QCE Chemistry - Unit 4 - Properties and structure of organic materials

Physical Properties and Trends | QCE Chemistry

Learn how intermolecular forces explain boiling point, solubility and volatility trends in QCE organic chemistry.

Updated 2026-05-18 - 5 min read

Organic molecules do not have physical properties by accident. Their boiling points, melting points, volatility and solubility come from the way molecules attract each other.

In QCE Chemistry, the strongest answers link a property to structure using this chain of reasoning:

structure -> intermolecular forces -> energy required -> observed property

Intermolecular forces

The main intermolecular forces for organic molecules are:

| Force | Occurs when | Relative importance | | --- | --- | --- | | Dispersion forces | all molecules | stronger for larger, longer, more polarisable molecules | | Dipole-dipole forces | polar molecules | important for functional groups with polar bonds | | Hydrogen bonding | $\mathrm{O-H}$, $\mathrm{N-H}$ or $\mathrm{F-H}$ donors interacting with lone pairs | often much stronger than ordinary dipole-dipole forces |

Every molecule has dispersion forces. The question is whether extra forces, such as dipole-dipole interactions or hydrogen bonding, are also present.

Boiling point trends

Boiling point is the temperature where enough energy has been supplied for molecules to separate into the gas phase. Stronger intermolecular forces require more energy to overcome, so boiling point increases.

Chain length

Longer carbon chains usually have higher boiling points because they have:

• more electrons
• greater surface area
• stronger dispersion forces

For example, pentane has a higher boiling point than propane because pentane molecules experience stronger dispersion forces.

Branching

Branching usually lowers boiling point for molecules with the same molecular formula. A branched molecule has a more compact shape, so there is less surface contact between molecules and weaker dispersion forces.

For example, 2-methylpropane has a lower boiling point than butane even though both are $\mathrm{C_4H_{10}}$.

Functional group

Functional groups can introduce polarity or hydrogen bonding. For similar-sized molecules, a typical boiling point comparison is:

alkane < haloalkane/ether/aldehyde/ketone < alcohol/carboxylic acid

This is a trend, not a memorised law. Always compare the actual molecules given.

Volatility

Volatility describes how easily a substance evaporates. If intermolecular forces are weak, molecules escape into the gas phase more easily, so volatility is higher.

This means boiling point and volatility usually move in opposite directions:

• stronger intermolecular forces -> higher boiling point -> lower volatility
• weaker intermolecular forces -> lower boiling point -> higher volatility

Solubility in water

Water is polar and forms hydrogen bonds, so substances dissolve well in water when they can form favourable interactions with water molecules.

Small alcohols and carboxylic acids often dissolve well because their polar functional groups can hydrogen bond with water. However, as the hydrocarbon chain gets longer, the non-polar part becomes more important and water solubility decreases.

For example, ethanol is very soluble in water, but hexan-1-ol is much less soluble. Both have an $\mathrm{-OH}$ group, but hexan-1-ol has a larger non-polar carbon chain.

Carboxylic acids can also become more soluble when they ionise in water or react with bases to form ionic salts.

Melting point

Melting point is harder to predict than boiling point because solid packing matters. Symmetrical molecules can sometimes pack more efficiently, increasing melting point. Stronger intermolecular forces still matter, but shape and crystal packing can complicate the trend.

In exam answers, avoid overclaiming. If the question gives data, use the data and explain it using structure.

Comparing molecules well

A good comparison keeps the molecules as similar as possible. If you compare methane with decanoic acid, several factors change at once: chain length, mass, polarity and hydrogen bonding. It is much clearer to compare similar-sized molecules, such as propane and propan-1-ol, when discussing the effect of a functional group.

Quick check

Exam traps worth knowing

• Explaining every trend using molecular mass only.
• Forgetting to compare molecules of similar size when discussing functional group effects.
• Saying hydrocarbons are insoluble because they cannot form bonds at all. They have dispersion forces, but they cannot form favourable interactions with water.
• Claiming all molecules with oxygen are water soluble. A long hydrocarbon chain can dominate the molecule's behaviour.

Sources

• QCAA Chemistry subject page
• QCAA Chemistry 2025 syllabus
• OpenStax Chemistry: Atoms First 2e: Intermolecular Forces
• OpenStax Chemistry 2e: Hydrocarbons
• OpenStax Chemistry 2e: Aldehydes, Ketones, Carboxylic Acids, and Esters