OAT Natural Sciences common mistakes
OAT Natural Sciences · Understand the traps and how to avoid them.
Solving a Mendelian or pedigree problem by instinct ("it skips a generation, so it must be recessive") instead of assigning alleles and working the cross out on paper.
Genetics items look familiar from introductory biology, so candidates pattern-match to memorised examples and stop tracking which individual is which. The OAT mixes in carriers, sex-linked traits, and incomplete dominance, where the intuitive answer is wrong.
What to do instead
For every genetics item, write the allele key first (e.g., A = affected, a = unaffected), then label each individual's known and unknown genotype above the pedigree or in the cross. Only accept an answer that follows from the genotypes you wrote, not from the phenotype pattern.
Confusing the roles of the sodium-potassium pump and the Na+/K+ gradients in neuronal action potentials, or placing depolarisation and repolarisation in the wrong order.
Textbook summaries compress the sequence into a few words, and candidates memorise "Na+ in, K+ out" without linking each ion to a specific channel and membrane potential change. Under time pressure the steps get shuffled.
What to do instead
Draw the action potential curve from memory with four labelled points: resting (-70 mV, K+ leak dominates), threshold, peak (+30 mV, voltage-gated Na+ channels inactivated), and hyperpolarisation (voltage-gated K+ channels slow to close). Annotate which pump restores gradients afterwards.
Treating the Calvin cycle and the light reactions as one continuous process, so questions about where ATP and NADPH are consumed, or where O2 is released, get answered with the wrong stage.
Both stages are taught together and share vocabulary ("photosynthesis"), so candidates blur the location (thylakoid membrane vs stroma) and the inputs and outputs of each.
What to do instead
Build a two-column table: light reactions (thylakoid membrane; inputs H2O, light, ADP, NADP+; outputs O2, ATP, NADPH) versus Calvin cycle (stroma; inputs CO2, ATP, NADPH; outputs G3P/sugar, ADP, NADP+). Recite the table before answering any photosynthesis item.
Calculating pH or pOH from a weak acid using the strong-acid formula [H+] = concentration, without setting up an ICE table or using Ka.
Strong-acid problems dominate practice sets, and the one-line formula is fast and memorable. Candidates apply it automatically even when the problem gives a Ka value or says "weak acid".
What to do instead
Before computing, check for a Ka or Kb value in the stem. If present, write the dissociation equation, set up an ICE table with x, and solve Ka = x^2/(C - x), approximating only when C/Ka > 100. Then convert to pH at the end.
Applying Le Chatelier's principle to changes in pressure or volume for reactions where the number of moles of gas is equal on both sides, or forgetting that adding an inert gas at constant volume changes nothing.
Candidates learn the shortcut "pressure up, shift to fewer moles" and apply it mechanically, without counting gas moles or checking whether the change actually alters partial pressures.
What to do instead
For every equilibrium shift question, first count gas moles on each side (Δn). If Δn = 0, pressure and volume changes do not shift the position. If the change is an inert gas at constant volume, state explicitly that partial pressures are unchanged, so no shift occurs.
Reading a titration curve or buffer problem and using the equivalence-point pH as if it were the pKa, or assuming the half-equivalence point is the equivalence point.
The two points sit on the same curve and both are marked on typical graphs. Candidates remember "the flat part matters" without attaching the correct quantity (pH = pKa at half-equivalence) to the correct location.
What to do instead
Label every curve before answering: mark equivalence point(s) at the steepest inflection, and mark half-equivalence at half that volume, noting pH = pKa there for a weak acid. For buffer items, confirm the Henderson-Hasselbalch ratio uses the half-equivalence condition.
Assigning R/S configuration by rotating a drawn molecule mentally without ranking the four substituents by CIP rules first, leading to inverted answers.
Candidates rush to the visual rotation step and skip the priority ranking, or misapply the rule that a lowest-priority group pointing toward the viewer reverses the result.
What to do instead
Follow a fixed sequence: rank substituents by atomic number (with tie-breakers by mass number and then by substituent atoms), place the lowest priority away from the viewer, then read 1→2→3. If you had to swap groups to do this, invert the result once. Write the ranking down.
Treating all alcohols and alkyl halides as equally reactive in substitution and elimination, ignoring substrate class (methyl/primary/secondary/tertiary) and the strength of the nucleophile or base.
Candidates memorise "SN2 is backside attack, SN1 has a carbocation" as facts, but do not connect the mechanism to the substrate and conditions, so they pick a mechanism that cannot occur for that starting material.
What to do instead
Before answering, classify the substrate and the reagent: strong nucleophile plus methyl/primary carbon points to SN2; weak nucleophile plus tertiary carbon points to SN1; strong bulky base points to E2; heat plus weak base points to E1. State the classification in one line before choosing.
Predicting aromaticity by counting double bonds or recognising a benzene ring, without checking Hückel's rule (4n + 2 π electrons) and planarity in the full ring system.
Benzene is the dominant example in coursework, so candidates equate "has alternating double bonds" with aromatic and misclassify charged or heteroatom rings such as cyclopentadienyl anion or pyridine.
What to do instead
For each ring, count all π electrons including lone pairs that participate, check that the count fits 4n + 2 (2, 6, 10, ...), and confirm the system is cyclic, planar, and fully conjugated. If any condition fails, label it antiaromatic or non-aromatic explicitly.
Skipping the units and significant figures in general chemistry calculations, reporting a molarity or enthalpy with the wrong power of ten or the wrong sign.
The OAT is timed and calculator use is restricted, so candidates do mental arithmetic and drop the unit tracking that would have caught a factor-of-1000 error or a sign flip in an exothermic reaction.
What to do instead
Carry units through every step and cancel them explicitly; for thermochemistry, write the sign convention (ΔH negative = exothermic) next to the value. Estimate the answer's order of magnitude before computing so a slipped decimal is caught immediately.