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Chemistry flashcards

150 cards covering every topic of the J248 syllabus (Version 4.0 (August 2026), for first assessment in 2018). Study one topic or mix them all, turn each card, and mark yourself honestly.

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Topic 1: Topic C1 – Particles

Particle model: Everything is built from tiny particles, which may be atoms, molecules or ions. Their arrangement, movement and spacing explain the states of matter.
Particles in a solid, a liquid and a gas: arrangement and movement: Solid: regular, touching, vibrate in place. Liquid: random, close (mostly touching), slide past each other. Gas: random, far apart, move fast in all directions. The gaps between gas particles are empty space.
Which changes of state take in energy, and which give it out?: Taken in: melting, boiling or evaporating, subliming. Given out: freezing, condensing. Mass stays the same, because no particles are gained or lost.
Physical change: The particles stay the same. No new substance forms.
Chemical change: Atoms are rearranged into new particles. At least one new substance forms.
Is "it can be reversed" enough to call a change physical?: No. Some chemical changes can be reversed. The test is whether new particles, and so new substances, form.
A substance has mp 6 °C and bp 81 °C. What state is it in at 2 °C, 25 °C and 95 °C?: Below the mp: solid at 2 °C. Between mp and bp: liquid at 25 °C. Above the bp: gas at 95 °C.
What does the inelastic sphere model of particles not take into account?Higher: The forces of attraction between particles, the size of the particles, and the space between them (gases are drawn far too crowded).
A gas takes up 1000 times the volume of the same mass of liquid. How much further apart are its particles?Higher: About the cube root: ∛1000 = 10 times further apart.
Thomson's model of the atom: Plum pudding: electrons embedded in positive charge.
Rutherford's model of the atom: Nuclear model: a small, dense, positive nucleus, with the atom mostly empty space.
Bohr's contribution to the model of the atom: Electrons in fixed shells at set distances from the nucleus.
Chadwick's contribution to the model of the atom: Discovery of the neutron.
Alpha scattering: three observations and the conclusion from each: Most passed straight through → the atom is mostly empty space. Some were deflected → the nucleus is positively charged. A very few bounced back → the nucleus is very small but contains most of the mass.
Relative mass, charge and location of protons, neutrons and electrons: Proton: mass 1, charge +1, nucleus. Neutron: mass 1, charge 0, nucleus. Electron: very small mass, charge −1, shells.
Isotopes, and why they react identically: Atoms with the same number of protons but different numbers of neutrons. Chemical behaviour depends on the electrons, which are the same.
Protons, neutrons and electrons in a sodium-23 atom (atomic number 11): 11 protons, 23 − 11 = 12 neutrons, 11 electrons.
Typical radius of an atom: About 10⁻¹⁰ m (0.1 nm).

