Quick Answer: The hardest IGCSE science questions are multi-step calculations and "explain" questions where marks depend on precise wording. Below are 10 original questions in the style of Cambridge IGCSE Physics (0625), Chemistry (0620) and Biology (0610) Extended papers and Pearson Edexcel International GCSE Higher papers: five Physics, three Chemistry and two Biology. Each has a fully worked answer and the mistake that costs students the most marks.
This post is for Year 10 and Year 11 students aiming for an A* or grade 8–9, and for parents who want to see what the top of the paper looks like. All ten questions were written for this post. They follow the style of exam questions but are not taken from past papers. Try each one on paper before reading the answer.
10 Hard IGCSE Science Questions With Answers
1. Physics: stopping distance
A car travels at 24 m/s. The driver's reaction time is 0.65 s. Once the brakes are applied, the car decelerates at a constant 6.0 m/s². Calculate the total stopping distance.
Answer:
Thinking distance = speed × reaction time = 24 × 0.65 = 15.6 m
Braking distance: the car slows from 24 m/s to 0, so v² = u² − 2as gives s = u² ÷ 2a = 576 ÷ 12 = 48 m
Stopping distance = 15.6 + 48 = 63.6 m ≈ 64 m (2 s.f.)
Common mistake: Forgetting the thinking distance, or treating the whole stop as constant deceleration. During the reaction time the car is still moving at full speed.
Typical of: Cambridge 0625 Extended and Edexcel International GCSE Physics. You could also find braking distance from the area under a speed–time graph.
2. Physics: specific heat capacity and efficiency
A 2.4 kW kettle contains 1.5 kg of water at 20 °C. The specific heat capacity of water is 4200 J/(kg °C). The water reaches 100 °C after 250 s. Calculate the efficiency of the kettle.
Answer:
Useful energy to the water = mcΔT = 1.5 × 4200 × 80 = 504,000 J
Electrical energy supplied = power × time = 2400 × 250 = 600,000 J
Efficiency = 504,000 ÷ 600,000 = 0.84, or 84%
Common mistake: Using 2.4 instead of 2400 W, or using the final temperature (100 °C) as ΔT instead of the change (80 °C).
Typical of: Both boards. Combining a thermal-energy equation with an electrical one is a common two-step question.
3. Physics: series and parallel circuit
A 6.0 V battery of negligible internal resistance is connected to a 4.0 Ω resistor in series with a parallel pair of resistors of 6.0 Ω and 12 Ω. Calculate (a) the current from the battery, (b) the current in the 12 Ω resistor and (c) the power dissipated in the 4.0 Ω resistor.
Answer:
Parallel pair: 1/R = 1/6 + 1/12 = 3/12, so R = 4.0 Ω. Total resistance = 4.0 + 4.0 = 8.0 Ω.
(a) I = V ÷ R = 6.0 ÷ 8.0 = 0.75 A
(b) Voltage across the parallel pair = 0.75 × 4.0 = 3.0 V. Current in 12 Ω = 3.0 ÷ 12 = 0.25 A (the 6.0 Ω resistor carries 0.50 A).
(c) P = I²R = 0.75² × 4.0 = 2.25 W ≈ 2.3 W
Common mistake: Adding parallel resistances (6 + 12 = 18 Ω), or using the full 6.0 V across the 12 Ω resistor. The voltage is shared between the series part and the parallel part.
Typical of: Cambridge 0625 Extended and Edexcel International GCSE Physics.
4. Physics: refraction and critical angle
A glass block has a refractive index of 1.52. The speed of light in air is 3.0 × 10⁸ m/s.
(a) Calculate the speed of light in the glass. (b) A ray enters the block from air at an angle of incidence of 30°. Calculate the angle of refraction. (c) Calculate the critical angle and state what happens to a ray meeting the glass–air surface at 45°.
Answer:
(a) v = c ÷ n = 3.0 × 10⁸ ÷ 1.52 = 2.0 × 10⁸ m/s (1.97 × 10⁸ m/s to 3 s.f.)
(b) sin r = sin 30° ÷ 1.52 = 0.329, so r = 19.2°
(c) sin c = 1 ÷ 1.52, so c = 41.1°. 45° is greater than the critical angle, so the ray undergoes total internal reflection.
Common mistake: Putting n the wrong way round (sin r = 1.52 × sin 30°), which gives a value above 1. Light bends towards the normal entering glass, so r must be smaller than i.
Typical of: Both boards. The critical angle and n = 1/sin c are Extended and Higher content.
