GCSE Chemistry Practice Questions

Answer: 24.3

Explanation: Multiply each isotope mass by its percentage abundance, add them, then divide by 100: (24 x 79) + (25 x 10) + (26 x 11) = 1896 + 250 + 286 = 2432. Then 2432 / 100 = 24.32, which rounds to 24.3. One mark for the method, one for the correct working, one for the final answer to 1 decimal place.

Answer: Diamond: each carbon forms 4 strong covalent bonds in a rigid tetrahedral lattice — no weak layers to slide, so very hard. Graphite: carbon atoms form layers of hexagons with weak intermolecular forces between layers — layers slide easily, so soft and slippery.

Explanation: Award marks for: (1) diamond has 4 covalent bonds per carbon in 3D lattice, (2) this rigid structure resists scratching, (3) graphite has layered hexagonal structure, (4) weak forces between layers allow sliding. The key contrast is strong covalent bonds throughout vs weak forces between layers.

Answer: 13.6 g

Explanation: Equation: Zn + 2HCl –> ZnCl2 + H2. Moles of Zn = 6.5 / 65 = 0.1 mol. Mole ratio Zn : ZnCl2 = 1:1, so 0.1 mol ZnCl2 formed. Mr of ZnCl2 = 65 + (2 x 35.5) = 136. Mass = 0.1 x 136 = 13.6 g. Marks for: moles of zinc, Mr of product, mole ratio, final mass.

Answer: Both Na+ and H+ (from water) are attracted to the negative electrode. Hydrogen is discharged because it is less reactive than sodium — sodium is too reactive to be discharged from aqueous solution, so H+ gains electrons instead: 2H+ + 2e- –> H2.

Explanation: Marks for: (1) both ions present at cathode, (2) hydrogen discharged due to reactivity series position, (3) correct half equation. The rule: if the metal is more reactive than hydrogen, hydrogen is produced at the cathode.

Answer: Measure volume of CO2 produced per unit time using a gas syringe. Higher temperature increases rate because particles have more kinetic energy, move faster, collide more frequently and a greater proportion have energy above the activation energy — so more successful collisions per second.

Explanation: Marks for: (1) gas syringe / volume-time method, (2) particles gain kinetic energy, (3) more frequent collisions, (4) more particles exceed activation energy, (5) more successful collisions per unit time. Collision theory answers need both frequency AND energy points.

Answer: C10H22 –> C2H4 + C8H18

Explanation: Cracking breaks a long alkane into a shorter alkene and a shorter alkane. Carbon atoms must balance (2 + 8 = 10) and hydrogen atoms must balance (4 + 18 = 22). One mark for correct products, one for balancing. Any valid combination totalling C10H22 is accepted.

Answer: Clean a nichrome wire in concentrated HCl, dip in the sample, hold in a blue Bunsen flame and observe the colour. Lithium: crimson red. Sodium: yellow/orange. Potassium: lilac.

Explanation: Marks for: (1) cleaning wire in acid, (2) Bunsen flame method, (3-4) two or three correct flame colours. All three colours needed for full marks. Common error: using a platinum wire — nichrome is standard.

Answer: Increased combustion of fossil fuels releases CO2. Deforestation reduces CO2 removal by photosynthesis. Consequence: enhanced greenhouse effect leading to global warming, causing sea-level rise / extreme weather / habitat loss.

Explanation: Marks for: (1) fossil fuel combustion, (2) deforestation / reduced photosynthesis, (3) greenhouse effect mechanism, (4) specific consequence. Link the cause to the effect clearly — vague “pollution” answers score poorly.

Answer: He left gaps for undiscovered elements. He predicted their properties based on position in the table. When these elements were later discovered with matching properties, it proved his arrangement was correct.

Explanation: Marks for: (1) gaps for unknown elements, (2) predicted properties, (3) later discovery confirmed predictions. This is a classic “how science works” question — the key idea is prediction followed by verification.

Answer: Nitrogen (5 outer electrons) shares one electron with each of three hydrogen atoms, forming three single covalent bonds. Nitrogen retains one lone pair. Each hydrogen has 2 electrons (full shell); nitrogen has 8 (octet).

Explanation: Marks for: (1) three shared pairs shown correctly, (2) lone pair on nitrogen, (3) correct electron counts (H=2, N=8). Use dots for one atom and crosses for the other to distinguish electron origin.

Answer: 0.0800 mol/dm3

Explanation: Moles HCl = 0.100 x (20.0/1000) = 0.00200 mol. Equation NaOH + HCl –> NaCl + H2O gives 1:1 ratio, so moles NaOH = 0.00200 mol. Concentration = 0.00200 / (25.0/1000) = 0.0800 mol/dm3. Marks for: moles of acid, mole ratio, volume conversion, final concentration.

