Lebanese Baccalaureate - General Sciences (GS) common mistakes
Lebanese Baccalaureate - General Sciences (GS) · Understand the traps and how to avoid them.
Writing the derivative of a composite function as f'(g(x)) only, dropping the inner factor g'(x) - e.g. giving (sin 2x)' = cos 2x instead of 2cos 2x.
Candidates memorise the table of standard derivatives and apply the outer rule mechanically; the chain rule is learned as a formula to recite rather than a procedure to execute at each step.
What to do instead
Before differentiating, explicitly name the inner function u = g(x) and write the derivative as f'(u)·u' on the script. Practise by differentiating a list of composites (e^{3x}, ln(2x+1), √(x²+1), sin(x²)) and marking the inner factor each time.
Solving an equation by squaring both sides (or multiplying by an expression containing the unknown) and then presenting all resulting roots as solutions, without checking them in the original equation.
Squaring is the obvious way to remove a square root, and candidates treat algebraic manipulation as reversible; the habit of verification is not built into their routine.
What to do instead
Adopt a fixed closing step: substitute every candidate root back into the original equation and write 'verified' or 'rejected' next to it. When the equation contains a square root, also state the domain condition (radicand ≥ 0) before solving.
In probability, writing P(A∪B) = P(A) + P(B) for events that are not disjoint, or P(A∩B) = P(A)·P(B) for events that are not independent, without justifying the hypothesis.
The simple addition and multiplication formulas are the first ones learned and are over-generalised; candidates do not habitually test disjointness or independence before choosing a formula.
What to do instead
Before computing, write one line stating whether the events are disjoint / independent and why (e.g. 'draws without replacement ⇒ not independent'). Use the general formulas P(A∪B) = P(A)+P(B)−P(A∩B) and P(A∩B) = P(A)·P(B|A) as the default, and reduce only after the hypothesis is stated.
Applying the conservation of mechanical energy to a system where friction or an external driving force does work, and then being surprised when the numerical answer disagrees with the kinematics calculation.
Energy conservation is drilled as a universal shortcut, so candidates reach for it automatically; identifying the system and listing the non-conservative forces is a step they skip under time pressure.
What to do instead
Start every energy problem by drawing the system boundary and listing the forces that do work across it. If a non-conservative force acts, write W_nc = ΔE_m explicitly and only set W_nc = 0 after confirming it is zero.
Using scalar magnitudes in vector quantities - adding velocities or forces as plain numbers, or writing a momentum equation without direction - so that a two-dimensional problem is treated as one-dimensional.
Candidates are comfortable with arithmetic on magnitudes and have not internalised that conservation laws are vector equations; the geometry of the situation is often drawn hastily or not at all.
What to do instead
For any collision, force, or field problem, draw axes and decompose each vector into components before writing any equation. Write the conservation law once per axis (p_x conserved, p_y conserved) and solve the components separately.
Mixing units or prefixes in a calculation - inserting a mass in grams into a formula that requires kilograms, or a length in centimetres into an SI expression - and reporting a numerically wrong result with the correct-looking method.
Exam statements deliberately use practical units (g, cm, mL, kJ) and candidates copy the numbers straight into the formula without a conversion step.
What to do instead
Convert every given quantity to SI immediately after reading the statement, writing the converted value beside the original. Carry units through the calculation and check that the final unit matches the quantity asked for.
Writing a full balanced equation for a reaction but then using the masses or moles of the reactants directly in the yield or concentration calculation, ignoring the stoichiometric coefficients.
Candidates see the equation as a formality to display rather than as the source of the mole ratios; the link between coefficients and amounts is not made explicit.
What to do instead
After balancing, write the mole ratio underneath the equation (e.g. 1 mol A reacts with 2 mol B). Build a table of amounts in moles before and after reaction, and convert to mass or concentration only at the end.
Confusing the equivalence point with the neutrality point in an acid–base titration, assuming pH = 7 at equivalence for a weak acid–strong base titration.
The rule 'acid + base gives salt and water, pH 7' is learned early and survives into titration problems; the hydrolysis of the conjugate base is treated as a footnote rather than the determining factor.
What to do instead
At the equivalence point, identify the species actually present in solution and decide whether it hydrolyses. Compute the pH from the conjugate species (e.g. pH of the salt solution using K_b of the conjugate base) rather than assuming neutrality, and state the reasoning in one sentence.
Describing a reaction as 'fast' or 'spontaneous' because it is exothermic, or claiming a catalyst increases the yield of an equilibrium reaction.
Everyday language ('releases heat, so it happens') and the word 'catalyst' are imported into chemistry without the thermodynamic and kinetic distinctions; ΔH, ΔG and activation energy are conflated.
What to do instead
Keep three separate statements in the answer: thermodynamic feasibility (sign of ΔG), kinetics (activation energy, rate), and equilibrium position (catalyst does not shift it). Whenever the words 'spontaneous' or 'catalyst' appear, write which of the three quantities is affected.
In genetics problems, giving the phenotypic ratio for a cross without deriving it from the genotypes, and confusing the ratio of genotypes with the ratio of phenotypes in a dihybrid cross.
The 9:3:3:1 ratio is memorised as a fact and reproduced even when the cross is a test cross or involves linked or sex-linked genes; the Punnett square is filled in mechanically.
What to do instead
Always write the parental genotypes, the gametes each produces, and the Punnett square, then count phenotypes explicitly. Before using 9:3:3:1, state the conditions under which it holds (independent assortment, complete dominance, both parents heterozygous).
Describing a physiological or molecular mechanism as a list of vocabulary terms ('the synapse transmits the message') rather than as an ordered causal sequence with the structures named at each step.
Biology answers are often written from memorised keywords; candidates do not practise turning a labelled diagram into a prose sequence, so the causal links between steps are missing.
What to do instead
Answer mechanism questions as numbered steps, each naming the structure and the change it undergoes (e.g. '1. Action potential arrives at the presynaptic terminal; 2. voltage-gated Ca²⁺ channels open; 3. vesicles fuse…'). Check that each step follows causally from the previous one.
In integrated multi-domain questions, answering each part in isolation - for example computing a numerical value in the physics part and then, in the biology part, quoting a different value or a value in inconsistent units.
The combined paper is attempted part by part, and candidates do not carry forward the result of one part into the next; the shared context of the question is treated as decoration.
What to do instead
Read the whole integrated question before writing. Underline the quantities that recur across parts and record intermediate results in a boxed summary line so the same value and unit are reused. Where a later part depends on an earlier one, state the dependency explicitly ('using the speed found in 1.b').