Topic 2: Topic C2 – Elements, Compounds and Mixtures

Pure substance (chemical sense), and how melting point shows purity: A single chemical substance. A pure substance melts and boils at a single, sharp temperature; impurities lower the melting point and widen its range. "Pure" fruit juice is a mixture.
Filtration separates…: An insoluble solid from a liquid.
Crystallisation separates…: A dissolved solid from its solvent: evaporate the solvent and the solid remains.
Simple distillation vs fractional distillation: Simple: a liquid from dissolved solids, or liquids with very different boiling points. Fractional: miscible liquids with similar boiling points; needs a fractionating column.
Rf value, and the Rf of a spot that moves 3.2 cm when the solvent front moves 8.0 cm: Rf = distance travelled by spot ÷ distance travelled by solvent. 3.2 ÷ 8.0 = 0.4
How does a chromatogram show purity, and when can Rf values be compared?: A pure substance gives a single spot; an impure sample gives several. Compare Rf values only under the same conditions (same solvent, paper, temperature), ideally on the same chromatogram.
A compound contains 2.4 g of carbon and 0.6 g of hydrogen. Empirical formula?: C: 2.4 ÷ 12 = 0.2. H: 0.6 ÷ 1 = 0.6. Divide by 0.2: 1 : 3, so CH₃.
How do you recover dry sand and dry salt from a mixture of sand and salt water?: Filter: the insoluble sand stays on the paper. Then crystallise the filtrate: heat to evaporate water until crystals form, cool and dry.
Ion: An atom or group of atoms with a charge, because electrons have been lost or gained. Protons never change.
Ionic bond: The electrostatic pull holding positive and negative ions together, acting in all directions.
Covalent bond: An electron pair held between two non-metal atoms, with both nuclei pulling on it.
Metallic bond: Positive metal ions in a regular lattice pulled towards the delocalised electrons that surround them.
Lone pair: A pair of outer electrons on one atom that is not shared.
Ion charges from the group number: Groups 1, 2, 3: +1, +2, +3. Groups 5, 6, 7: −3, −2, −1.
Deduce the formula of calcium nitride: Each Ca loses 2 electrons; each N gains 3. Lowest common number 6: 3 Ca²⁺ and 2 N³⁻, so Ca₃N₂.
Metal oxides vs non-metal oxides: Metal oxides are basic (they neutralise acids; soluble ones are alkaline), e.g. Li₂O. Non-metal oxides are acidic, e.g. CO₂.
What a dot and cross diagram does not show: The 3D shape, that all electrons are identical, and the relative sizes of the atoms or ions.
Why does carbon form so many compounds?: It forms four covalent bonds, it bonds strongly to other carbon atoms, so it makes chains (straight or branched) and rings, and families of similar compounds differing by CH₂.
Diamond: bonding and properties: Each carbon bonded to 4 others in a giant 3D tetrahedral network. Very hard, very high melting point; does not conduct (no delocalised electrons).
Graphite: bonding and properties: Each carbon bonded to 3 others in stacked hexagonal layers. Soft and slippery (weak forces between layers); conducts along the layers (one delocalised electron per carbon).
Graphene and fullerenes: Graphene: a single graphite sheet; very strong and conducts. Fullerenes: molecules such as hollow C₆₀ spheres or nanotubes. C₆₀ is soft (weak forces between molecules); nanotubes are strong and conduct.
When water boils, what is overcome?: The weak intermolecular forces between water molecules. The covalent bonds inside the molecules do not break.
Size of a nanoparticle, and 1 nm in metres: Nanoparticles are 1 nm to 100 nm across. 1 nm = 1 × 10⁻⁹ m.
Surface area to volume ratio of a cube of side 4 nm: Area 6 × 4² = 96 nm²; volume 4³ = 64 nm³; ratio 96 ÷ 64 = 1.5 nm⁻¹. Smaller particles have a larger ratio (6 ÷ x).
Two uses of nanoparticles, and one risk linked to a property: Silver: antibacterial (wound dressings). Titanium dioxide or zinc oxide: block UV in sunscreens. Risk: they are so small they may pass through skin or be inhaled, and long-term effects are not fully known.

