Bihar BSEB Matric Science common mistakes
Bihar BSEB Matric Science · Understand the traps and how to avoid them.
While balancing an equation such as Fe + H2O → Fe3O4 + H2, changes the subscript in Fe3O4 to FeO or writes H2O as H2O2 so that the atom counts appear to match.
The candidate treats the chemical formula as adjustable and does not distinguish a coefficient, which changes the amount, from a subscript, which changes the identity of the substance.
What to do instead
Balance only by placing coefficients in front of unchanged formulae; list each element on both sides, recount after every trial, and keep polyatomic ions such as SO4 and NO3 intact as units.
Reads pH 9 as acidic because 9 is larger than 7, or says a solution with pH 7 turns blue litmus red.
The candidate memorises indicator colour changes without linking pH to hydrogen-ion concentration and the logarithmic nature of the pH scale.
What to do instead
Apply the rule pH below 7 acidic, pH 7 neutral, pH above 7 basic; convert pH values to relative H+ concentration, and practise neutralisation examples using both litmus and pH paper.
Predicts that copper will displace zinc from zinc sulphate solution because copper is a 'heavy' metal.
The reactivity series is memorised as a list but is not used as an ordering rule; the candidate uses density or appearance instead of relative reactivity.
What to do instead
Before writing products, locate both metals on the reactivity series; only a metal above can displace one below. Write the ionic equation and check that charge is conserved.
Names the next member of the alkane homologous series but gives C5H10 instead of C5H12, or treats CH3OH and CH3COOH as homologues.
The candidate memorises the phrase 'difference of CH2' without applying the general formula CnH2n+2 and the requirement of the same functional group.
What to do instead
For alkanes use CnH2n+2; for any homologous series verify one-CH2 difference and identical functional group; write molecular formulae by counting carbon and hydrogen atoms explicitly.
In the modern periodic table, arranges elements by increasing atomic mass and then cannot explain why argon comes before potassium even though argon has the greater atomic mass.
Mendeleev's atomic-mass rule is mixed with Moseley's atomic-number rule, so the candidate applies the older criterion to the modern table.
What to do instead
Use atomic number Z as the modern ordering rule: argon Z=18 precedes potassium Z=19. Link group number to valence-shell electrons and period number to the number of occupied shells.
Draws a concave mirror ray diagram with the object between the pole and focus and labels the image as real, inverted and on the same side as the object.
The candidate applies ray rules from memory without tracing the reflected rays, so divergence behind the mirror is mistaken for convergence in front.
What to do instead
For an object inside the focal length, extend the reflected rays backward to locate a virtual, erect and magnified image behind the mirror. Confirm with 1/v + 1/u = 1/f using the sign convention.
Calculates the equivalent resistance of two 6 Ω resistors in parallel as 12 Ω by adding them.
The series formula is overgeneralised, and the candidate ignores the fact that a parallel connection provides additional paths for current.
What to do instead
Use 1/Rp = 1/R1 + 1/R2; for two equal resistors in parallel the equivalent is half of one resistor. Always check that Rp is smaller than the smallest branch resistance.
For a straight current-carrying wire in a magnetic field, uses Fleming's right-hand rule to find the direction of the force, or says magnetic field lines outside a magnet run from south to north.
The right-hand rule for induced current and the left-hand rule for motor force are confused, and the field-direction convention is reversed.
What to do instead
Use Fleming's left-hand rule for force on a current-carrying conductor: thumb force, first finger field, second finger current. Outside a magnet, field lines run from north to south; inside, from south to north.
States that all arteries carry oxygenated blood and all veins carry deoxygenated blood, then marks the pulmonary artery as an error.
The general rule is learned without the pulmonary and umbilical exceptions, so the candidate applies it universally.
What to do instead
Define an artery by direction away from the heart and a vein by direction toward the heart. Trace the pulmonary artery carrying deoxygenated blood to the lungs and the pulmonary vein carrying oxygenated blood to the left atrium.
In a monohybrid cross, writes the F2 phenotypic ratio as 1:1 and the genotypic ratio as 3:1, or says the recessive trait disappears in the F2 generation.
The 3:1 phenotypic ratio and the 1:2:1 genotypic ratio are swapped, and the candidate does not draw a Punnett square to follow the alleles.
What to do instead
Draw gametes for Tt × Tt, fill the four boxes, and count phenotypes as 3 tall : 1 short and genotypes as 1 TT : 2 Tt : 1 tt. Label dominant and recessive phenotypes explicitly.