5. Physics: half-life with background radiation
A detector reads 520 counts per minute near a radioactive source. The background count is 40 counts per minute. Fifteen hours later, the detector reads 100 counts per minute. Calculate the half-life of the source.
Answer:
Corrected count rates: 520 − 40 = 480 and 100 − 40 = 60.
480 → 240 → 120 → 60 is three half-lives.
Half-life = 15 ÷ 3 = 5.0 hours
Common mistake: Not subtracting the background count. 520 to 100 doesn't give a whole number of halvings, which is the clue that something has been missed.
Typical of: Cambridge 0625 Extended and Edexcel International GCSE Physics.
6. Chemistry: limiting reactant and gas volume
5.00 g of magnesium is added to 50.0 cm³ of 2.00 mol/dm³ hydrochloric acid.
Mg + 2HCl → MgCl₂ + H₂
(a) Show which reactant is in excess. (b) Calculate the volume of hydrogen produced at room temperature and pressure. (c) Calculate the mass of the excess reactant left over. (Aᵣ: Mg = 24. One mole of gas occupies 24 dm³ at r.t.p.)
Answer:
Moles of Mg = 5.00 ÷ 24 = 0.208 mol
Moles of HCl = 2.00 × (50.0 ÷ 1000) = 0.100 mol
(a) 0.100 mol of HCl reacts with only 0.0500 mol of Mg, so magnesium is in excess and HCl is the limiting reactant.
(b) Moles of H₂ = 0.100 ÷ 2 = 0.0500 mol. Volume = 0.0500 × 24 = 1.20 dm³ (1200 cm³)
(c) Mg used = 0.0500 × 24 = 1.20 g. Mg left = 5.00 − 1.20 = 3.80 g
Common mistake: Ignoring the 1 : 2 ratio and comparing 0.208 with 0.100 directly, or forgetting to convert cm³ to dm³ (dividing by 1000).
Typical of: Both boards. Limiting-reactant questions are a regular top-grade item on Cambridge 0620 Extended and Edexcel International GCSE Chemistry.
7. Chemistry: water of crystallisation
2.50 g of hydrated copper(II) sulfate, CuSO₄·xH₂O, is heated until all the water is removed. 1.60 g of anhydrous CuSO₄ remains. Find x. (Mᵣ: CuSO₄ = 160, H₂O = 18)
Answer:
Mass of water lost = 2.50 − 1.60 = 0.90 g
Moles of CuSO₄ = 1.60 ÷ 160 = 0.0100 mol
Moles of H₂O = 0.90 ÷ 18 = 0.0500 mol
Ratio CuSO₄ : H₂O = 0.0100 : 0.0500 = 1 : 5, so x = 5 (CuSO₄·5H₂O)
Common mistake: Dividing the masses (0.90 ÷ 1.60) instead of the moles. You must convert both to moles before taking the ratio. In practicals, students also forget to heat to constant mass.
Typical of: Both boards, usually alongside a practical write-up.
8. Chemistry: bond energies
Use the bond energies below to calculate the enthalpy change for the combustion of methane, and state whether the reaction is exothermic or endothermic.
CH₄ + 2O₂ → CO₂ + 2H₂O
| Bond | C–H | O=O | C=O | O–H |
| Bond energy (kJ/mol) | 412 | 496 | 743 | 463 |
Answer:
Energy to break bonds: 4(C–H) + 2(O=O) = 4(412) + 2(496) = 1648 + 992 = 2640 kJ
Energy released making bonds: 2(C=O) + 4(O–H) = 2(743) + 4(463) = 1486 + 1852 = 3338 kJ
ΔH = 2640 − 3338 = −698 kJ/mol. The reaction is exothermic.
Common mistake: Counting only one O=O and one O–H instead of using the balancing numbers, or getting the sign the wrong way round. "Broken minus made" gives a negative answer when more energy is released than taken in.
Typical of: Both boards. Cambridge 0620 and Edexcel International GCSE both test bond-energy calculations.
9. Biology: sex-linked inheritance
Red–green colour blindness is caused by a recessive allele on the X chromosome. A woman with normal vision who is a carrier (XᴮXᵇ) has children with a man with normal vision (XᴮY).
(a) What is the probability that their next child is a colour-blind boy? (b) Given that the child is a boy, what is the probability that he is colour-blind?
Answer:
The four equally likely outcomes are XᴮXᴮ, XᴮY, XᵇXᴮ and XᵇY.
(a) Only XᵇY is a colour-blind boy: 1 in 4 (0.25)
(b) The boys are XᴮY and XᵇY, and one of the two is colour-blind: 1 in 2 (0.5)
No daughter can be colour-blind, because every daughter gets the father's Xᴮ.