Topic 3: Topic C3 – Chemical Reactions

Which elements exist as diatomic molecules?: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂ (the Group 7 elements plus hydrogen, nitrogen and oxygen).
Formula of iron(III) oxide and of magnesium nitrate: Fe³⁺ and O²⁻: lowest common multiple 6, so Fe₂O₃. Mg²⁺ and two NO₃⁻: Mg(NO₃)₂.
The four state symbols: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water. Pure water is (l); a precipitate is (s).
Half equation for chloride ions forming chlorine: 2Cl⁻ → Cl₂ + 2e⁻ (atoms and charges both balance).
Why might a balance reading change during a reaction in an open container?: Falls: a gas product escapes (e.g. CO₂ from an acid and a carbonate). Rises: a gas from the air joins on (e.g. oxygen forming a metal oxide). Mass is still conserved.
Ionic equation for barium chloride solution + sodium sulfate solutionHigher: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s). Na⁺ and Cl⁻ are spectator ions.
The mole and the Avogadro constantHigher: One mole contains exactly 6.02214076 × 10²³ elementary entities (classically, as many as atoms in 12 g of carbon-12). Avogadro constant ≈ 6.02 × 10²³ per mole. amount = mass ÷ Mr.
Mass of one methane molecule (Mr 16)Higher: 16 ÷ (6.02 × 10²³) = 2.66 × 10⁻²³ g
2H₂ + O₂ → 2H₂O with 0.30 mol H₂ and 0.12 mol O₂. Which is limiting, and how much water forms?Higher: 0.30 mol H₂ needs 0.15 mol O₂, but only 0.12 mol is present, so oxygen is limiting. Water = 2 × 0.12 = 0.24 mol.
Exothermic vs endothermic (in terms of the surroundings): Exothermic: energy transferred from the chemicals to the surroundings; their temperature rises. Endothermic: energy transferred from the surroundings to the chemicals; their temperature falls.
Examples of exothermic and endothermic reactions: Exothermic: combustion, neutralisation, many metals + dilute acids. Endothermic: thermal decomposition of a carbonate, hydrogencarbonate + citric or ethanoic acid.
Profile: reactants 140, peak 200, products 90 kJ/mol. Activation energy and energy change?: Activation energy = 200 − 140 = 60 kJ/mol. Energy change = 90 − 140 = −50 kJ/mol, so exothermic.
A candle needs a match to light. Does that make burning endothermic?: No. The match only supplies the activation energy. Overall more energy is released than taken in, so it is exothermic.
Measuring a temperature change in a polystyrene cup: main systematic error and how to reduce it: Heat transfer to or from the room makes every measured change too small. Insulate the cup and use a lid.
Bond breaking and bond making in energy calculationsHigher: Breaking bonds takes in energy; making bonds releases it. Energy change = bonds broken − bonds made. Negative = exothermic.
2H₂O₂ → 2H₂O + O₂. O–H 464, O–O 146, O=O 498 kJ/mol. Energy change?Higher: Broken: 4 × 464 + 2 × 146 = 2148. Made: 4 × 464 + 498 = 2354. Change = 2148 − 2354 = −206 kJ/mol (exothermic).
Why does a value from bond energies differ from a measured one?Higher: Bond energies are average values from many compounds, so the calculation gives only an estimate.
