PAU General Sciences (Castilla y León) common mistakes
PAU General Sciences (Castilla y León) · Understand the traps and how to avoid them.
When converting units in physics problems (e.g., km/h to m/s), the candidate multiplies by 3.6 instead of dividing, or forgets to convert hours to seconds before calculating acceleration.
The conversion factor is memorized as a rule without understanding its derivation; under time pressure, the direction is reversed. Also, mixed units in a formula (e.g., speed in km/h, time in minutes) are not harmonized before substitution.
What to do instead
Always write the conversion as a fraction with units: (100 km/h) × (1000 m / 1 km) × (1 h / 3600 s) = 27.8 m/s. Before any calculation, list all given quantities with their units and convert them to SI (m, s, kg) explicitly. Practice 5 mixed-unit problems daily until the process is automatic.
In chemistry, the candidate balances the molecular equation but forgets to balance the ionic charges in a redox reaction, or fails to identify the oxidizing and reducing agents correctly.
The candidate focuses only on atom balance and overlooks charge balance, especially in acidic or basic medium. The terms 'oxidizing agent' and 'reducing agent' are confused because the agent is the species that is reduced/oxidized, not the one that does the reducing/oxidizing.
What to do instead
Use the half-reaction method: split into oxidation and reduction half-reactions, balance atoms other than O and H, balance O with H2O, balance H with H+, balance charge with electrons, then combine and cancel. Always verify that total charge is equal on both sides. Memorize: the oxidizing agent is reduced (gains electrons); the reducing agent is oxidized (loses electrons).
When solving genetics problems, the candidate assumes independent assortment for genes that are actually linked, or misinterprets the results of a test cross by not distinguishing between parental and recombinant phenotypes.
The candidate applies Mendel's laws universally without checking whether genes are on the same chromosome. In dihybrid cross problems, the 9:3:3:1 ratio is used even when the problem states or implies linkage. Test cross data are misread because the candidate does not recall that offspring phenotypes directly reflect gamete frequencies.
What to do instead
Before solving, ask: are the genes on different chromosomes or far apart? If linkage is mentioned or the observed ratio deviates from 9:3:3:1, use recombination frequencies. In a test cross, the offspring phenotypes correspond exactly to the gametes produced by the heterozygous parent; the most frequent classes are parental, the least frequent are recombinant. Draw a map if recombination frequencies are given.
In geology, the candidate misidentifies the relative ages of rock layers when an intrusion or fault cuts across them, applying the principle of superposition incorrectly.
The candidate memorizes superposition as 'older on bottom' but forgets that cross-cutting relationships, inclusions, and unconformities override simple superposition. Intrusions are younger than the rocks they cut, but candidates often label the intrusion as older because it appears 'inside' older layers.
What to do instead
Apply the principles in order: superposition (undisturbed sedimentary layers: oldest at bottom), original horizontality, cross-cutting relationships (a fault or intrusion is younger than the rocks it cuts), inclusions (fragments inside a rock are older than the host), and unconformities (a gap in time). For each diagram, list events as a sequence (deposition, folding, intrusion, erosion, etc.) and check that the order is logically consistent.
When calculating the pH of a weak acid solution, the candidate uses the strong acid formula [H+] = c, ignoring the acid dissociation constant (Ka) and the equilibrium expression.
The candidate overgeneralizes the strong acid approximation. Weak acids dissociate only partially, so [H+] is much less than the initial concentration. The candidate also forgets to check whether the approximation (x << c) is valid, leading to large errors when Ka is relatively high or concentration is low.
What to do instead
For a weak acid HA with concentration c and constant Ka, set up the equilibrium: Ka = x^2 / (c - x). Solve for x = [H+]. If c/Ka > 100, use the approximation x ≈ √(Ka·c); otherwise solve the quadratic. Then pH = -log[H+]. Always state the approximation and verify its validity. Practice with acetic acid (Ka = 1.8×10^-5) at various concentrations.
