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Science · 2026 · 5 marks
CBSE 2026 · Region 4 · Set 1 · Q29
Attempt either option (A) or (B): (A) (i) Carbon cannot form C^4+ cation and C^4- anion. Explain. (ii) Define homologous series of carbon compounds.…
Attempt either option (A) or (B) :
(i)
Carbon cannot form $\displaystyle \mathrm{C}^{4+}$ cation and $\displaystyle \mathrm{C}^{4-}$ anion. Explain.
(ii)
Define homologous series of carbon compounds. Why are the melting and boiling points of $\displaystyle \mathrm{C}_{4} \mathrm{H}_{10}$ higher than $\displaystyle \mathrm{C}_{3} \mathrm{H}_{8}$ or $\displaystyle \mathrm{C}_{2} \mathrm{H}_{6}$ ?
(iii)
What happens when ethanol is heated with acidified potassium dichromate? $\displaystyle \mathbf{2}+\mathbf{2}+\mathbf{1}$
(i)
How would you name the following : I.
II. $\displaystyle \mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{CH}$
(ii)
When ethanol reacts with ethanoic acid in the presence of conc. $\displaystyle \mathrm{H}_{2} \mathrm{SO}_{4}$, a substance with sweet smell is produced. Answer the following : I. State the class of compounds to which the sweet smelling compounds belong. II. Write the chemical equation for the reaction.
(iii)
Why soaps do not produce foam with hard water ?
Marking-scheme solution
(i)
Carbon cannot form \(\displaystyle \mathrm{C}^{4+}\) cation because removal of four electrons is energetically not possible. Carbon cannot form \(\displaystyle \mathrm{C}^{4-}\) anion because nucleus with six protons cannot hold ten electrons.
(ii)
A series of compounds in which the same functional group substitutes for hydrogen in carbon chain is called a homologous series.
Because of higher molecular mass of \(\displaystyle \mathrm{C}_{4} \mathrm{H}_{10}\) than \(\displaystyle \mathrm{C}_{3} \mathrm{H}_{8}\) or \(\displaystyle \mathrm{C}_{2} \mathrm{H}_{6}\).
(iii)
Ethanoic acid / acetic acid/ \(\displaystyle \mathrm{CH}_{3} \mathrm{COOH}\) is formed/
(i) I. Propanal II. Propyne
(ii) I. Ester group II.
(iii) Soaps reacts with calcium and magnesium salts present in hard water to form scum (insoluble substance).
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CBSE Class 10 Science past-paper question from the 2026 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.