ECAT common mistakes
ECAT · Understand the traps and how to avoid them.
Setting up a projectile or kinematics problem without first fixing a coordinate system and sign convention, then mixing upward-positive and downward-positive values in the same equation.
Candidates plug numbers into memorised formulas immediately, so gravity's sign is chosen ad hoc per line rather than once at the start, and negative displacements get silently dropped.
What to do instead
Before writing any equation, draw the motion and state 'up is positive, g = -9.8 m/s^2'. Substitute all vector quantities with explicit signs and check that the final velocity sign matches the physical direction of travel.
Applying energy conservation to a system where non-conservative forces (friction, applied push) do work, and omitting those work terms from the equation.
The formula mgh + 1/2mv^2 = constant is learned as a universal rule, so students do not scan the problem statement for friction or external forces.
What to do instead
Write the full statement W_nc = change in (KE + PE) every time. If the problem mentions friction, air resistance, or a pulling force, the non-conservative term must appear; solve for it explicitly rather than setting it to zero.
Computing the mole ratio from a balanced equation using the masses given, without first converting every reactant to moles and identifying the limiting reagent.
Stoichiometry practice often uses amounts that are already in moles, so candidates carry the habit of dividing grams directly by coefficients or assuming the first-listed reactant limits.
What to do instead
Always run the same four steps: mass to moles for each reactant, divide by the stoichiometric coefficient, compare quotients to find the limiting reagent, then convert the product moles back to the requested unit. Do this even when the answer 'looks obvious'.
Predicting the sign of an enthalpy or entropy change by intuition about whether the reaction 'feels' energetically favourable, instead of counting bonds broken versus formed and gas moles.
Candidates conflate spontaneity with enthalpy and skip the entropy term, so exothermic reactions are labelled spontaneous regardless of temperature or phase change.
What to do instead
For each reaction, tally bonds broken (endothermic) against bonds formed (exothermic) for the enthalpy sign, and compare total moles of gas on each side for the entropy sign. Then apply Gibbs free energy with the temperature given before concluding spontaneity.
Solving a quadratic or rational equation and keeping every root without testing it in the original equation, letting extraneous solutions through.
The habit of solving by formula and stopping at the discriminant is reinforced by drills that only ask for roots, so the verification step is never practised.
What to do instead
After obtaining roots, substitute each back into the original equation (not the transformed one) and discard any that make a denominator zero or a square root negative. State the rejection explicitly.
Differentiating or integrating a composite function by treating it as a product of separate derivatives, e.g. writing d/dx of sin(3x) as cos(3x).
Chain rule is learned as a formula but not as an operation, so the inner derivative is forgotten under time pressure, especially with nested trigonometric or exponential terms.
What to do instead
Name the outer and inner functions aloud before differentiating, write the outer derivative evaluated at the inner function, then multiply by the inner derivative. For integrals, check the answer by differentiating it back.
Reading a passage question and choosing an option that is true in general knowledge but not stated or implied in the passage.
Candidates answer from background knowledge or personal opinion because they skim the passage for keywords rather than locating the specific supporting sentence.
What to do instead
For every option, point to the line in the passage that supports or contradicts it. If no line supports an option, eliminate it even if it is factually correct. Practise marking the passage as you read.
Choosing an answer based on the sound of a word rather than its grammatical role, e.g. selecting 'effect' for 'affect' or a plural verb after a singular collective subject.
These pairs are memorised as spelling lists without sentence-level practice, so candidates rely on familiarity rather than parsing subject, verb, and object.
What to do instead
Identify the part of speech the blank requires first (noun, verb, adjective, adverb), then choose the word that fits that slot. For subject-verb agreement, underline the true subject and ignore intervening phrases.
Spending disproportionate time on one hard numerical item early in the paper, leaving too few minutes for later questions that are quicker to earn.
Candidates treat every question as equally weighted and are reluctant to leave a problem unsolved, so sunk cost keeps them on a single item.
What to do instead
Adopt a two-pass strategy: first pass answers everything solvable within about a minute, flagging the rest; second pass returns to flagged items with the remaining time. Set a personal per-question cap and enforce it.
Losing marks on unit conversions and significant figures, e.g. leaving a length in centimetres inside an SI formula or reporting more digits than the data justify.
Formulas are practised with clean SI numbers, so conversion is treated as trivial and skipped under exam pressure.
What to do instead
Write the unit next to every substituted number and convert to SI before substitution. Carry units through the calculation to check they cancel to the expected unit, and round only at the final step to the precision of the least precise given value.