Kerala SSLC Chemistry common mistakes
Kerala SSLC Chemistry · Understand the traps and how to avoid them.
When writing electron configuration, the candidate fills the 3d subshell before the 4s subshell, writing Sc as [Ar]3d1 4s2 instead of [Ar]4s2 3d1.
The candidate memorises the sequence of subshells as 1s 2s 2p 3s 3p 4s 3d but then applies the aufbau principle literally in order of increasing n, forgetting that 4s has lower energy than 3d and is filled first.
What to do instead
Use the (n+l) rule: for 4s, n+l=4+0=4; for 3d, n+l=3+2=5. Lower n+l fills first; when n+l is equal, lower n fills first. Practise writing configurations for K, Ca, Sc, and Fe using the rule, and verify with the periodic table: the 4s block (groups 1–2) comes before the 3d block (groups 3–12).
In mole calculations, the candidate divides or multiplies by Avogadro’s number in the wrong direction, for example calculating the number of molecules in 0.5 mol of CO2 as 0.5 ÷ 6.022×10^23 instead of 0.5 × 6.022×10^23.
The candidate memorises the formula ‘n = N/NA’ but does not anchor it to the meaning of the mole: one mole contains 6.022×10^23 particles, so the number of particles is always larger than the number of moles.
What to do instead
Always set up the calculation with units: N = n × NA = 0.5 mol × 6.022×10^23 mol⁻¹ = 3.011×10^23. Check the order of magnitude: the answer must be around 10^23, not 10^-24. Do a quick ‘does this make sense?’ check before writing the final answer.
When asked to balance a chemical equation, the candidate changes subscripts in a formula (e.g., writing H2O as H2O2) to make the atoms balance, instead of placing coefficients in front of the formulas.
The candidate treats balancing as a purely numerical puzzle and forgets that subscripts define the identity of the substance; changing them changes the compound.
What to do instead
Follow a fixed procedure: (1) write the correct formulas, (2) count atoms of each element on both sides, (3) place coefficients only in front of formulas, (4) recheck by counting again. Practise with equations such as Fe + H2O → Fe3O4 + H2 and C3H8 + O2 → CO2 + H2O, and explicitly state that subscripts are never altered.
In the laboratory preparation of hydrogen chloride, the candidate writes the drying agent as sodium hydroxide or quicklime instead of concentrated sulphuric acid.
The candidate generalises ‘drying agents remove water’ without considering that HCl is an acidic gas and reacts with basic drying agents such as NaOH, KOH, or CaO.
What to do instead
Learn the rule: acidic gases (HCl, CO2, SO2, Cl2) are dried with concentrated H2SO4 or P2O5; basic gases (NH3) are dried with CaO or NaOH. For HCl specifically, state that concentrated H2SO4 is used because it does not react with HCl. Write the reaction HCl + NaOH → NaCl + H2O to show why NaOH is unsuitable.
When predicting the products of electrolysis of aqueous solutions, the candidate writes the products as if the electrolyte were molten, for example predicting sodium metal at the cathode during electrolysis of aqueous NaCl.
The candidate memorises electrode reactions for molten electrolytes and does not apply the preferential discharge series for aqueous solutions, where H+ and OH− from water compete with the ions of the electrolyte.
What to do instead
For aqueous solutions, always list all ions present, including H+ and OH− from water. Apply the discharge series: at the cathode, the ion lower in the reactivity series is discharged (H+ is discharged in preference to Na+); at the anode, Cl− is discharged in preference to OH− in concentrated NaCl. Practise with aqueous NaCl, CuSO4, and dilute H2SO4, writing the half-equations.
When distinguishing between a physical change and a chemical change, the candidate classifies a change as chemical only if a gas is evolved or a precipitate forms, and labels changes such as dissolving sodium chloride in water as chemical because ‘the salt disappears’.
The candidate relies on a limited set of visual cues and does not check the defining criterion: whether a new substance with different properties is formed.
What to do instead
Use the criterion ‘new substance formed?’ and test reversibility: dissolving NaCl can be reversed by evaporation, so it is physical. Practise a table of changes (melting ice, burning magnesium, rusting iron, dissolving sugar, souring milk) and for each state the evidence and whether the original substance can be recovered by a physical method.
In organic chemistry, the candidate writes the IUPAC name of a branched alkane by numbering the carbon chain from the end that gives the substituent the lower number, but chooses the wrong parent chain (not the longest continuous chain).
The candidate focuses on the substituent and forgets the first rule of IUPAC nomenclature: select the longest continuous carbon chain as the parent.
What to do instead
Apply the rules in order: (1) find the longest continuous chain, (2) number from the end that gives the lowest locant to the first substituent, (3) name substituents alphabetically. Practise with structures such as 2-methylbutane and 3-ethyl-2-methylpentane, and for each draw the chain and mark the numbering explicitly.
When calculating the pH of a solution, the candidate uses the concentration of the acid directly as [H+] without considering the basicity of the acid, for example calculating the pH of 0.01 M H2SO4 as 2 instead of approximately 1.7.
The candidate memorises pH = −log[H+] but does not account for the number of H+ ions released per molecule of acid, especially for strong diprotic acids.
What to do instead
For a strong acid, write the ionisation equation first: H2SO4 → 2H+ + SO4^2−. Then [H+] = 2 × 0.01 = 0.02 M, so pH = −log(0.02) ≈ 1.7. For weak acids, state that partial ionisation requires Ka. Practise with HCl, HNO3, and H2SO4, and always write the ionisation equation before substituting into the formula.
When describing the reaction of metals with acids, the candidate writes that copper reacts with dilute hydrochloric acid to produce hydrogen gas, or that all metals displace hydrogen from acids.
The candidate generalises from reactive metals such as zinc and magnesium and does not use the reactivity series to decide whether a metal is above hydrogen.
What to do instead
Memorise the reactivity series in order (K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Ag, Au) and apply the rule: only metals above hydrogen displace H2 from dilute acids. Copper, silver, and gold do not react with dilute HCl or dilute H2SO4. Practise by predicting and justifying the outcome for Mg, Zn, Fe, Cu, and Ag with dilute HCl.
In questions on the periodic table, the candidate states that atomic radius increases across a period from left to right, or that metallic character increases across a period.
The candidate confuses the trend down a group (radius increases, metallic character increases) with the trend across a period, where increasing nuclear charge pulls electrons closer and increases non-metallic character.
What to do instead
Draw a blank periodic table and annotate the trends: across a period, atomic radius decreases, ionisation energy increases, metallic character decreases; down a group, atomic radius increases, ionisation energy decreases, metallic character increases. Test the rule with a specific comparison, e.g., Na vs Cl across period 3 and Li vs Cs down group 1, and explain each in terms of nuclear charge and shielding.