Oxidising agent and reducing agent: Oxidising agent: oxidises something else and is itself reduced. Reducing agent: reduces something else and is itself oxidised.
NiO + H₂ → Ni + H₂O: what is reduced, and what is the reducing agent?: Nickel(II) oxide loses oxygen, so it is reduced (the oxidising agent). Hydrogen gains oxygen, so it is oxidised and is the reducing agent.
Redox in terms of electrons: Mg + Zn²⁺ → Mg²⁺ + ZnHigher: OIL RIG: oxidation is loss, reduction is gain of electrons. Mg → Mg²⁺ + 2e⁻ (oxidised). Zn²⁺ + 2e⁻ → Zn (reduced).
What ions do acids and alkalis form in water?: Acids form H⁺ ions (HCl → H⁺ + Cl⁻). Alkalis form OH⁻ ions (LiOH → Li⁺ + OH⁻). An alkali is a soluble base.
Balanced equation: calcium carbonate + nitric acid: CaCO₃ + 2HNO₃ → Ca(NO₃)₂ + H₂O + CO₂
Making a pure, dry salt from an insoluble base (PAG C7): Warm the acid; add the base in excess, stirring; filter off the unreacted solid; heat the filtrate to boil off part of the water and leave to crystallise; filter and dry the crystals.
Strong vs weak, and concentrated vs dilute (acids)Higher: Strong/weak: degree of ionisation (hydrochloric, nitric, sulfuric are fully ionised; ethanoic, citric, carbonic partly). Concentrated/dilute: amount of acid in a given volume.
pH falls from 6 to 3. How does the H⁺ concentration change?Higher: Each fall of 1 pH unit is 10 times more H⁺: 10 × 10 × 10 = 1000 times greater.
One advantage of a pH probe over universal indicator: Any one: more precise; not affected by the colour of the solution; can record pH continuously; no subjective colour matching.
Electrolysis and electrolyte: Electrolysis: decomposition of a liquid (molten or aqueous) by passing a direct current through it. Electrolyte: an ionic substance, melted or in solution, that carries the current and is broken down.
Why must an ionic compound be molten or dissolved to be electrolysed?: In the solid, the ions are locked in the lattice and cannot move. Melted or dissolved, the ions are free to move and carry charge.
Products of electrolysing molten sodium chloride with inert electrodes: Sodium at the cathode (negative); chlorine at the anode (positive).
Rules for the products of an aqueous solution (inert electrodes): Cathode: the metal only if it is less reactive than hydrogen (Cu, Ag); otherwise hydrogen. Anode: a halogen if Cl⁻, Br⁻ or I⁻ is present; otherwise oxygen.
Products of electrolysing NaCl(aq) and CuSO₄(aq) with inert electrodes: NaCl(aq): hydrogen at the cathode, chlorine at the anode; sodium hydroxide is left. CuSO₄(aq): copper at the cathode, oxygen at the anode; sulfuric acid is left and the blue colour fades.
Half equation for oxygen forming at the anode from an aqueous solution: 4OH⁻ → O₂ + 2H₂O + 4e⁻ (charge 4− on each side).
Why do hydrogen and oxygen form in a 2 : 1 volume ratio?: Each H₂ needs 2 electrons and each O₂ releases 4, so twice as many hydrogen molecules form as oxygen molecules.
Inert vs non-inert anode in copper sulfate solution: Graphite gives oxygen and a fading colour; copper dissolves and the colour stays.
In electrolysis, what moves through the liquid and what moves through the wires?: Ions move through the liquid; electrons move through the wires.

