RBSE Praveshika Science common mistakes
RBSE Praveshika Science · Understand the traps and how to avoid them.
Writes formulae such as CaOH₂, AlSO₄, NaCO₃ or Ca₂O₂ by criss-crossing valencies without putting brackets round polyatomic ions or reducing subscripts to the simplest ratio.
The criss-cross method is memorised as a ritual rather than as a way of building a neutral particle. Students never check whether the result is charge-balanced, and they treat a polyatomic ion like (SO₄) or (OH) as separate atoms.
What to do instead
Add two compulsory checks after criss-crossing. First, reduce the subscripts to the lowest whole-number ratio (Ca₂O₂ becomes CaO). Second, put brackets round any polyatomic ion used more than once (Ca(OH)₂, Al₂(SO₄)₃). Then verify the charge balance out loud: total positive charge equals total negative charge.
Substitutes mass in grams and speed in km/h straight into F = ma, v = u + at or s = ut + ½at², and reports answers such as '20 g m/s²' or '5000 N' for a 500 g mass.
Numericals are treated as formula-plugging exercises. The SI requirement (kg, m, s) is never linked to the definition of the newton, so unit conversion feels optional rather than part of the calculation.
What to do instead
Before substituting, write a 'given' line that converts every quantity to SI with its unit shown (250 g = 0.25 kg; 72 km/h = 20 m/s). Carry the units through the working. If the final unit is not kg·m/s², the conversion is incomplete. Do this even when the question gives 'nice' numbers.
Puts a positive value for f in the concave mirror or concave lens formula, or takes the object distance u as positive, and then 'corrects' the answer by silently dropping the minus sign.
The sign convention is learned as a list of signs to memorise, not as a coordinate system with origin at the pole or optical centre and distances measured along the incident light direction.
What to do instead
Draw the principal axis once, mark the pole and the direction of incident light. Write a small sign table for u, v and f before substituting. Interpret the sign of v in words: negative means a real image in front of the mirror, positive means a virtual image behind it. Never delete a sign; explain it.
Adds resistances in parallel directly (R = R₁ + R₂) or computes 1/R and forgets to invert, so the equivalent resistance comes out larger than the smallest branch resistor.
The series rule is over-learned and rehearsed, while the reciprocal in the parallel formula is treated as decoration rather than as the physics of shared voltage.
What to do instead
For parallel combinations, compute each 1/R, add them, then invert as a clearly written separate step. Sanity-check the answer: the equivalent resistance must be smaller than the smallest branch. Anchor the rule with one sentence: same voltage across parallel branches, same current through series elements.
'Balances' an equation by changing subscripts, writing H₂ + O₂ → H₂O₂ or leaving Fe + O₂ → Fe₂O₃ as if it were balanced, instead of adding coefficients.
The task is understood as 'make the numbers on both sides equal', not 'conserve atoms of each element while keeping every compound chemically unchanged'.
What to do instead
Balance only with coefficients. Keep an atom tally table (element | LHS | RHS) and adjust coefficients until every row matches. Then re-check that no formula was altered, and finish with state symbols and conditions if the question asks for them.
Reads pH in the wrong direction - 'pH 9 is more acidic than pH 5' - and treats acid strength and acid concentration as the same thing.
pH is taught as a colour-scale number without the idea that it is a negative logarithm of hydrogen-ion concentration, so 'bigger number, bigger effect' feels natural.
What to do instead
Anchor three points: pH 7 neutral, below 7 acidic, above 7 basic; the smaller the pH, the greater the H⁺ concentration. Separate strength (degree of ionisation) from concentration (amount per litre) using HCl as a strong but possibly dilute acid and CH₃COOH as a weak but possibly concentrated acid.
Writes displacement reactions that cannot happen, such as Cu + ZnSO₄ → CuSO₄ + Zn, or predicts hydrogen gas from copper plus dilute hydrochloric acid.
The reactivity series is memorised as a list of names for recall, not used as a decision rule, and every metal-plus-acid case is assumed to give hydrogen.
What to do instead
Before writing any displacement, place both metals on the series (K Na Ca Mg Al Zn Fe Pb H Cu Hg Ag Au) and apply the rule: a metal above displaces one below; only metals above hydrogen release H₂ from dilute acid. Then add one line stating the reason for the answer.
Swaps xylem and phloem (says xylem carries food downwards) and describes stomata as the site of photosynthesis rather than as pores for gas exchange and water loss.
Functions are revised as isolated facts, with no link between structure, direction of transport and the plant's overall water and food economy.
What to do instead
Learn them as contrasting pairs: xylem carries water and minerals from roots upwards, is dead and needs no energy; phloem carries sucrose and amino acids from leaves to all parts, is living and uses energy. Sketch a leaf cross-section and mark where gas exchange, transpiration and food loading each occur.
Uses the F₂ ratios interchangeably, answering '3 tall : 1 dwarf' when the question asks for genotypes, or writing 1:2:1 for phenotypes in a monohybrid cross.
Only the visible ratio is memorised, and the Punnett square is copied from the book rather than read, so genotype (genetic makeup) and phenotype (appearance) are never separated.
What to do instead
Always draw the Punnett square, then count twice: once by genotype (TT : Tt : tt = 1:2:1) and once by phenotype (tall : dwarf = 3:1). Underline that Tt looks tall but is genetically different. Treat the test-cross 1:1 result as a separate fact to be learned.
Applies the 10% law backwards, claiming that energy increases at higher trophic levels, or saying the top consumer receives 10% of the producer's original energy.
'10%' is memorised as a bare number without the direction of transfer, so the accompanying 90% loss as heat, respiration and excretion is never attached to it.
What to do instead
State the rule with its direction every time: only about 10% of the energy at one trophic level passes to the next, and about 90% is lost. Practise with a worked pyramid (10,000 J → 1,000 J → 100 J → 10 J) and use it to explain why food chains rarely have more than four or five links.