Common mistake: Giving the same answer to (a) and (b). Read whether the question asks about any child or only about the sons. Students also put the allele on the Y chromosome.
Typical of: Cambridge 0610 Extended, which names sex linkage and colour blindness in the syllabus. Edexcel International GCSE Biology covers sex determination and genetic crosses, so check how far your specification goes.
10. Biology: osmosis and percentage change in mass
Potato strips were left in sucrose solutions for 24 hours.
| Sucrose concentration (mol/dm³) | 0.0 | 0.2 | 0.4 | 0.6 |
| Percentage change in mass | +16 | +6 | ? | −12 |
(a) A strip in 0.4 mol/dm³ went from 2.50 g to 2.40 g. Calculate its percentage change in mass. (b) Estimate the concentration at which there would be no change in mass. (c) Explain the result at 0.6 mol/dm³.
Answer:
(a) (2.40 − 2.50) ÷ 2.50 × 100 = −4.0%
(b) The change goes from +6 at 0.2 to −4 at 0.4, so zero is 6/10 of the way across: 0.2 + 0.6 × 0.2 = about 0.32 mol/dm³. At this concentration the potato cells and the solution have the same water potential.
(c) The solution has a lower water potential than the potato cells, so water moves out of the cells by osmosis through the partially permeable cell membrane. The strip loses mass.
Common mistake: Dividing by the final mass instead of the starting mass. In (c), saying "sucrose moves into the potato" or "water moves from low to high concentration" without saying water concentration or water potential.
Typical of: Both boards. Percentage change and the "no change" estimate are common on practical papers.
How to Get Better at Hard IGCSE Science Questions
- Write the equation first, then substitute. Many mark schemes give a mark for the correct equation even if the arithmetic slips.
- Convert units before you calculate. kW to W, cm³ to dm³, minutes to seconds. Most lost marks in Questions 2 and 6 come from units.
- Use the command word. "Explain" needs a reason ("because the water potential is lower"), not just a description of what happened.
- Sense-check the size. A refraction angle bigger than the incidence angle, or a sine above 1, tells you something went wrong.
- Practise under time. Timed past papers from your own board show which question types you need to work on.
How Improve ME Institute Helps
Want questions pitched at your exam? Our IGCSE groups teach Physics, Chemistry and Biology in small groups of a maximum of six, matched to each student's board, whether Cambridge or Edexcel. With 40+ specialist tutors and 1,000+ students every year, we start every student with a free Diagnostic Test, so teaching targets the topics that are losing marks. If maths is the weaker subject, try our hard GCSE maths questions with answers and our post on hard maths questions that actually make you better at maths.
Frequently Asked Questions
Q: How hard is IGCSE Chemistry?
A: Many students find IGCSE Chemistry the hardest of the three sciences because of mole calculations, which build on each other. Once moles, reacting masses and gas volumes are secure, most of the remaining topics become much more manageable.
Q: Which IGCSE science is the hardest?
A: It depends on the student. Physics and Chemistry have more calculations, while Biology has more extended writing where precise words earn the marks. Students who struggle with maths tend to find Physics hardest, and students who struggle with wording tend to find Biology harder than expected.
Q: Are Cambridge and Edexcel IGCSE science questions the same?
A: The core science is very similar, but the papers are structured differently and some topics appear in one specification and not the other. Always practise with past papers from the board your school uses.
Q: What is the difference between Core and Extended in Cambridge IGCSE science?
A: Extended covers the Core content plus supplementary material, such as the critical angle in Physics, and gives access to grades up to A*. Core papers have a lower maximum grade. Edexcel International GCSE science doesn't use this split in the same way.
Q: How should I revise for IGCSE science calculations?
A: Learn the equations, then do short sets of mixed calculations every few days rather than one long session. When you get one wrong, find the exact step that failed, such as a unit conversion or a mole ratio, and redo it from there.
Ready for the Top of the Paper?
Pick the question you found hardest and try it again tomorrow without looking at the answer. If the same topics keep costing marks, a free Diagnostic Test at Improve ME Institute in Gold & Diamond Park, Al Quoz, will show exactly where the gaps are.
Sources
- Cambridge IGCSE Physics (0625) syllabus page (syllabuses 2026–2028 and 2029 listed)
- Cambridge IGCSE Chemistry (0620) syllabus page
- Cambridge IGCSE Biology (0610) syllabus page
- Pearson Edexcel International GCSE Physics (2017)
- Pearson Edexcel International GCSE Chemistry (2017)
- Pearson Edexcel International GCSE Biology (2017)
