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In a board numerical, the examiner marks the lines you write, not only the number at the end. A clear layout lets them find every step that earns a mark, even if your final arithmetic slips.
How are chemistry numericals marked in CBSE Class 12?
Solutions, Electrochemistry and Chemical Kinetics, the three units where most numericals sit, carry 7, 9 and 7 marks: 23 of the 70 theory marks in the CBSE Chemistry curriculum for 2026-27.
The CBSE 2026-27 sample paper marking scheme shows how those marks are split across steps:
In the Nernst equation answer (Question 33), the equation E = E° − (0.059/2) log Qc, a working line and the final value of 1.07045 V each sit beside a separate ½ mark.
In the activation energy answer (Question 23), the last two half marks are labelled "Correct answer and correct unit".
In the vapour pressure numerical (Question 28), four half marks run from the relative lowering formula to the final molar mass.
The 2026-27 sample question paper also states that log tables and calculators are not allowed. So expect numbers that can be simplified by hand, and show that simplification in your working rather than jumping to the answer.
What should each line of a numerical show?
Use the same five lines for every numerical, whatever the chapter:
Given: list the data with units, and convert to the units the formula needs (minutes to seconds, grams of solvent to kilograms, mL to L).
Formula: write the equation in symbols before any number appears.
Substitution: put in every value, with its unit, in the same order as the formula.
Simplification: one or two lines, showing any log or power of ten you cancelled.
Answer: underline or box it, with its unit and sensible significant figures.
The units most often lost are chapter-specific:
Solutions: molality is moles of solute per kilogram of solvent, so a solvent mass given in grams must be converted first.
Electrochemistry: cell potentials are in volts, and the marking scheme's conductivity answer notes that molar conductivity is quoted in both S cm² mol⁻¹ and S m² mol⁻¹, where 1 S m² mol⁻¹ = 10⁴ S cm² mol⁻¹ (marking scheme, Question 33).
Chemical Kinetics: the unit of the rate constant depends on the order of the reaction, which we explain in our guide to working out the units of a rate constant.
How many significant figures should the final answer have?
The NCERT Class 11 Chemistry textbook, Unit 1, sets the rule: in multiplication and division, the result must have no more significant figures than the measurement with the fewest. Its example is 2.5 × 1.25 = 3.125, reported as 3.1 because 2.5 has two significant figures. For addition and subtraction, the same section says the result cannot have more digits after the decimal point than either of the original numbers. Counted numbers, such as 20 eggs, have infinite significant figures, and defined conversions such as 60 seconds in a minute are treated the same way.
In practice, carry one extra digit through your working and round only in the final line.
A marked example: first-order kinetics
This is our own board-style question, with a mark split modelled on the sample scheme. It is not an official key.
Question (3 marks): A first-order reaction is 90% complete in 46.06 min. Calculate its rate constant in min⁻¹ and in s⁻¹.
Given: t = 46.06 min. 90% complete, so [R] = [R]₀/10 and [R]₀/[R] = 10.
Formula: k = (2.303 / t) log([R]₀/[R]). (½)
Substitution: k = (2.303 / 46.06 min) × log 10. (½)
Simplification: log 10 = 1, and 2.303 / 46.06 = 0.05000. (½)
Answer: k = 5.000 × 10⁻² min⁻¹. (½ for value and unit)
Conversion: k = (5.000 × 10⁻²) / (60 s). (½)
Answer: k = 8.333 × 10⁻⁴ s⁻¹. (½ for value and unit)
The answers keep four significant figures because 46.06 and 2.303 both have four, and the 60 is exact.
If you know the formula but your substitutions keep going wrong, work through similar problems in the Fermi Problem Library, whose step-by-step solver explains the reasoning rather than printing an answer. If the gap is the idea itself, such as why a log appears in the first-order equation, the Fermi Concept Library gives each concept in chemistry its own lesson.
How should you practise this layout before the boards?
Take the official sample paper and solve only the numericals, using the five-line layout.
Mark your answers against the official scheme, half mark by half mark, as described in our guide to using the CBSE marking scheme.
For every half mark you lost, write which line was missing: formula, substitution, unit or rounding.
A week later, repeat with a fresh set.
For where numericals fit in your wider revision, see our CBSE Class 12 chemistry paper strategy.
