Q1. [3] medium thorough-understanding
A student is given two unlabelled compounds with molecular formulas C₃H₈ and C₃H₆. (i) Using the general formula for alkanes and alkenes, determine which compound is saturated and which is unsaturated. (ii) Describe one chemical test the student could perform to confirm this, stating the expected observation for each compound.
Generated by claude-sonnet-4-6 · 2026-06-26 01:09 · grounding rag
Model Answer
(i) The general formula for alkanes is $C_nH_{2n+2}$ and for alkenes is $C_nH_{2n}$.
- $C_3H_8$: For $n=3$, $2(3)+2 = 8$ → matches alkane formula. It is saturated (propane).
- $C_3H_6$: For $n=3$, $2(3) = 6$ → matches alkene formula. It is unsaturated (propene).
(ii) Test: Pass each compound through bromine water (or bromine solution).
- $C_3H_6$ (unsaturated): Bromine water gets decolourised due to addition reaction across the double bond.
- $C_3H_8$ (saturated): Bromine water shows no colour change as saturated hydrocarbons do not undergo addition reactions.
Source: Carbon and its Compounds, Section 4.2.2 and Exercise Q.13–14
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Explanation
- Examiners expect you to explicitly apply the general formulas $C_nH_{2n+2}$ (alkanes) and $C_nH_{2n}$ (alkenes) and substitute $n=3$ — don't just state the answer without showing the check.
- For the chemical test, bromine water (decolourisation) is the standard NCERT-expected answer. The textbook Exercise Q.14 directly asks for a test to differentiate saturated and unsaturated hydrocarbons — this is it.
- Avoid writing about the combustion/flame test here; it is less reliable as a distinguishing test compared to bromine water.