Galicia PAU General Sciences common mistakes
Galicia PAU General Sciences · Understand the traps and how to avoid them.
In genetics problems, the student writes the genotype of the parents but never states the gametes produced, then jumps straight to offspring ratios and loses track of which allele came from which parent.
Punnett squares are memorised as a drawing exercise rather than as a probability model, so students skip the step that connects meiosis (segregation and independent assortment) to the square. Without an explicit gamete list, linked genes, sex-linked traits and dihybrid crosses all collapse into guesswork.
What to do instead
Force a written line 'gametes: ...' before drawing any square, listing each gamete with its probability. For dihybrids, derive gametes by FOIL from each parent genotype. For sex-linked traits, always write X and Y chromosomes explicitly (e.g. X^A Y). Check that the gamete probabilities sum to 1 before proceeding.
When asked to explain an osmolarity or transport experiment, the student describes what happened ('the potato got heavier') but never names the mechanism (osmosis, water potential gradient, aquaporins, active transport against a gradient) or states the direction of net water movement relative to solute concentration.
Students conflate observation with explanation because the data table invites description. They also carry a vague 'water goes to the salty side' rule without linking it to water potential or to the difference between tonicity and osmolarity.
What to do instead
Use a fixed three-sentence template for every transport answer: (1) the gradient or potential difference that exists, (2) the membrane protein or process that allows movement, (3) the direction of net flux and the predicted mass/volume change. Practise with a sucrose series and state water potential values in MPa where the syllabus requires it.
In kinematics and dynamics problems, the student substitutes numbers into a formula before choosing a coordinate system and sign convention, producing sign errors in acceleration, velocity and work, and mixing up displacement with distance travelled.
Formulas are treated as plug-in recipes. Students rarely draw a free-body diagram or a velocity-time sketch, so the direction of g, friction or normal force is decided implicitly and inconsistently. Distance versus displacement confusion persists from earlier courses.
What to do instead
Make a diagram mandatory: axes, positive direction, and every force arrow labelled, before any algebra. Solve symbolically first, substitute last, and carry units through the calculation. For any motion with a reversal, compute displacement by integration or area under v-t and distance by summing absolute areas.
In electricity and magnetism, the student applies Ohm's law and Kirchhoff's laws to circuits without identifying series/parallel structure, and applies the right-hand rule to magnetic forces while ignoring the sign of the charge or the direction of conventional current.
Circuit diagrams in exams are often drawn non-rectangularly, so students guess at node structure. The right-hand rule is memorised as a gesture without the underlying vector cross product, so negative charges and reversed currents give wrong directions.
What to do instead
Redraw every circuit as a clean rectangular schematic, label nodes, and reduce resistors step by step, writing the equivalent resistance at each stage. For magnetic force, write F = qv × B explicitly, substitute the sign of q, and verify with a second method (e.g. Fleming's left-hand rule) before committing.
In stoichiometry and titration problems, the student balances the equation but then ignores the mole ratio, or uses the volume in mL directly as if it were litres, and reports an answer with no unit or with the wrong number of significant figures.
The arithmetic is rushed under time pressure, and the mole ratio is seen as a formality rather than the bridge between substances. Unit conversion (mL to L, g to mol via molar mass) is a frequent silent failure, and significant figures are treated as decoration.
What to do instead
Use a fixed four-line layout: balanced equation, moles of known, mole ratio, moles and mass of unknown. Write the conversion factor (1 L = 1000 mL) explicitly each time. Determine the limiting reagent before computing yield. State the final answer with units and significant figures matching the least precise given data.
In acid-base and equilibrium questions, the student calculates pH from concentration without considering whether the acid is strong or weak, or uses the Henderson-Hasselbalch equation at the equivalence point of a titration where it does not apply.
The pH = -log[H+] formula is over-learned and applied indiscriminately. Students do not distinguish the four regions of a titration curve (initial, buffer, equivalence, excess titrant) and so choose the wrong model for each.
What to do instead
Before any calculation, classify the species (strong/weak acid or base, buffer, salt of weak acid) and write which region of the titration curve you are in. For weak acids, set up the Ka expression with an ICE table and the small-x approximation, checking its validity. At equivalence, compute pH from the hydrolysis of the conjugate base, not from Henderson-Hasselbalch.
In organic chemistry, the student names or draws a structure without checking the principal functional group priority, and gives a name with wrong locants because numbering was started from the wrong end of the chain.
IUPAC rules are memorised as a list rather than a decision procedure, so priority order (carboxylic acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine > alkene > alkyne) is applied inconsistently. Numbering is often started at the end nearest a substituent rather than the end giving the lowest locant set to the principal group.
What to do instead
Follow a written checklist: identify the principal group, find the longest chain containing it, number to give the principal group the lowest locant, then apply lowest locant set to substituents, then alphabetical order. Redraw the molecule with numbered carbons before writing the name, and reverse the process to verify.
In geology, the student reads a stratigraphic column or block diagram and states the order of events without applying the principles of superposition, cross-cutting relationships and faunal succession explicitly, or confuses relative dating with absolute (radiometric) dating.
The principles are known as slogans but not used as an ordered reasoning tool. Students also treat any numerical age as absolute dating even when it comes from an index fossil, and they overlook that an intrusion or fault postdates the units it cuts.
What to do instead
For every geological history question, write an ordered list of events and justify each step by naming the principle used (superposition, original horizontality, cross-cutting, inclusions, faunal succession). Separate the answer into 'relative sequence' and 'absolute ages' sections, and state which method supports each numerical claim.
In Earth science questions on plate tectonics or climate, the student describes a phenomenon (e.g. 'earthquakes happen at plate boundaries') without naming the mechanism (ridge push, slab pull, elastic rebound, thermohaline circulation, albedo feedback) or the type of boundary involved.
Answers are written at the level of a news headline because the underlying causal chain was never rehearsed. Students also mix up convergent boundary subtypes (ocean-ocean, ocean-continent, continent-continent) and their associated features.
What to do instead
Build a one-page table of boundary type, relative motion, resulting landforms, seismicity and volcanism, and rehearse it until recall is automatic. For climate questions, always name the forcing, the feedback and the sign of the feedback. Answer in causal chains ('because ... therefore ...'), not single clauses.
In experimental-design and data-analysis questions, the student identifies variables but fails to state a control, a null hypothesis, or how the measurement reduces error, and describes a graph by reading points aloud instead of interpreting the trend and its uncertainty.
Practical work is often done as a recipe, so students have not internalised why a control group or a repeat measurement exists. Graph interpretation is treated as transcription, and uncertainty (error bars, resolution, systematic versus random error) is rarely quantified.
What to do instead
For every design question, write four labelled lines: independent variable, dependent variable, controlled variables with how each is controlled, and control group or blank. State the hypothesis as a testable prediction with a direction. When describing data, give the trend, the quantitative rate or relationship, and the uncertainty, and distinguish systematic from random error with an example of each.