Alkane Physical Properties for NEET: Key Points, Tricks & MCQs
Alkane PropertiesHydrocarbonsNEET ChemistryNEET 2025Physical PropertiesBoiling Point TrendsSolubility
Alkane Physical Properties for NEET: Key Points, Tricks & MCQs
Hydrocarbons·2 min read·NEET 2026
What NEET Asks
Questions primarily focus on trends in boiling point, melting point, and density with increasing molecular weight and branching.
Solubility characteristics of alkanes are also frequently tested.
Expect direct conceptual questions or comparative analysis MCQs.
Key Points
Alkanes are nonpolar molecules due to similar electronegativities of C and H and symmetrical structure.
Intermolecular forces are weak London Dispersion Forces (LDFs) or van der Waals forces.
The first four alkanes (C1-C4) are gases, C5-C17 are liquids, and C18 onwards are solids at room temperature.
Boiling point increases with increasing molecular weight (number of carbon atoms) due to stronger LDFs.
Boiling point decreases with increasing branching due to reduced surface area for intermolecular interaction.
Melting point generally increases with molecular weight, but also depends on packing efficiency in the solid state.
Alkanes are insoluble in water (polar solvent) but soluble in nonpolar organic solvents.
Must-Know Formula / Reaction
There's no specific 'formula' for physical properties. The key concept is:
Strength of Intermolecular Forces (LDFs) ∝ Molecular Weight ∝ Surface Area
LDFs: London Dispersion Forces, the only intermolecular forces in nonpolar alkanes.
Molecular Weight: Sum of atomic masses; more mass means more electrons, leading to stronger LDFs.
Surface Area: Greater contact area allows for more extensive LDFs.
Common Mistakes
Students often confuse the effect of branching on boiling point vs. melting point. Branching decreases boiling point but can increase melting point for symmetrical molecules due to better packing.
Don't assume alkanes are insoluble in all solvents; they are soluble in nonpolar organic solvents like CCl₄.
Forgetting that alkanes are less dense than water is a common oversight.
Rapid Revision
Alkanes are nonpolar, held by weak LDFs. BP & MP generally increase with MW. Branching decreases BP by reducing surface area. Insoluble in water, soluble in organic solvents. Density is less than water.
Frequently Asked Questions
Why do alkanes have low boiling points compared to other organic compounds?▾
Alkanes are nonpolar molecules and only possess weak London Dispersion Forces (LDFs) between them. These forces require less energy to overcome, resulting in lower boiling points compared to compounds with stronger intermolecular forces like dipole-dipole interactions or hydrogen bonding.
How does branching affect the boiling point of alkanes?▾
Increasing branching in an alkane decreases its boiling point. Branching makes the molecule more spherical, reducing its surface area. A smaller surface area leads to fewer points of contact between molecules, thus weakening the London Dispersion Forces and requiring less energy to boil.
Are alkanes soluble in water? Why or why not?▾
Alkanes are insoluble in water. Water is a highly polar solvent, while alkanes are nonpolar. The 'like dissolves like' principle applies here; nonpolar alkanes cannot form strong interactions with polar water molecules, which prefer to interact with themselves via hydrogen bonding.
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