PAU General Sciences (Aragón) common mistakes
PAU General Sciences (Aragón) · Understand the traps and how to avoid them.
Writing the full electron configuration from scratch and miscounting electrons in the 3d/4s block when asked for the configuration or for the number of unpaired electrons of a transition-metal ion.
Candidates memorise noble-gas shorthand but never practise filling orbitals after ionisation, so they forget that 4s electrons are removed before 3d and lose track of the total electron count.
What to do instead
For every ion, write the neutral atom configuration first, mark the total Z, then subtract electrons explicitly and re-fill using the (n+l) rule, checking that 4s is emptied before 3d; verify unpaired electrons by drawing the orbital boxes for the last subshell.
Balancing redox equations in acidic or basic medium without adding H2O and H+/OH-, producing an equation that balances atoms of the metals but not oxygen or charge.
Candidates balance by inspection from the formula and skip the half-reaction method, so they never add the water and proton terms that the medium requires.
What to do instead
Always split into oxidation and reduction half-reactions, balance O with H2O, H with H+, and charge with electrons; in basic medium add OH- to both sides to neutralise H+, then cancel and re-check both atoms and net charge on each side.
Calculating pH or equilibrium concentrations by assuming the weak acid is fully dissociated, or by neglecting the water contribution, and reporting a pH off by more than one unit.
Candidates default to strong-acid formulas (pH = -log C) because they are simpler, and do not check whether Ka or Kb is small enough to justify the approximation.
What to do instead
Set up the equilibrium table with x, solve x^2/(C-x) = Ka, and only then test whether x/C < 5% to drop the x in the denominator; state the approximation explicitly in the answer and give pH with the correct number of significant figures.
Describing an organic reaction by name only ('it undergoes addition') without drawing the mechanism arrows or the intermediate, and misassigning the major product in Markovnikov/anti-Markovnikov additions.
Candidates learn reaction names as vocabulary and never practise drawing curly arrows, so they cannot decide which carbon bears the positive charge or where the substituent ends up.
What to do instead
For each reaction type, draw the carbocation or radical intermediate and apply the stability order (tertiary > secondary > primary); for peroxide-initiated HBr, draw the radical addition to show the anti-Markovnikov outcome, then name the product with IUPAC rules.
Solving kinematics or dynamics problems by plugging numbers into a memorised formula without drawing the free-body diagram or fixing a sign convention, producing wrong signs for acceleration or friction.
Under time pressure candidates treat physics as formula recall and skip the diagram, so they cannot see that friction opposes motion or that a component of weight acts along the incline.
What to do instead
Before any calculation, draw the free-body diagram, choose and label axes and the positive direction, decompose forces into components, then write Newton's second law per axis; keep signs consistent through the whole solution and check the result's direction and magnitude for plausibility.
Applying conservation of mechanical energy to a system where friction or an inelastic collision occurs, instead of using the work-energy theorem or momentum conservation.
Candidates see 'height' or 'speed' and automatically write mgh = 1/2 mv^2, ignoring that energy is dissipated or that momentum, not kinetic energy, is conserved in the collision.
What to do instead
Classify the process first: if non-conservative forces do work, use W_nc = ΔE; if a collision is involved, conserve momentum and compute the kinetic-energy loss separately; only use pure mechanical-energy conservation when the statement confirms no friction and no inelasticity.
Confusing electric potential with electric field, or adding potentials and fields as scalars when direction matters, when calculating the field at a point due to several charges.
Both quantities are introduced together and candidates memorise V = kq/r and E = kq/r^2 without internalising that potential is scalar and field is a vector.
What to do instead
Compute the field by adding vector components (draw the vectors and use geometry) and the potential by algebraic scalar sum with signs; state units (V vs N/C or V/m) in the answer and use the sign of each charge explicitly.
Writing a chemical or physics answer with no units, or with units that do not follow from the calculation, and losing marks on magnitude or dimension.
Candidates focus on getting the number and treat units as decoration, so they never convert cm to m, g to kg, or check dimensional consistency.
What to do instead
Convert all data to SI at the start, carry units through every step, and finish by checking dimensions of the final expression; write the final answer as number with unit and the correct significant figures.
In genetics problems, confusing genotype and phenotype ratios or failing to distinguish linked genes from independent assortment, giving wrong expected proportions in a dihybrid cross.
Candidates memorise 9:3:3:1 as a fact without deriving it from independent assortment, so they apply it even when genes are linked or when a test cross is asked for.
What to do instead
Always write the parental genotypes, list the gametes with their probabilities, and build the Punnett square or branch diagram; state whether genes assort independently, and if linkage is given, use recombination frequency to weight gametes instead of the 9:3:3:1 ratio.
Describing cell division or metabolic pathways as a list of stage names without locating the key events (crossing over, independent assortment, ATP yield per stage) and mixing up mitosis and meiosis outcomes.
Candidates revise from summaries and never practise tracing chromosomes or counting ATP, so they cannot answer questions about ploidy changes or where each process occurs.
What to do instead
Draw the chromosome complement at each stage for a cell with 2n = 4, mark where crossing over and independent assortment occur, and tabulate ATP/NADH/FADH2 per stage of respiration and photosynthesis; state the final ploidy and number of cells explicitly.