Answer: Aluminium reacts with oxygen to form a thin, tough layer of aluminium oxide on its surface. This oxide layer is unreactive and prevents further reaction — it acts as a protective barrier.

Explanation: Marks for: (1) oxide layer forms, (2) layer is protective / prevents further reaction, (3) explains the apparent contradiction with its reactivity series position. Key term: “protective oxide layer”.

Answer: Greater surface area exposes more reactant particles to collisions. This increases the frequency of collisions between reactant particles, so there are more successful collisions per second and the rate increases.

Explanation: Marks for: (1) more particles exposed, (2) increased collision frequency, (3) more successful collisions per unit time. Note: surface area affects frequency, NOT energy — do not mention activation energy here.

Answer: Addition reaction. Product: CH2Br-CHBr-CH3 (1,2-dibromopropane). The Br-Br bond breaks and each bromine adds across the C=C double bond, which becomes a single bond.

Explanation: Marks for: (1) “addition” named, (2) correct product structure, (3) displayed formula showing all bonds. The orange bromine decolourises — a classic test for alkenes.

Answer: Bubble the gas through limewater (calcium hydroxide solution). Positive result: limewater turns milky/cloudy white.

Explanation: One mark for limewater, one for “milky”. The cloudiness is calcium carbonate precipitate: CO2 + Ca(OH)2 –> CaCO3 + H2O. With excess CO2 the milkiness disappears — a common follow-up question.

Answer: Poly(ethene) has long chains of strong C-C single bonds that microorganisms cannot break down — no enzymes exist to digest it. Problem: accumulates in landfill taking up space, or enters oceans harming marine life.

Explanation: Marks for: (1) long hydrocarbon chains / strong bonds, (2) microorganisms lack enzymes to break them, (3) specific environmental problem, (4) elaboration. Link structure to property to environmental impact.

Answer: Rutherford fired alpha particles at thin gold foil. Most passed straight through, some deflected, a few bounced back. This showed: atoms are mostly empty space (passed through), with a small dense positively charged nucleus (deflection/bounce-back) — contradicting the plum pudding model’s uniform positive charge.

Explanation: Marks for: (1) experimental set-up, (2) observations, (3) empty space conclusion, (4) nucleus conclusion. Link each observation to its conclusion — this “evidence to model” reasoning is what examiners reward.

Answer: In solid NaCl, ions are locked in a fixed lattice and cannot move to carry charge. When molten, the lattice breaks down and ions are free to move towards oppositely charged electrodes, conducting electricity.

Explanation: Marks for: (1) ions fixed in solid lattice, (2) ions free/mobile when molten, (3) moving ions carry charge. The key principle: conductivity requires mobile charged particles.

Answer: 90%

Explanation: Moles Mg = 12.0/24 = 0.50 mol. Equation 2Mg + O2 –> 2MgO gives 1:1 ratio, so 0.50 mol MgO. Mr MgO = 24 + 16 = 40. Theoretical mass = 0.50 x 40 = 20.0 g. Percentage yield = (18.0/20.0) x 100 = 90%. Marks for: moles, theoretical mass, formula, final percentage.

Answer: A: acidic (pH 1-3). B: neutral (pH 7). C: alkaline (pH 8-14).

Explanation: One mark per correct identification. Red = strongly acidic, green = neutral, purple = strongly alkaline. Remember: universal indicator gives a spectrum — red/orange/yellow (acid), green (neutral), blue/purple (alkali).

Answer: Mass decreases because CO2 gas escapes from the flask. The graph starts steep (fast rate, high reactant concentration), curves, then levels off horizontal when the reaction finishes (one reactant used up).

Explanation: Marks for: (1) gas escapes, (2) steep initial gradient, (3) curve flattening, (4) horizontal at end with reason. The gradient of the curve at any point equals the rate — steepest at the start.

Answer: Crude oil is vaporised and fed into a fractionating column with a temperature gradient (hot at bottom, cool at top). Long-chain hydrocarbons have high boiling points and condense near the bottom; short chains rise higher before condensing. Each fraction is collected at its condensation level.

Explanation: Marks for: (1) vaporisation, (2) temperature gradient, (3) boiling point linked to chain length, (4) condensation at different levels. Key link: longer chains = stronger intermolecular forces = higher boiling point.

Answer: 0.60

Explanation: Rf = distance moved by spot / distance moved by solvent = 4.8 / 8.0 = 0.60. One mark for the formula/substitution, one for the answer. Rf has no units and is always between 0 and 1.

Answer: Benefit: nanoparticles (e.g. titanium dioxide) block UV effectively while remaining transparent — no white residue. Risk: particles are so small they may penetrate skin cells; long-term health effects unknown.

Explanation: “Evaluate” requires balanced argument: (1-2) benefit with explanation, (3-4) risk with explanation. High surface-area-to-volume ratio is the key property — link it to both the benefit (effective UV blocking) and the risk (cell penetration).