Valencian Community PAU General Sciences common mistakes
Valencian Community PAU General Sciences · Understand the traps and how to avoid them.
Writing the final numerical answer without stating units, or mixing units mid-calculation (e.g., leaving a force in grams-force but reporting newtons, or giving a wavelength in nm while the formula expects m).
Candidates focus on getting the number and treat units as decoration; the PAU physics and chemistry problems often give data in mixed prefixes (cm, g, mL, kJ) precisely to test conversion discipline.
What to do instead
Adopt a fixed routine: convert every given quantity to SI base units in the first line of working, carry units through each algebraic step, and write the final answer as a number with its unit and correct significant figures. Practise by re-checking the dimensional consistency of each equation before substituting numbers.
Applying conservation laws without checking the conditions under which they hold - e.g., using conservation of mechanical energy on a surface with friction, or momentum conservation in a collision where an external impulse acts.
Students memorise the energy and momentum equations as universal tools and skip the preliminary analysis of forces and non-conservative work; the exam deliberately includes a friction or external-force variant.
What to do instead
Before writing any equation, list the forces acting and classify each as conservative or non-conservative; state explicitly whether mechanical energy is conserved or whether the work-energy theorem with friction is needed. For collisions, verify the system is isolated over the collision interval. Write the justifying sentence in the answer, since it earns method marks.
Drawing or describing a ray diagram or field diagram that violates the sign convention - e.g., placing a virtual image on the same side as the outgoing rays, or drawing electric field lines crossing each other.
The geometric conventions (real/virtual, object/image distances positive or negative) are taught quickly and rarely practised with diagrams; field-line rules are known verbally but not applied when sketching.
What to do instead
Learn one sign convention (the Cartesian one used in Spanish curricula) and apply it consistently: measure all distances from the optical centre or vertex, with the direction of incident light positive. For fields, redraw from the source charges outward and verify lines never cross and are perpendicular to equipotentials. Practise by labelling every distance and sign on the diagram itself.
Balancing a redox equation only by inspection and then failing to balance charge and mass in each half-reaction, or forgetting to add H⁺/OH⁻ and water for the medium specified.
Students are comfortable with simple combustion balancing and assume redox is the same; the ion-electron method requires tracking electrons, charge, oxygen and hydrogen simultaneously, and the acidic versus basic medium changes the additives.
What to do instead
Use the ion-electron method mechanically: split into half-reactions, balance atoms other than O and H, balance O with H₂O, balance H with H⁺, balance charge with electrons, equalise electrons, then add. If the medium is basic, neutralise H⁺ with OH⁻ at the end and simplify water. Verify that total charge is equal on both sides before combining.
Confusing equilibrium shifts with changes in the equilibrium constant - stating that adding a catalyst or changing concentration changes K, or that changing temperature only shifts the position without changing K.
Le Chatelier's principle is learned as a set of directional rules, and the distinction between a shift in position and a change in the constant is subtle; catalysts are wrongly assumed to affect yield.
What to do instead
Memorise the single rule: only temperature changes K (via van 't Hoff/ΔH sign); concentration, pressure and catalysts change the position or the rate but not K. For each perturbation, state which quantity changes, then predict the shift, then state whether K changes. Write the equilibrium expression and substitute to check.
Calculating pH or solubility without accounting for dilution, weak-acid partial dissociation, or the common-ion effect - e.g., using the initial concentration directly as [H⁺] for a weak acid, or ignoring the added ion in a solubility problem.
The simple strong-acid formula is over-practised, and students do not set up the ICE table or the Ka/Ksp expression with the correct equilibrium concentrations; dilution is forgotten when a solution is mixed.
What to do instead
Always compute the post-mixing concentration first (moles divided by total volume). For weak acids and bases, write the ICE table and use Ka or Kb with the approximation checked (dissociation under about 5%). For solubility, write Ksp with the common ion included and solve for the unknown. Show the approximation check explicitly.
Describing genetics problems by phenotype alone without writing genotypes and Punnett squares, and failing to handle linkage, sex-linkage, or the distinction between the observed and expected ratios.
Students jump to ratios they have memorised (3:1, 9:3:3:1) and misapply them to dihybrid crosses with linkage or to sex-linked traits, where the ratios differ and reciprocal crosses are not equivalent.
What to do instead
For every cross, define alleles with clear symbols, write parental genotypes, draw the Punnett square or branch diagram, and only then read off ratios. Check whether genes are linked or sex-linked from the wording, and if so use the appropriate chromosome notation and state that reciprocal crosses differ. Compare observed to expected with a chi-square reasoning step when data are given.
Explaining a physiological or cellular process by naming the structure only - e.g., 'mitochondria produce energy' or 'the kidney filters blood' - without the mechanism, the membrane transport, or the direction of flow.
Revision relies on label-the-diagram recall, and open-response questions require causal chains (gradients, carriers, ATP, osmosis) that students have not rehearsed in prose.
What to do instead
Practise writing each process as a numbered causal chain: what enters, through which membrane protein or channel, driven by what gradient or energy source, what is produced and where it goes. Include the terms that earn marks (active transport, facilitated diffusion, osmotic gradient, chemiosmosis). Test yourself by explaining the process aloud without the diagram.
Interpreting a geological cross-section or stratigraphic column without applying the principles of superposition, cross-cutting relationships and faunal succession in the correct order, so relative ages are assigned wrongly.
The principles are known as definitions but are not applied as an ordered procedure; students date a fold or fault before the beds it affects, or ignore an unconformity.
What to do instead
Work through the section in a fixed sequence: establish the undisturbed sequence by superposition, identify unconformities, then apply cross-cutting relationships (a feature is younger than what it cuts), then use index fossils for relative age, and only then assign absolute ages if radiometric data are given. Write the reasoning for each assigned age.
Designing or evaluating an experiment without identifying the independent, dependent and controlled variables, and without a control group or a statement of how the measurement reduces error.
The scientific-methods section is treated as common sense, but the exam rewards explicit variable identification, replication, and discussion of sources of error; students describe a procedure without these elements.
What to do instead
Use a template for every experimental design answer: hypothesis, independent variable, dependent variable, at least three controlled variables, control group, sample size and replicates, measurement method and instrument precision, and two named sources of error with how to reduce them. Apply the same template when criticising a given design.