Combined Science — IGCSE

Answer: 12.5 m/s

1. Speed = distance ÷ time
2. 150 ÷ 12 = 12.5
3. Unit: m/s

Estimate before you divide: 150 m in 12 s must be a little over 10 m/s, which rules out three of the four options immediately.

Answer: 300 W

1. Work done = mgh = 45 × 10 × 4.0 = 1800 J
2. Power = work ÷ time = 1800 ÷ 6.0
3. Power = 300 W

Energy first, power second. Answers that go straight to a power formula without showing the 1800 J normally lose the first mark.

Answer: 3.0 Ω

1. 1/R = 1/6.0 + 1/6.0 = 2/6.0
2. 1/R = 0.333…, so R = 3.0 Ω
3. Sanity check: less than either single resistor

Two equal resistors in parallel always give half the value. Knowing that lets you check every answer in one second.

Answer: Ammonia particles move randomly and spread out (diffuse) from where there are many to where there are few, colliding with air particles on the way.

1. Particles of ammonia are in constant random motion
2. They move from a region of higher concentration to lower concentration (diffusion)
3. Repeated collisions with air particles spread them through the room

"Gas spreads" earns nothing. Name the particles, say they move randomly, and say the direction of movement is from high to low concentration.

Answer: 4Fe + 3O₂ → 2Fe₂O₃; oxidation (iron gains oxygen)

1. Oxygen atoms need to match: 3 O₂ gives 6 oxygen atoms, so 2Fe₂O₃
2. Iron then needs 4 atoms on the left
3. Gaining oxygen is oxidation

Balance the odd element last. Starting with iron leaves you adjusting it twice, which is where the second mistake usually happens.

Answer: Chloroplast

1. Chloroplasts are the site of photosynthesis
2. Animal cells do not photosynthesise
3. So chloroplasts are plant-only

Watch the wording: cell wall is plant-only, but cell membrane is not. Questions like this are testing whether you read the word.

Answer: Carbon dioxide was the limiting factor at first; once it was supplied in excess, light became the limiting factor, so only more light increased the rate.

1. Rate increased, so carbon dioxide was limiting at the start
2. Adding more had no effect, so it was no longer limiting
3. Increasing light raised the rate, so light had become the limiting factor

Limiting factor questions are answered by elimination: whatever change produces an increase is the factor that was limiting. Say that logic out loud in your answer.

Answer: Series: one path, same current through both lamps, p.d. shared, both dimmer. Parallel: two paths, same p.d. across each lamp, larger total current from the battery, each lamp at normal brightness.

1. Series: a single path, so the current through both lamps is the same (1 mark)
2. Series: the battery p.d. is shared between the lamps, so each lamp is dimmer (1 mark)
3. Parallel: each lamp is connected across the battery, so each has the full p.d. and normal brightness (1 mark)
4. Parallel: the total current from the battery is the sum of the branch currents, so it is larger and the battery drains faster (1 mark)

Write comparisons as pairs — one sentence with "whereas" in the middle scores two marks that two separate paragraphs do not.

Answer: Root hair (extension) — increases surface area for water uptake. Large permanent vacuole — stores cell sap and keeps the cell rigid. No chloroplasts — the cell is underground, in darkness, so photosynthesis cannot occur. Many mitochondria — release energy for active transport of mineral ions.

1. Names the root hair extension and gives its function: surface area for absorption (1 mark)
2. Names the permanent vacuole and gives its function: stores sap, helps keep the cell firm (1 mark)
3. States that chloroplasts are absent and links it to the absence of light underground (1 mark)
4. Names the mitochondria and links the number to active transport and energy release (1 mark)

Label questions are the cheapest marks on the paper and the most commonly rushed. Draw the line, name the part, then add its job in three or four words — the job is usually the mark you are actually being tested on.