O/L Science 2023 — past paper questions and answers

1. #1 · Paper II · 6 marks
   Question 4 describes a destarched green leaf kept in the dark, then illuminated, then tested for starch with iodine. It asks you what result to expect and why, to write the word equation for photosynthesis, and to name two factors that limit the rate of photosynthesis in a plant kept indoors.
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   (i) The iodine solution turns BLUE-BLACK where starch is present — expected in the illuminated leaf because the plant performed photosynthesis during the 6 hours of light, producing and storing starch. (ii) Carbon dioxide + Water → (light energy) → Glucose + Oxygen (iii) Low light intensity; low carbon dioxide concentration (temperature could also be limiting if indoor temp is low).

   Theory notes on this topic: Photosynthesis →

   Source: Department of Examinations, Sri Lanka — G.C.E. O/L Science 2023, Paper II, Question 4 · https://doenets.lk/pastpapers
2. #2 · Paper II · 7 marks
   Question 9 connects three resistors of stated values in parallel across a battery of stated voltage. It asks you to draw the circuit diagram, to calculate the total resistance of the combination, to calculate the total current drawn from the battery, and to state Ohm's Law.
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   (ii) 1/R_total = 1/4 + 1/6 + 1/12 = 3/12 + 2/12 + 1/12 = 6/12 → R_total = 2 Ω (iii) I = V / R = 12 / 2 = 6 A (iv) Ohm's Law: the current through a conductor is directly proportional to the potential difference across it, provided the temperature remains constant (V = IR).

   Theory notes on this topic: Electricity and Simple Circuits →

   Source: Department of Examinations, Sri Lanka — G.C.E. O/L Science 2023, Paper II, Question 9 · https://doenets.lk/pastpapers
3. #3 · Paper II · 6 marks
   Question 6 shows a nephron as a diagram. It asks you to state the kidney's two main functions, to describe what happens during filtration in the glomerulus, to name two substances selectively reabsorbed in the proximal convoluted tubule, and to name the hormone that controls how much water the kidney reabsorbs.
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   (i) Excretion of nitrogenous waste (urea); Osmoregulation (control of water balance in blood). (ii) High blood pressure in the glomerulus forces small molecules (water, glucose, urea, salts) out of the capillaries into the Bowman's capsule. Large molecules (proteins, blood cells) remain in the blood. (iii) Glucose and amino acids (also accept water, Na⁺). (iv) ADH (Antidiuretic hormone / Vasopressin).

   Theory notes on this topic: Reproduction, Excretion and the Skeletal System →

   Source: Department of Examinations, Sri Lanka — G.C.E. O/L Science 2023, Paper II, Question 6 · https://doenets.lk/pastpapers
4. #4 · Paper I (MCQ) · 2 marks
   Two multiple-choice questions. Question 31 asks which gas in a list of four is NOT a greenhouse gas. Question 42 describes copper sulphate solution mixed with iron nails — the blue colour fading and the iron becoming coated — and asks which reaction type that best describes.
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   Q31: Answer B — Nitrogen (N₂). Methane, CO₂ and N₂O are greenhouse gases; nitrogen makes up ~78% of the atmosphere but does NOT trap heat significantly. Q42: Answer C — Displacement. Iron is more reactive than copper, so it displaces copper from copper sulphate solution: Fe + CuSO₄ → FeSO₄ + Cu. The blue colour fades as copper ions are removed; red/brown copper coats the iron nail.

   Theory notes on this topic: Past Papers and Model Answers →

   Source: Department of Examinations, Sri Lanka — G.C.E. O/L Science 2023, Paper I, Question 31 & Question 42 · https://doenets.lk/pastpapers

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