Science 2021

1. #1 · Paper II · 6 marks
   A student tests four solutions A, B, C and D with universal indicator. The colours obtained are: A — red (pH 2), B — green (pH 7), C — blue (pH 11), D — orange (pH 4). (i) Identify the acidic, neutral and alkaline solutions. (ii) Write the word equation for the neutralisation reaction between solutions A and C. (iii) Name ONE everyday use of an alkali.
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   (i) Acidic: A (pH 2) and D (pH 4); Neutral: B (pH 7); Alkaline: C (pH 11). (ii) Acid + Alkali → Salt + Water (i.e. hydrochloric acid + sodium hydroxide → sodium chloride + water). (iii) Toothpaste / baking soda for cooking / lime to neutralise acidic soil (any valid example accepted).
   Source: NIE O/L Science 2021 Paper II Q5
2. #2 · Paper II · 6 marks
   Describe how food is digested in the human digestive system. In your answer include: (i) The organs where digestion occurs and the enzymes involved. (ii) The role of villi in the small intestine. (iii) One feature of the circulatory system that transports digested nutrients to body cells.
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   (i) Mouth — salivary amylase breaks starch to maltose. Stomach — pepsin (protease) digests proteins in acidic conditions. Small intestine — pancreatic amylase, lipase (fats → fatty acids + glycerol), protease complete digestion. (ii) Villi greatly increase the surface area of the small intestine for absorption. They have a rich blood capillary network and a lacteal (lymph vessel) for fat absorption. (iii) Blood plasma carries dissolved nutrients (glucose, amino acids) via blood vessels to all body cells.
   Source: NIE O/L Science 2021 Paper II Q8
3. #3 · Paper II · 5 marks
   Methane (CH₄) is the simplest hydrocarbon. (i) Draw the structural formula of methane. (ii) Write the word equation for the complete combustion of methane in air. (iii) Name ONE useful product obtained from petroleum refining and state one use of that product.
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   (i) H−C−H with four H atoms (tetrahedral around C). (ii) Methane + Oxygen → Carbon dioxide + Water CH₄ + 2O₂ → CO₂ + 2H₂O (iii) Petrol (gasoline) — used as fuel in motor vehicles. (Accept: kerosene / diesel / LPG with correct use.)
   Source: NIE O/L Science 2021 Paper II Q11
4. #4 · Paper II · 5 marks
   A sound wave has a frequency of 440 Hz and travels at 340 m/s in air. (i) Calculate the wavelength of the sound wave. (ii) Distinguish between a transverse wave and a longitudinal wave. State which type sound is. (iii) Explain why sound cannot travel through a vacuum.
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   (i) λ = v / f = 340 / 440 ≈ 0.77 m (ii) Transverse wave — particles vibrate perpendicular to the direction of wave travel (e.g. light). Longitudinal wave — particles vibrate parallel to the direction of travel (e.g. sound). Sound is longitudinal. (iii) Sound requires a material medium (particles) to compress and rarefy. A vacuum has no particles, so sound cannot be transmitted.
   Source: NIE O/L Science 2021 Paper II Q6

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Past papers are reproduced from the Department of Examinations, Sri Lanka, for educational use. The 2026 model paper and its marking scheme are original Idasara Academy work.

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