Lesson 4 of 13 · 6 min
Physical properties of alkanes
NCERT §9.2.3
Butane boils at 272.4 K, just below the freezing point of water. That is why it stays liquid in a pressurised cylinder but turns to gas the moment it escapes into a warm kitchen.
The lesson in notes
In short
C–C and C–H bonds are covalent and carbon and hydrogen differ little in electronegativity, so alkanes are nearly non-polar and are held together only by weak van der Waals forces.
At 298 K, C₁ to C₄ are gases, C₅ to C₁₇ are liquids and C₁₈ onwards are solids. Alkanes are colourless and odourless.
Being non-polar they dissolve in non-polar solvents, not in water: 'like dissolves like'. This is why petrol removes grease (a mixture of higher alkanes) in dry cleaning.
Boiling point rises steadily with molecular mass because a larger molecule has more surface area and stronger van der Waals attraction: methane 111.0 K, ethane 184.4 K, propane 230.9 K, hexane 341.9 K, decane 447.1 K.
Among isomers, branching lowers the boiling point: pentane 309.1 K, 2-methylbutane 300.9 K, 2,2-dimethylpropane 282.5 K. More branching makes the molecule more nearly spherical, so it touches its neighbours over a smaller area.
Melting points do not follow the same simple pattern: 2,2-dimethylpropane melts at 256.4 K, far above pentane at 143.3 K.