Assam ASSEB HSLC General Science common mistakes
Assam ASSEB HSLC General Science · Understand the traps and how to avoid them.
When writing a chemical formula from a name such as aluminium sulphate or calcium hydroxide, the candidate writes AlSO4 or CaOH2 by simply pairing the symbols without balancing charges, then cannot derive the correct molecular mass.
The candidate never internalised valency as a combining capacity number; they memorise a few formulae and try to pattern-match new ones by copying symbol order from the name.
What to do instead
For every formula, write the two symbols with their valencies above them, cross the numbers down, then divide by the highest common factor. Practise ten salt names daily and check by adding charges to zero. Keep a one-page valency table (Al 3, Ca 2, SO4 2, OH 1, CO3 2, PO4 3) and use it until recall is automatic.
The candidate balances a chemical equation by changing subscripts (turning H2O into H2O2 or Fe2O3 into FeO) to make the atom counts match.
They treat balancing as an arithmetic puzzle and forget that a subscript defines the identity of the substance; coefficients feel like an extra, unfamiliar tool.
What to do instead
Adopt a fixed rule: only numbers in front of formulae may change, never below or inside. Balance by the hit-and-trial order metals, then non-metals, then hydrogen, then oxygen, and finish by counting every element on both sides. Redo five unbalanced equations this way and write 'subscripts locked' at the top of the page.
Asked to separate a mixture such as salt, sand and iron filings, or to obtain pure water from ink, the candidate names one technique (usually 'filtration' or 'evaporation') for the whole mixture instead of ordering a sequence with reasons.
Techniques are memorised as isolated definitions (what is filtration) rather than as decisions driven by the physical property that differs (particle size, magnetism, boiling point, solubility).
What to do instead
For each mixture, first list the components and the property that distinguishes them, then choose the technique that exploits that property. Practise the standard chains: magnet for iron, dissolve-and-filter for sand plus salt, evaporate or distil for salt from water, fractional distillation for miscible liquids with different boiling points. Always state the property exploited next to each step.
The candidate classifies changes and substances by surface appearance: calling rusting, burning of magnesium ribbon, or curdling of milk a physical change because the substance 'looks different', or calling a solution a compound, or an alloy a compound.
The definitions (new substance formed, fixed composition by mass, components separable by physical means) are recited but not applied, and no test is run against the observation.
What to do instead
Use a two-question test every time: (1) can the original substance be recovered by a physical method, and (2) is a new substance with new properties formed? For mixtures versus compounds, ask whether the components keep their own properties and whether the ratio is fixed. Apply the test to ten everyday examples and write the answer with the deciding evidence.
On pH and indicator questions, the candidate states that a strong acid always has pH 0 or that dilution with water makes an acidic solution basic, or says litmus turns red in a base, and mislabels neutralisation products.
The pH scale is memorised as a list of numbers without the underlying idea of hydrogen ion concentration, and indicator colours are learnt as unordered pairs that get swapped under exam pressure.
What to do instead
Anchor the scale: pH below 7 acidic, 7 neutral, above 7 basic, and dilution moves a solution towards 7, never past it. Build a colour table (litmus red/blue, methyl orange red/yellow, phenolphthalein colourless/pink) and recite it in one direction only. Write neutralisation as acid + base gives salt + water and name the salt from the acid used (HCl gives chloride, H2SO4 gives sulphate, HNO3 gives nitrate).
The candidate predicts that copper will displace zinc from zinc sulphate, or that silver will liberate hydrogen from dilute acid, by guessing 'more reactive' from shine or cost instead of using the reactivity series position.
The reactivity series is memorised as a song without being linked to the two concrete rules: a metal displaces only metals below it, and only metals above hydrogen displace hydrogen from dilute acids.
What to do instead
Write the series once (K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Hg, Ag, Au) and mark the hydrogen line. For every displacement question, locate both metals, check relative position, and state the rule used. Memorise that copper, silver and gold sit below hydrogen, so they do not react with dilute acids.
The candidate confuses the three ways of representing motion: treating distance and displacement as interchangeable, writing speed when velocity is asked, or computing acceleration as distance over time.
The words are used loosely in everyday language, so the vector nature (direction matters) and the distinction between total path length and straight-line change are never fixed.
What to do instead
Before solving, draw the path and mark start and end points. Write the definitions in symbols: displacement is final minus initial position with direction; speed is distance over time; velocity is displacement over time; acceleration is change in velocity over time. Practise one circular-track question (displacement zero after a full round) and one where a body returns to its start, so the distinction becomes concrete.
When applying Newton's laws, the candidate writes F = ma for a body moving uniformly, or claims a moving object must have a force acting on it, and forgets the reaction force in paired-force questions.
Everyday experience suggests motion requires effort, so the inertia idea is not accepted; the candidate also treats force as a property of one body rather than an interaction between two.
What to do instead
State the law before substituting: zero net force means zero acceleration, so constant velocity is as natural as rest. For each problem, draw a free-body diagram, list all forces with directions, and find the net force before dividing by mass. For paired forces, name both bodies and check the forces are equal, opposite and on different bodies.
In ray diagrams for mirrors and lenses, the candidate draws rays that do not follow the rules (a ray through the centre of curvature bent at the mirror, a ray through the focus drawn parallel before reaching the lens) and then reports a virtual image as real or the wrong magnification.
The diagrams are copied from textbook figures rather than constructed from the three standard rays, so small changes in object position break the remembered picture.
What to do instead
Construct every diagram from a fixed ray set: for a lens, a ray parallel to the axis through the focus, a ray through the optical centre undeviated, and a ray through the near focus emerging parallel; for a mirror, the corresponding three. Then apply the sign convention and the lens or mirror formula to confirm the diagram. Label the image as real or virtual from whether the rays actually meet.
On electricity numericals, the candidate adds resistances in parallel by simple addition, or mixes up the heating effect formula, using H = VIt when current is unknown but voltage and resistance are given, and misplaces ammeter and voltmeter in circuit diagrams.
The series and parallel rules are memorised as formulas without the underlying idea that parallel branches share voltage and add current, and the power formulas are not derived from one another.
What to do instead
Derive the parallel rule once from equal voltage across branches to see why the reciprocals add. Keep the three power forms (P = VI = I squared R = V squared over R) and choose by the known quantities. Practise two numericals daily, writing knowns, unknown, formula, substitution, unit. Draw circuits with the ammeter in series and the voltmeter across the component.