Question

Which of the statement(s) regarding alkanes is/are correct?

• A. All the alkanes are solids at room temperature ( 298 K )
• B. Alkanes are colourless.
• C. For a given molar mass , as the branching in alkane increases, boiling point increases.
• D. Alkanes are insoluble in polar solvents.

Answer 1

Answer: (B), (D)

The correct options are
B Alkanes are colourless.
D Alkanes are insoluble in polar solvents.

Theory :
Physical properties of alkanes :

1. Physical state : At 298 K

$C_1-C_4\to \text{Gases}$
$C_5-C_{17}\to \text{Liquids}$
$\ge C_{18}\to \text{Solids}$

2.Alkanes are colourless.

3. Alkanes are odourless.

4. Generally, polar substances are soluble in polar solvents and non-polar substances are soluble in non-polar solvents. Alkanes are non-polar in nature. So, alkanes are insoluble in polar solvents like water.

5. Boiling point: For the same molar mass, as branching increases, the overall surface area decreasesThis leads to a reduction in the intermolecular forces (London dispersion forces), thereby decreasing boiling point.

6. Melting point depends on:
(a) Intermolecular attraction.
(b) Packing in crystal lattice.
In general, alkanes with odd number of carbons are packed less tightly than alkanes with even number of carbons. Hence, they have weaker intermolecular attractions and lower melting points.