Topic 4: Topic C4 – Predicting and Identifying Reactions and Products

Group 1 metals: physical properties and how they are stored: Soft, low density, low melting point (falling down the group). Stored under oil because they react with air and water vapour.
Why does potassium react faster with water than lithium?: Its outer electron is further from the nucleus with more inner shells shielding it, so it is attracted less strongly and lost more easily.
Chlorine, bromine and iodine: state and colour at room temperature: Chlorine: pale green gas. Bromine: red-brown liquid (orange in water). Iodine: grey-black solid (brown in water).
Chlorine water is added to sodium bromide solution. Equation and observation: Cl₂ + 2NaBr → 2NaCl + Br₂. The colourless solution turns orange as bromine forms.
Group 0 (noble gases): properties and the reason for them: Colourless, monatomic, very unreactive gases. Full outer shells (2 for helium, 8 for the rest). Boiling point and density increase down the group.
Transition metals compared with Group 1: Higher melting point and density, much less reactive, ions with different charges (Fe²⁺/Fe³⁺, Cu⁺/Cu²⁺), coloured compounds, and many are catalysts.
Three transition metal catalysts and their reactions: Iron: the Haber process. Nickel: adding hydrogen to alkenes. Manganese(IV) oxide: decomposing hydrogen peroxide.
Metal Q displaces copper from copper(II) sulfate but not iron from iron(II) sulfate. Where is Q?: Between them: iron > Q > copper. A metal displaces only a less reactive metal from its salt.
1.10 g of manganese forms 1.74 g of oxide (Mn = 55, O = 16). Empirical formula?: O = 1.74 − 1.10 = 0.64 g. Mn: 1.10 ÷ 55 = 0.020 mol. O: 0.64 ÷ 16 = 0.040 mol. Ratio 1 : 2, so MnO₂.
Precipitate: An insoluble solid that forms when two solutions are mixed.
Tests for oxygen, hydrogen and carbon dioxide: Oxygen: glowing splint relights. Hydrogen: lighted splint at the mouth of the tube gives a squeaky pop. Carbon dioxide: bubble through limewater, which turns milky.
Test for chlorine: Damp blue litmus paper (in a fume cupboard) turns red, then is bleached white.
Flame test colours: Li⁺, Na⁺, K⁺, Ca²⁺, Cu²⁺. Why use a blue flame?: Li⁺ crimson, Na⁺ yellow, K⁺ lilac, Ca²⁺ orange-red, Cu²⁺ green. A yellow flame would add its own colour.
Sodium hydroxide test: results for Ca²⁺, Zn²⁺, Cu²⁺, Fe²⁺, Fe³⁺Higher: Ca²⁺ white, stays in excess. Zn²⁺ white, dissolves in excess. Cu²⁺ blue. Fe²⁺ green. Fe³⁺ orange-brown.
Tests for halide ions and their resultsHigher: Acidify with dilute nitric acid, add silver nitrate solution. Cl⁻ white (AgCl), Br⁻ cream (AgBr), I⁻ yellow (AgI).
Tests for sulfate and carbonate ionsHigher: Sulfate: acidify with dilute hydrochloric acid, add barium chloride solution → white BaSO₄. Carbonate: dilute acid → fizzing; the gas turns limewater milky.
The three advantages of instrumental methods named in the specification: Sensitivity (detects very small amounts), accuracy (reliable, correct results) and speed (fast, often automatic).
Molecular ion peak: The peak at the highest m/z in a mass spectrum; its value is the Mr of the compound.
Base peak: The tallest peak in a mass spectrum. It is not always the molecular ion.

