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"Why does titanium melt at a lower temperature than chromium?" Many students answer: "Because chromium has a higher melting point." That earns nothing, because it repeats the question. Give-reasons questions in Class 12 chemistry test whether you can connect a property to its cause. This post shows a three-step method, works through questions from CBSE's 2026-27 sample paper with the official answers, and ends with a practice routine.
Where do give-reasons questions appear?
The CBSE Class 12 Chemistry sample paper for 2026-27 has 33 questions for 70 marks, and notes no change in the paper design for 2026-27. Reasoning questions appear across sections: "Why does...", "Account for the following", "Justify your answer", and assertion-reason items. They cluster in units such as d- and f-block elements, coordination compounds, and haloalkanes and haloarenes, all in the CBSE 2026-27 chemistry syllabus.
How are these answers marked?
The 2026-27 marking scheme shows the split. For Question 20 (which substituent, nitro or methoxy, makes chlorobenzene more reactive towards nucleophilic substitution, and why), it gives:
1 mark: "Nitro group will increase the reactivity."
½ mark: the nitro group is strongly electron-withdrawing (−I and −M effects), whereas methoxy is electron-donating.
½ mark: at the ortho or para position, the nitro group withdraws electron density and stabilises the negative charge through resonance.
So half the marks are for the reasoning chain, split into steps. A correct claim with no reason earns half.
Worked example 1: titanium and chromium
Question 17(A)(II) asks why titanium melts at a lower temperature than chromium. The marking scheme answer: chromium has more unpaired electrons than titanium, giving stronger interatomic attractions, so more energy is needed to break them.
In three steps:
Fact: Cr has a higher melting point than Ti.
Cause: Cr (3d⁵4s¹) has more unpaired electrons than Ti (3d²4s²) available for metallic bonding.
Link: more unpaired electrons, stronger interatomic bonding, more energy to melt.
This matches the NCERT chapter on the d- and f-block elements, which links high melting points to the number of unpaired electrons taking part in interatomic metallic bonding.
Worked example 2: magnetic moment with numbers
Question 22(I) asks which of Ti²⁺, Co²⁺ and Fe²⁺ has the highest magnetic moment. A full answer counts electrons first:
Ti²⁺: 3d², 2 unpaired electrons
Co²⁺: 3d⁷, 3 unpaired electrons
Fe²⁺: 3d⁶, 4 unpaired electrons
The marking scheme gives Fe²⁺, because it has the most unpaired electrons. You can support it with NCERT's spin-only formula, μ = √(n(n + 2)) BM:
Fe²⁺: √(4 × 6) = √24 ≈ 4.90 BM
Co²⁺: √(3 × 5) = √15 ≈ 3.87 BM
Ti²⁺: √(2 × 4) = √8 ≈ 2.83 BM
The numbers are not required for the mark, but they make the reason impossible to misread.
Worked example 3: account for a practical fact
Question 21(I): underground iron pipelines are often connected to a more reactive metal such as magnesium. The marking scheme: magnesium, being more reactive, acts as a sacrificial anode and is oxidised (Mg → Mg²⁺ + 2e⁻), supplying electrons to the iron, which behaves as the cathode and does not corrode.
Notice the chain: reactivity, then which electrode, then why iron is protected.
Which slips cost marks?
Restating the question as the reason.
Naming the wrong factor, such as atomic size when the question is about unpaired electrons.
Skipping the comparison. If the question compares two things, say what each has.
Missing the electron count. For transition metal ions, write the configuration.
Writing a paragraph when three clear lines are expected.
A practice routine
Collect questions. List every "why", "account for" and "justify" item in the sample paper and NCERT exercises for one unit.
Write three-step answers without the book, then check against the marking scheme or NCERT text.
Mark yourself as CBSE does: one mark for the fact, the rest for each step.
Fix concept gaps first. If you cannot find the cause at all, the problem is the concept, not the writing. The Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can reread the idea before trying again.
Our posts on using CBSE marking schemes and a Class 12 chemistry plan cover the rest of the paper.
Tonight, take five "account for" questions from the d- and f-block chapter and answer each in exactly three lines.
