O/L Science 2021 — past paper questions and answers

1. #1 · Paper II · 6 marks
   Question 5 gives the universal-indicator colours and pH values a student obtained for four unnamed solutions. It asks you to sort them into acidic, neutral and alkaline, to write the word equation for the neutralisation between the strongest acid and the alkali, and to 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).

   Theory notes on this topic: Acids, Bases and the pH Scale →

   Source: Department of Examinations, Sri Lanka — G.C.E. O/L Science 2021, Paper II, Question 5 · https://doenets.lk/pastpapers
2. #2 · Paper II · 6 marks
   Question 8 is a structured essay on digestion in the human body. It asks you to cover the organs where digestion happens and the enzymes involved, the role of the villi in the small intestine, and one feature of the circulatory system that carries digested nutrients to the 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.

   Theory notes on this topic: Digestive, Circulatory, Respiratory and Nervous Systems →

   Source: Department of Examinations, Sri Lanka — G.C.E. O/L Science 2021, Paper II, Question 8 · https://doenets.lk/pastpapers
3. #3 · Paper II · 5 marks
   Question 11 opens on methane as the simplest hydrocarbon. It asks you to draw its structural formula, to write the word equation for its complete combustion in air, and to name one useful product of petroleum refining with a use for it.
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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.)

   Theory notes on this topic: Hydrocarbons and Organic Compounds →

   Source: Department of Examinations, Sri Lanka — G.C.E. O/L Science 2021, Paper II, Question 11 · https://doenets.lk/pastpapers
4. #4 · Paper II · 5 marks
   Question 6 gives a sound wave's frequency and its speed in air. It asks you to calculate the wavelength, to distinguish a transverse wave from a longitudinal wave and say which sound is, and to explain why sound cannot travel through a vacuum.
   Show worked solution

   (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.

   Theory notes on this topic: Waves, Sound and Light →

   Source: Department of Examinations, Sri Lanka — G.C.E. O/L Science 2021, Paper II, Question 6 · https://doenets.lk/pastpapers

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