Natural Sciences: Biology, Chemistry, and Physics
The Natural Sciences paper in ESSLCE draws on the Grades 9–12 Ethiopian curriculum in biology, chemistry, and physics. It rewards candidates who can move between a remembered rule and an unfamiliar context: a diagram of the nephron, a titration table, or a circuit with two resistors. Calculations are usually short, but they test units, signs, and limiting conditions.
Biology: information flow and systems
- Genetics: a monohybrid cross of two heterozygotes gives a 3:1 phenotype ratio; a test cross with a homozygous recessive identifies an unknown genotype. Work in probabilities, not 'what must happen'.
- Physiology: follow the path - nephron (filtration → reabsorption → secretion), heart (right side to lungs, left side to body), synapse (electrical → chemical → electrical).
- Ecology: energy transfer between trophic levels is about 10%; pyramids of energy are always upright, while pyramids of numbers need not be.
Chemistry: amount, structure, change
- The mole is the hinge: n = m/M, n = V/22.4 at STP for gases, n = CV for solutions.
- Balance equations before using ratios. Periodic trends: atomic radius falls across a period and rises down a group; ionisation energy does the reverse.
- Organic families are identified by functional group, not by name alone.
Physics: laws applied to a situation
- Use the SUVAT equations only for constant acceleration; choose a sign direction and keep it.
- Ohm's law and power: V = IR, P = VI = I²R. In series, current is common; in parallel, voltage is common.
Worked example
0.50 mol of calcium carbonate reacts completely with excess hydrochloric acid. What volume of carbon dioxide is produced at STP?
CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O. The equation shows 1 mol CaCO₃ gives 1 mol CO₂, so n(CO₂) = 0.50 mol. At STP, V = n × 22.4 L mol⁻¹ = 0.50 × 22.4 = 11.2 L. The reasoning is the balanced ratio, not the mass of acid.
Traps
- Using 24 L mol⁻¹ (room temperature) when the question says STP.
- Forgetting that the limiting reagent controls the product.
- Mixing up genotype and phenotype ratios.
- In physics, dropping the sign of g or treating parallel resistors as series.
- In biology, saying energy is 'recycled' - matter cycles, energy flows.