Punjab PSEB Matric Science common mistakes
Punjab PSEB Matric Science · Understand the traps and how to avoid them.
Balancing a chemical equation by changing subscripts in a formula-for example, writing H2 + O2 → H2O2 to make oxygen atoms equal-instead of placing coefficients in front of the correct formulas.
Candidates treat the equation as an atom-count puzzle and alter the formula to force equality, forgetting that a subscript is part of the substance’s identity.
What to do instead
Balance only with coefficients. Write a tally table for each element on both sides, change one coefficient at a time, and recheck. Never touch a subscript. Practise with H2 + O2 → H2O, Fe + O2 → Fe2O3 and combustion of CH4.
Reading pH backwards or equating acid strength with concentration-for example, saying pH 14 is the strongest acid, or that a dilute solution of a strong acid must have pH 1.
Everyday words such as “strong” and “concentrated” are used interchangeably, and the logarithmic pH scale is memorised as a list rather than as an H+ concentration scale.
What to do instead
Anchor the scale: pH below 7 is acidic, and lower pH means more H+ ions. Define strength as complete or partial ionisation in water and concentration as amount per volume. Compare HCl (strong, can be dilute) with acetic acid (weak, can be concentrated).
Drawing stomata, nephron, heart or reflex-arc diagrams as unlabelled pictures, or labelling guard cells as the stomatal pore and the nephron’s Bowman’s capsule as the collecting duct.
Diagrams are memorised as shapes rather than as structures with functions, so labels are attached by position instead of by what each part does.
What to do instead
Learn each diagram through function: guard cells open and close the pore; Bowman’s capsule filters blood; the collecting duct reabsorbs water. Practise blank diagrams and write one function beside every label. Draw arrows for gas or filtrate movement.
Confusing reproduction terms-saying the placenta mixes the mother’s and embryo’s blood, calling the umbilical cord the site of exchange, or mislabelling ovary and ovule in a flower diagram.
The terms appear together in textbook diagrams and are remembered as a cluster without separate definitions.
What to do instead
Write one-line definitions: placenta is the exchange surface; umbilical cord is the conduit; fetal and maternal blood do not mix. In flowers, ovary contains ovules and becomes fruit, while ovules become seeds. Label a blank flower diagram from memory.
Using a Punnett square without writing parental genotypes and gametes, then reporting a 3:1 ratio as the genotype ratio or a 1:2:1 ratio as the phenotype ratio.
The square is drawn as a symbol pattern and the two ratios are memorised as a pair without linking them to genotype and phenotype.
What to do instead
Always state parental genotypes, place gametes across the top and side, fill the square, then count genotypes first and phenotypes second. For a monohybrid cross, state clearly: genotype ratio 1:2:1, phenotype ratio 3:1.
In mirror and lens numericals, substituting u as positive for a real object or taking f as positive for a concave mirror, then accepting a positive image distance without checking whether the image should be real or virtual.
Several formula variants are memorised, but the Cartesian sign convention is skipped under time pressure.
What to do instead
At the top of every solution write: distances measured from pole or optical centre, against incident light negative. Use 1/v + 1/u = 1/f with u negative; concave mirror f negative; convex lens f positive. Check the sign against a quick ray diagram.
Correctly naming myopia or hypermetropia but giving the wrong correcting lens, or drawing the correction with the lens placed behind the eye and the image still focused in front of the retina.
The defect is remembered as a word, not as where the image forms relative to the retina and which lens diverges or converges light.
What to do instead
Link defect to image position: myopia-image in front of retina-concave lens diverges light; hypermetropia-image behind retina-convex lens converges light. Draw the lens in front of the eye and show the final image on the retina.
In series-parallel numericals, dividing the supply voltage by a single resistance in a mixed circuit, or adding resistances in parallel as R1 + R2 instead of using 1/R = 1/R1 + 1/R2.
Candidates pattern-match to the last formula seen and do not redraw the circuit into simple series and parallel parts.
What to do instead
Redraw and label the circuit, reduce it step by step to an equivalent resistance, write R_eq before calculating current, then work back for branch currents. Convert mA to A and cm to m before substitution.
Applying Fleming’s right-hand rule to the force on a current-carrying conductor in a magnetic field, or drawing magnetic field lines that cross each other or run from south to north outside a magnet.
The left-hand and right-hand rules are learned together and the motor/generator contexts are not firmly attached to each rule.
What to do instead
Anchor the rules by device: motor effect-force on conductor-Fleming’s left-hand rule; generator effect-induced current-Fleming’s right-hand rule. Draw field lines from north to south outside the magnet, never crossing, and label the force, field and current directions.
Drawing food-chain arrows from predator to prey, or confusing ozone-layer depletion with the greenhouse effect and naming carbon dioxide as the cause of the ozone hole.
Arrows are read as “who eats whom” rather than as energy flow, and both environmental issues are grouped as “pollution” without separate mechanisms.
What to do instead
Draw arrows from the organism eaten to the eater, so energy flows to the higher trophic level. Separate the issues: ozone depletion is caused mainly by CFCs, while the greenhouse effect is enhanced by CO2, methane and water vapour. Practise a four-trophic-level chain with labels.