Topic 5: Topic C5 – Monitoring and Controlling Chemical Reactions

Titration technique (PAG C6): Pipette a measured volume into a conical flask with a few drops of indicator; fill a rinsed burette, read the bottom of the meniscus at eye level; add while swirling, drop by drop near the end point; repeat until titres are concordant.
Concordant titres: Accurate titres that agree closely, usually within 0.10 cm³. Only these are averaged; the rough titre is left out.
Colours of phenolphthalein and methyl orange: Phenolphthalein: pink in alkali, colourless in acid. Methyl orange: yellow in alkali, red in acid.
Why is the actual yield less than the theoretical yield?: The reaction is incomplete or reversible; product is lost during filtering, transferring or crystallising; side reactions form unwanted products.
Atom economy: formula, and why it matters: (total Mr of desired product ÷ total Mr of reactants) × 100, from the balanced equation. High atom economy means less waste. A reaction with one product has 100%.
2.92 g of HCl (Mr 36.5) is dissolved to make 400 cm³. Concentration in mol/dm³?Higher: 2.92 ÷ 36.5 = 0.0800 mol; 0.0800 ÷ 0.400 = 0.200 mol/dm³.
25.0 cm³ of KOH needs 16.00 cm³ of 0.125 mol/dm³ HNO₃ (1 : 1). Concentration of KOH?Higher: HNO₃: 0.125 × 0.01600 = 0.00200 mol = KOH. 0.00200 ÷ 0.0250 = 0.0800 mol/dm³.
Molar gas volume at RTP. What volume does 0.0250 mol of oxygen occupy?Higher: 24 dm³ (24 000 cm³) per mole at room temperature and pressure. 0.0250 × 24 = 0.600 dm³.
Factors to weigh when choosing a reaction pathwayHigher: Atom economy, percentage yield, rate, equilibrium position, and the usefulness of by-products.
Three ways to measure the rate of a reaction that gives off a gas: Gas volume at set times (gas syringe, or upturned measuring cylinder over water); mass loss on a balance (good for a heavy gas such as CO₂, poor for hydrogen); or time for a fixed change (1/t).
105 cm³ of gas forms in 70 s. Mean rate?: 105 ÷ 70 = 1.5 cm³/s
A tangent passes through (10 s, 28 cm³) and (50 s, 76 cm³). Rate at that time?: (76 − 28) ÷ (50 − 10) = 48 ÷ 40 = 1.2 cm³/s
Which factors change only collision frequency, and which change collision energy too?: Concentration, pressure and surface area: more frequent collisions only. Temperature: more frequent and more energetic collisions, so more reach the activation energy.
Surface area to volume ratio of a 5 mm cube, and of it cut into 1 mm cubes: 5 mm cube: 150 mm² ÷ 125 mm³ = 1.2 : 1. 1 mm cubes: 125 × 6 = 750 mm² ÷ 125 mm³ = 6 : 1. Smaller pieces react faster.
Characteristics of a catalyst, and how it works: Increases the rate, is not used up (same mass at the end), does not change the amount of product; needed in small amounts. It provides an alternative pathway with a lower activation energy.
Enzyme: A biological catalyst: it speeds up reactions in living things and is not used up.
Run X takes 20 s and run Y takes 80 s for the same change. How many times faster is X?: Rate ∝ 1/t. 1/20 = 0.050 s⁻¹ and 1/80 = 0.0125 s⁻¹, so X is 4 times as fast.
Heating hydrated cobalt(II) chloride, then adding water to the product: Heat: pink → blue as water is driven off (forward, endothermic). Add water: blue → pink, heat released (reverse, exothermic).
Dynamic equilibrium: In a closed system, the forward and reverse reactions happen at equal rates, so the concentrations stay constant. Both reactions continue.
At equilibrium, are the concentrations of reactants and products equal?: No. They are constant, rarely equal. It is the rates of the two reactions that are equal.
Le Chatelier's principleHigher: A change made to a system at equilibrium makes the position shift to oppose the change.
PCl₅(g) ⇌ PCl₃(g) + Cl₂(g). Effect of raising the pressure?Higher: The position shifts left, to the side with fewer gas molecules (1 on the left, 2 on the right). Count gas molecules, not atoms.
A forward reaction is endothermic. What happens to the amount of product when the temperature is lowered?Higher: The position shifts in the exothermic (reverse) direction, so there is less product.
CO(g) + 3H₂(g) ⇌ CH₄(g) + H₂O(g), forward exothermic. Conditions for the most methane?Higher: High pressure (4 gas molecules → 2), low temperature (forward is exothermic), and excess hydrogen.

