O/L Science 2022 — past paper questions and answers

1. #1 · Paper II · 5 marks
   Question 3 asks you to explain from particle theory why a gas fills its whole container while a liquid takes the container's shape but keeps a fixed volume, and to include a labelled particle diagram for each state.
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   In a GAS: particles are far apart, move rapidly and randomly, exert negligible attractive forces on each other. They spread to fill all available space — hence fill the container fully. In a LIQUID: particles are close together with moderate attractive forces. They can slide past each other (flow) so the liquid conforms to the container shape. The volume is fixed because particles cannot easily escape the attraction of neighbours. Diagram: Gas — particles spread throughout box; Liquid — particles close together, no fixed arrangement but with a flat surface at the top.

   Theory notes on this topic: Matter, Elements and Compounds →

   Source: Department of Examinations, Sri Lanka — G.C.E. O/L Science 2022, Paper II, Question 3 · https://doenets.lk/pastpapers
2. #2 · Paper II · 6 marks
   Question 7 gives a car of stated mass accelerating from rest to a stated speed in a stated time on a straight road. It asks you to calculate the acceleration, then the resultant force while stating the formula used, and to state Newton's First Law with one everyday example.
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   (i) a = (v − u) / t = (30 − 0) / 12 = 2.5 m/s² (ii) F = ma = 1 200 × 2.5 = 3 000 N (iii) An object remains at rest or in uniform motion in a straight line unless acted upon by a resultant (unbalanced) force. Example: a book lying still on a table stays still because the normal reaction and gravity are balanced.

   Theory notes on this topic: Motion, Force and Newton's Laws →

   Source: Department of Examinations, Sri Lanka — G.C.E. O/L Science 2022, Paper II, Question 7 · https://doenets.lk/pastpapers
3. #3 · Paper II · 5 marks
   Question 4 states that magnesium ribbon burns in oxygen to form magnesium oxide. It asks you to write the balanced chemical equation, to classify the reaction among combination, decomposition, displacement and double displacement, and to name two signs that a chemical reaction has occurred in that experiment.
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   (i) 2Mg + O₂ → 2MgO (ii) Combination (synthesis) reaction — two substances combine to form one product. (iii) Bright white light / flame produced; white ash (MgO) forms (any two of: light, heat, new substance formed, colour change accepted).

   Theory notes on this topic: Chemical Compounds and Equations →

   Source: Department of Examinations, Sri Lanka — G.C.E. O/L Science 2022, Paper II, Question 4 · https://doenets.lk/pastpapers
4. #4 · Paper II · 6 marks
   Question 8 states that in pea plants the tall allele is dominant over the short allele. It asks you to write the genotype and phenotype of a homozygous tall plant, to use a Punnett square to predict the tall-to-short ratio from crossing two heterozygous tall plants, and to define dominant and recessive allele.
   Show worked solution

   (i) Genotype: TT. Phenotype: tall. (ii) Cross: Tt × Tt → TT : Tt : tt = 1:2:1. Phenotype ratio: 3 tall : 1 short. (iii) Dominant allele: the allele that is expressed in the phenotype even when only one copy is present (heterozygous). Recessive allele: only expressed in the phenotype when two copies are present (homozygous recessive).

   Theory notes on this topic: DNA, Genes and Principles of Heredity →

   Source: Department of Examinations, Sri Lanka — G.C.E. O/L Science 2022, Paper II, Question 8 · https://doenets.lk/pastpapers

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