Physical Properties of Alkanes

Introduction

Methane is a gas that boils at −162 °C, petrol is a runny liquid that evaporates quickly, and candle wax is a solid. Yet all three are alkanes, made of the same kind of molecule with the same bonds. The difference lies in the size of the molecules and the strength of the forces between them. This page explains how chain length controls the physical properties of alkanes and why those properties matter when we choose a fuel.

Core explanation

Two kinds of force. In an alkane there are strong covalent bonds inside each molecule (C–C and C–H) and weak intermolecular forces between neighbouring molecules. When an alkane melts or boils, only the weak intermolecular forces are overcome. The covalent bonds do not break — boiling methane gives methane gas, not carbon and hydrogen.

Boiling point increases with chain length.

Alkane Carbons Boiling point (°C) --- --- --- methane 1 −162 propane 3 −42 pentane 5 36 heptane 7 98 decane 10 174 icosane 20 about 343

Longer molecules have more electrons and a larger surface area in contact with their neighbours. This makes the intermolecular forces stronger, so more energy is needed to pull the molecules apart, and the boiling point is higher.

State at room temperature. Alkanes with 1 to 4 carbons are gases, those with about 5 to 17 carbons are liquids, and those with more are waxy solids.

Viscosity. Longer chains attract each other more strongly and also tangle together, so liquids made of long-chain alkanes flow more slowly. Petrol is runny; lubricating oil is thick.

Volatility and flammability. Short-chain alkanes evaporate easily, because their intermolecular forces are weak. As a result they produce plenty of vapour to mix with air, and they ignite easily. Long-chain alkanes are less volatile and harder to ignite, which is why a candle needs a wick.

Branching lowers boiling point. Pentane (straight) boils at 36 °C, but its compact isomer 2,2-dimethylpropane boils at about 10 °C. The more spherical molecule has less surface in contact with its neighbours, so its intermolecular forces are weaker.

Solubility and density. Alkanes are non-polar, so they do not mix with water, which is polar. They are also less dense than water, so liquid alkanes float on it. They do dissolve well in other non-polar solvents.

Step-by-step reasoning

To explain why one alkane has a higher boiling point than another:

1. Compare the sizes of the molecules (number of carbons). 2. State that the larger molecule has stronger intermolecular forces. 3. State that more energy is needed to overcome these forces. 4. Conclude that the larger alkane has the higher boiling point.

Visual explanation

Picture short alkane molecules as small balls touching only at a point, and long alkane molecules as strands of spaghetti lying side by side along their whole length. The spaghetti strands have much more contact, and it takes much more effort to pull them apart.

Real-world analogy

Short strips of sticky tape peel off easily; long strips stuck along their full length need a strong pull. The glue in each centimetre is equally weak, but the longer the contact, the greater the total force — just like intermolecular forces between long alkane chains.

Real-world example

Oil spills at sea spread as a thin film on the surface because the hydrocarbons are less dense than water and do not dissolve in it. The lighter, more volatile fractions evaporate within days, while the heavier, less volatile fractions remain and form sticky tar.

Why?

Why do alkanes not dissolve in water? Water molecules attract each other strongly through hydrogen bonds. Non-polar alkane molecules cannot form comparable attractions to water, so there is no energetic benefit in breaking up the water structure to let them in. The two liquids separate.

Common misconception

"When an alkane boils, its covalent bonds break." Boiling only separates whole molecules by overcoming weak intermolecular forces. If the C–C and C–H bonds broke, new substances would form — that would be a chemical change such as cracking, not boiling.

Worked example

Question: Hexane boils at 69 °C and octane at 126 °C. Predict whether heptane is a liquid or a gas at 25 °C and explain.

Reasoning: Heptane lies between hexane and octane in the series, so its boiling point lies between 69 °C and 126 °C (it is 98 °C). This is well above 25 °C.

Answer: A liquid, because its boiling point is above room temperature.

Quick check

1. What must be overcome when an alkane boils? Answer: The weak intermolecular forces between the molecules.

Exam focus

When explaining boiling points, always say that intermolecular forces are overcome — never that bonds break. Link longer chain to stronger intermolecular forces to more energy needed to higher boiling point. Remember the three trends with increasing chain length: boiling point up, viscosity up, flammability down.

Advanced insight

The intermolecular forces between alkanes are London dispersion forces. They arise because the electron cloud of a molecule fluctuates, creating a temporary dipole that induces a dipole in a neighbour. More electrons and more contact area give larger fluctuations and stronger attractions, which is why branching and chain length have such clear effects.

Summary

Alkanes have strong covalent bonds within molecules but weak intermolecular forces between them. As chain length increases, intermolecular forces get stronger, so boiling point and viscosity rise, while volatility and flammability fall. Branching lowers the boiling point. Alkanes are non-polar, insoluble in water and less dense than it.

Practice questions

1. State how the boiling point of alkanes changes as the number of carbon atoms increases. Answer: It increases. 2. Explain why decane has a higher boiling point than pentane. Answer: Decane molecules are larger, so the intermolecular forces between them are stronger and more energy is needed to overcome them. 3. Which is more viscous, an alkane with 6 carbons or one with 16 carbons? Explain. Answer: The one with 16 carbons, because its longer molecules attract each other more strongly and tangle, so it flows less easily. 4. Explain why petrol vapour ignites more easily than candle wax. Answer: Petrol contains short-chain alkanes that are volatile and readily form vapour to mix with air; wax molecules are long and have strong intermolecular forces, so little vapour forms.