In physics, the candidate confuses mass and weight, using kilograms in force calculations or newtons in mass calculations, and fails to distinguish between the two in free-body diagrams.
Everyday language uses 'weight' for mass (e.g., 'I weigh 70 kg'). The candidate carries this into physics problems, especially when calculating normal force, tension, or gravitational force. Free-body diagrams are drawn with 'kg' labels instead of 'N'.
What to do instead
Memorize: mass (kg) is a scalar measure of inertia; weight (N) is the force of gravity, W = m·g. In every free-body diagram, label forces in newtons and include the weight vector pointing downward with magnitude m·g. When a problem gives mass in kg, immediately convert to weight if a force is needed. Do a set of problems mixing mass and weight to build the habit.
When interpreting a speed-time graph, the candidate calculates distance as the slope of the line instead of the area under the curve, or confuses acceleration with velocity.
The candidate memorizes 'slope = speed' from distance-time graphs and transfers it incorrectly to speed-time graphs. The geometric meaning of area under the curve is not internalized, so the candidate uses the wrong graphical operation.
What to do instead
Create a reference card: on a distance-time graph, slope = velocity; on a velocity-time graph, slope = acceleration and area under the curve = displacement. For each graph, first identify the axes, then decide what the slope and area represent. Practice by calculating both slope and area for the same graph and checking units (slope of v-t gives m/s², area gives m).
In biology, the candidate describes the process of photosynthesis and cellular respiration as exact reverses of each other, ignoring that they occur in different organelles, use different electron carriers, and that respiration is not simply the reverse of photosynthesis.
The simplified summary 'photosynthesis stores energy, respiration releases it' leads to the misconception that the reactions are reversible. The candidate overlooks the role of NADPH vs NADH, the location of ATP synthase, and the fact that respiration involves glycolysis, the Krebs cycle, and oxidative phosphorylation, whereas photosynthesis involves light reactions and the Calvin cycle.
What to do instead
Build a comparison table with columns: location, inputs, outputs, electron carrier, ATP-producing step, and energy source. Emphasize that photosynthesis occurs in chloroplasts (thylakoid and stroma) and respiration in mitochondria (matrix and inner membrane). Note that the overall equations look reversed, but the pathways and intermediates are different. Use diagrams to trace carbon and electrons separately.
When solving stoichiometry problems, the candidate uses the mass of the limiting reactant directly as the mass of product, without converting to moles and using the mole ratio.
The candidate skips the mole concept and tries to solve by inspection or by simple proportion of masses. This works only when the molar masses happen to be equal, so it fails in most cases. The candidate also forgets to identify the limiting reactant when two reactant amounts are given.
What to do instead
Follow a fixed four-step method: (1) convert all given masses to moles using n = m/M; (2) divide by the stoichiometric coefficients to find the limiting reactant; (3) use the mole ratio from the balanced equation to find moles of product; (4) convert product moles to mass. Always show the units and the balanced equation. Practice with problems that give two reactant amounts to reinforce limiting reactant identification.
In Earth and environmental science, the candidate confuses the greenhouse effect with ozone depletion, attributing global warming directly to the ozone hole or blaming ozone depletion on CO2 emissions.
Both are atmospheric environmental issues often discussed together in the media, so the candidate conflates them. The mechanisms are distinct: the greenhouse effect involves infrared absorption by CO2, CH4, and H2O, while ozone depletion involves chlorine and bromine radicals from CFCs breaking down O3 in the stratosphere.
What to do instead
Create a clear contrast: greenhouse gases (CO2, CH4, N2O, water vapor) trap outgoing infrared radiation, warming the troposphere; ozone-depleting substances (CFCs, halons) release Cl/Br radicals in the stratosphere that catalytically destroy O3. Note that the Montreal Protocol addressed ozone depletion, while the Kyoto Protocol and Paris Agreement address greenhouse gases. Use a two-column table to compare causes, locations, and effects.