC: Positive Anode, Negative Is Cathode.
- Aqueous electrolysis includes ions from water (H⁺ and OH⁻), so products depend on the ions present and their reactivity.
9. Periodic Table + Reactivity Series (Fast Marks)
- Group I: 1 outer electron; react with water; reactivity increases down the group.
- Group VII: 7 outer electrons; diatomic molecules; reactivity decreases down the group; more reactive halogen displaces a less reactive halide.
- Reactivity series (memorise): K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Ag.
10. Energy Changes + Rate of Reactions (Fast Marks)
Energy changes
- Exothermic: heat released to surroundings; Δ H is negative.
- Endothermic: heat absorbed from surroundings; Δ H is positive.
- Activation energy: minimum energy needed for a reaction to start.
Rate of reaction (collision theory keywords)
- rate increases when collisions are more frequent and more energetic.
- factors: concentration, pressure (gases), surface area, temperature, catalyst.
11. Environmental Chemistry (Air)
Composition of clean, dry air (by volume)
| Gas | Approx. % |
|---|---|
| N₂ | 78% |
| O₂ | 21% |
| noble gases (mainly Ar) | 0.9% |
| CO₂ | 0.04% |
Common pollutants (name them precisely)
- CO, SO₂, NOₓ, O₃ (ground-level), unburnt hydrocarbons, particulates.
Controls + big-picture links (syllabus-safe)
- Catalytic converters reduce CO, NOₓ and unburnt hydrocarbons in car exhaust.
- Flue gas desulfurisation uses limestone (CaCO₃) to remove SO₂.
- Greenhouse gases (especially CO₂ and CH₄) trap IR radiation and contribute to global warming.
- Ozone layer (stratosphere) absorbs UV; CFCs can cause ozone depletion.
Exam Tip
Memorise the solubility rules, ion tests, and colour observations first. These appear repeatedly in Paper 1 and practical-style questions.