Topic 6: Topic C6 – Global Challenges

Ore: Rock from which a metal compound can be extracted at a profit.
Which metals are extracted by heating with carbon, and which by electrolysis?: Metals below carbon in the reactivity series (e.g. iron, zinc, copper, lead): heat the oxide with carbon, which is the reducing agent. Metals above carbon (e.g. aluminium): electrolysis, which needs large amounts of electricity.
Extracting aluminium: electrolyte and electrode half equations: Aluminium oxide dissolved in molten cryolite (lower operating temperature). Cathode: Al³⁺ + 3e⁻ → Al. Anode: 2O²⁻ → O₂ + 4e⁻; the oxygen burns the carbon anodes to CO₂, so they are replaced.
Bioleaching vs phytoextractionHigher: Bioleaching: bacteria produce a leachate of metal ions from low-grade ore. Phytoextraction: plants absorb metal ions and are burned, leaving metal-rich ash. Both are low energy but slow; the metal is then got by scrap iron displacement or electrolysis.
Haber process conditions, and why the pressure is about 200 atmHigher: N₂ + 3H₂ ⇌ 2NH₃ (exothermic): iron catalyst, about 450 °C, about 200 atm. Fewer gas molecules on the right, so high pressure helps yield and rate; higher still would cost too much.
Contact process: equation and conditionsHigher: 2SO₂ + O₂ ⇌ 2SO₃ (exothermic): vanadium(V) oxide catalyst, about 450 °C, pressure close to atmospheric (the yield is already high).
Fertilisers: the three elements, and one property every fertiliser compound needs: Nitrogen, phosphorus and potassium (NPK). The compounds must be soluble.
The four stages of a life-cycle assessment: Raw materials and manufacture; use; transport; disposal (landfill, incineration or recycling).
Composition of brass, bronze and solder: Brass: copper + zinc. Bronze: copper + tin. Solder: tin + lead (or tin with small amounts of copper or silver).
What does rusting need, and how does sacrificial protection work?: Oxygen and water. A more reactive metal (zinc or magnesium) attached to the iron oxidises first, in place of the iron. Galvanising gives a barrier and sacrificial protection.
Homologous series: A family with one general formula and the same functional group; each member adds CH₂.
General formulae of alkanes, alkenes, alcohols and carboxylic acids: Alkane CₙH₂ₙ₊₂. Alkene CₙH₂ₙ. Alcohol CₙH₂ₙ₊₁OH. Carboxylic acid CₙH₂ₙ₊₁COOH.
How does bromine water distinguish an alkene from an alkane?: The alkene adds bromine across the C=C, turning bromine water from orange to colourless. An alkane leaves it orange.
Oxidising an alcohol: reagent, observation and product: Warm with acidified potassium manganate(VII): purple → colourless. Ethanol → ethanoic acid.
Monomer and repeat unit of PTFE: Monomer: tetrafluoroethene, CF₂=CF₂. Repeat unit: –[CF₂–CF₂]– with n outside the brackets. The C=C opens to C–C.
Condensation polymers: what the monomers need, and the two typesHigher: Each monomer has two functional groups, and a small molecule (usually water) is released at each link. Polyester: diol + dicarboxylic acid. Polyamide (nylon): diamine + dicarboxylic acid.
Monomers of DNA, starch and cellulose, and proteins: DNA: four nucleotides (bases adenine, thymine, cytosine, guanine). Starch and cellulose: sugars (glucose). Proteins: amino acids.
Fractions of crude oil, from the top of the column to the bottom: Refinery gases, petrol, kerosene, diesel, fuel oil, bitumen. Bigger molecules have stronger intermolecular forces and higher boiling points, so they are collected lower.
Cracking: products, conditions and reasons: Long alkanes → shorter alkanes + alkenes. Vapour over a hot catalyst (zeolite at about 500 °C in industry; aluminium oxide in the lab). Demand for short-chain fuels exceeds supply, and alkenes make polymers.
A chain of poly(butene) has Mr 56 000. How many C₄H₈ monomers joined?: Mr of C₄H₈ = 48 + 8 = 56. 56 000 ÷ 56 = 1000.
Earth's first atmosphere: thought composition and source: Mainly carbon dioxide and water vapour, some nitrogen, possibly a little methane and ammonia, almost no oxygen. Released by volcanic outgassing.
How the early atmosphere changed: Water vapour condensed to form oceans; CO₂ dissolved and was locked into carbonate rocks; photosynthesis removed more CO₂; carbon was stored in fossil fuels; unreactive nitrogen built up.
Composition of dry air today: About 78% nitrogen, 21% oxygen, 0.9% argon and 0.04% carbon dioxide.
How did oxygen build up in the atmosphere?: Cyanobacteria, then algae and plants, photosynthesised. Early oxygen reacted with dissolved iron (banded iron formations), then built up in the air; an ozone layer formed. It took billions of years.
How do greenhouse gases keep the Earth warm?: Mostly short-wavelength sunlight warms the surface, which emits longer-wavelength infrared. Greenhouse gases absorb the infrared and re-emit it in all directions, so less energy escapes to space.
Why does a correlation between CO₂ and temperature not prove a human cause on its own?: Correlation is not causation: a mechanism is needed. Natural factors also change climate, past data come from proxies with larger error, and models simplify complex systems.
Convert 0.038% carbon dioxide into ppm: 1% = 10 000 ppm, so 0.038 × 10 000 = 380 ppm.
How carbon monoxide, sulfur dioxide and oxides of nitrogen form, and their problems: CO: incomplete combustion; toxic, no colour or smell. SO₂: sulfur impurities in fuels burn; acid rain. NO and NO₂: nitrogen from the air reacts at high engine temperatures; acid rain and breathing problems.
How is fresh surface water made potable, and which source needs most energy?: Sedimentation → filtration (sand, gravel) → sterilisation (chlorine, ozone or UV). Salt water needs most energy: desalination by distillation or reverse osmosis.

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Written for OCR GCSE Chemistry A (Gateway Science) (J248), syllabus J248 (Version 4.0 (August 2026), for first assessment in 2018). 27 cards cover Higher tier content only; Foundation students can hide them with “Foundation only”.

Cards by Marlbridge Academic Team; 19 restate definitions from the J248 revision notes. This deck was reviewed by Nouman Ahmed on 2026-11-05. Report an error.

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