Cambodia Bac II Science common mistakes
Cambodia Bac II Science · Understand the traps and how to avoid them.
On the function-study question, the candidate computes the derivative and then jumps straight to a conclusion such as 'the function increases then decreases', without writing the sign table, without the limits at the boundaries of the domain, and without the asymptotes, even though each of those is a separate scored step.
Most revision is done from answer booklets and tutoring notes that present only the finished solution. The student therefore rehearses the conclusion rather than the chain of justification, and under exam pressure reproduces whatever line of it they happen to remember.
What to do instead
Adopt one fixed skeleton for every function study and use it unchanged: (1) domain, (2) derivative, its sign and its roots, (3) table of variation with the limits written inside the table, (4) asymptotes and intercepts, (5) graph. Practise past papers by drawing the table first, before writing any sentence of conclusion.
Solving an equation containing a square root, a logarithm or a variable denominator, the candidate squares both sides or clears the denominator, then accepts every root obtained, without stating the condition of validity and without testing the candidates against it.
The condition line is mentally filed as optional decoration. Students have memorized the manipulation steps but not the logical reason for the condition, namely that squaring and multiplying can manufacture solutions that were never in the original equation.
What to do instead
Make the condition the first written line of every such solution (radicand greater than or equal to zero, logarithm argument strictly positive, denominator not zero) and keep it visible to the end. Finish with one verification sentence that names the rejected value and says why it fails, because that sentence itself usually carries a mark.
In a numerical physics problem the candidate writes only arithmetic and a bare final number: no formula line, no substitution line, and the data left in the units of the question (cm, g, cm squared, minutes) instead of SI units.
Students equate physics with getting the right number, and question papers often quote data in convenient units, so the habit of converting never forms. The formula line feels like wasted time when the examination is tightly timed.
What to do instead
Use a three-line template on every problem without exception: formula written symbolically, substitution with units shown beside each number, result with unit and the correct power of ten. Convert the data to SI on the line where it is copied from the question, before any calculation, so the conversion cannot be forgotten.
Faced with a circuit containing a battery of e.m.f. E and internal resistance r, the candidate applies U = RI to the whole circuit, or adds the resistance of a parallel branch as though the branches were in series, producing a current that is too small or too large.
Many textbook exercises and classroom demonstrations use ideal batteries, so the internal resistance term never became part of the student's mental model of a circuit. Series and parallel rules are also memorized as formulas rather than as pictures, so they get swapped under time pressure.
What to do instead
Redraw the circuit in the answer book and label each group as series or parallel before computing anything. Always write the loop relation E = U + rI explicitly, reduce the circuit one step at a time, and use the check that an equivalent resistance of a parallel group must be smaller than the smallest resistor in it.
The candidate writes a skeleton equation with unbalanced formulae, then takes the mole ratio straight from those formulae, and assumes that whichever reactant is named first in the question is the one that runs out.
Balancing is treated as a separate formality belonging to a different chapter from mole calculations, and the limiting reagent is memorized as a vocabulary item rather than as a step in a procedure.
What to do instead
Balance and re-count every element before a single number is used. Then compute the moles of each reactant, divide each by its coefficient in the balanced equation, and let the smaller quotient identify the limiting reagent before any product amount is calculated. Write that comparison down; it is usually awarded marks on its own.
In redox questions the candidate assigns oxidation numbers by resemblance to a previously seen exercise rather than by derivation, so oxygen in a peroxide, hydrogen in a hydride, or the total charge of a polyatomic ion comes out wrong, and the electrons lost and gained are never equalized.
Oxidation number is taught as a set of examples rather than as a priority rule, so any unfamiliar species triggers guessing. The half-equation bookkeeping also collapses when the student is working from memory of a similar answer.
What to do instead
Assign numbers from a written priority list every time: element in its free state is 0, a monatomic ion equals its charge, oxygen is minus 2 except in peroxides and with fluorine, hydrogen is plus 1 except in hydrides, and the sum equals the overall charge. Then write the two half-equations separately with the electron counts shown, and multiply them until the electrons cancel.
In a Mendelian genetics problem the candidate writes the phenotype ratio straight away, skipping the parental genotypes, the gametes and the Punnett square, and uses the labels F1 and F2 interchangeably or applies them to the wrong generation.
The ratio 3:1 is the memorable fact, so it is reproduced from memory even when the cross is a test cross, a dihybrid cross or a sex-linked case. Generation notation is rarely written out in class notes, only spoken.
What to do instead
Use an invariant written format: define the allele symbols and say which is dominant, write both parental genotypes, list the gametes each produces, draw the square, then state the genotype ratio and the phenotype ratio as two separate lines, each labelled with its generation. If the ratio memorized does not match the square, trust the square.
Describing a process such as photosynthesis, a stage of mitosis or meiosis, blood circulation or a nerve impulse, the candidate writes a general everyday sentence, for example that the cell divides or that the plant makes food, instead of naming the stages, structures and molecules the question is testing.
Comprehension is checked orally in class, so the student can explain the idea in plain words and recognizes the technical term on the page, but cannot retrieve the term when writing under pressure.
What to do instead
For each process, memorize an ordered list of named steps and structures, with what happens to the chromosomes, membranes or molecules at each step, and answer by walking through the list. Use the exact term even when a plain word would be understood, and add a labelled diagram where the question allows one, since the labels themselves earn marks.
Given the essay question on a poem or a prose extract, the candidate retells the content, saying who did what and what the passage is about, with no quotation from the text, no named figure of speech, and no statement of the author's intention.
Revision for this paper usually consists of memorizing model essays from a booklet and reproducing them, so the student owns a stock summary of famous works but no method for building an argument about an extract placed in front of them.
What to do instead
Practise a four-part answer shape: one sentence stating what the extract conveys, two short quotations each followed by what it shows, the device named with the textbook Khmer term (for example ឧបមា for simile, អតិសយោក្តិ for hyperbole), and a closing sentence on the author's purpose or the social message. Spend a few minutes planning before writing, and proofread the Pali-Sanskrit loanwords, whose spelling is a frequent and avoidable loss.
On the reading comprehension section the candidate answers from general knowledge or from the apparent topic rather than from the passage, and in the writing task opens with a memorized paragraph that does not match the situation, register or required format (formal letter, notice, note, short paragraph).
Reading questions on familiar themes invite the student to answer what they know about the subject instead of what the text says, and memorized opening formulas are taught as a safety net, so they get inserted whether or not they fit.
What to do instead
For comprehension, locate and underline the sentence that answers each question before writing, and phrase the answer so that it would be false if the sentence were removed. For the writing task, first identify the required format and the reader, then write a single opening line that names the purpose, and keep the memorized phrases only for greetings and closings where the format genuinely calls for them.