IB Chemistry HLTopic 2 — Models of Bonding & StructurePaper 1 & 2Core skill~9 min read
Physical Properties of Covalent Substances
This page is where the whole topic pays off. Once you know the shape of a molecule, whether it is polar, and which intermolecular forces it has, you can predict how it behaves without ever having seen it — whether it is a gas, what it dissolves in, and whether it conducts.
📘 What you need to know
The physical properties of molecular covalent substances are decided by their intermolecular forces, not their covalent bonds.
Stronger intermolecular forces → higher melting and boiling point → less volatile.
Force strength depends on the number of electrons, the shape, and the polarity of the molecule.
Ranking: London < dipole–dipole < hydrogen bonding.
Solubility follows “like dissolves like”: polar in polar, nonpolar in nonpolar.
Molecular covalent substances do not conduct, because they have no free electrons or ions.
Exceptions: some polar molecules ionise in water (HCl), and graphite and graphene conduct because of delocalised electrons.
Melting point, boiling point and volatility
To boil a molecular substance you have to pull the molecules apart from each other. You do not have to break anything inside them. So the question is always the same: how strongly do these molecules grip one another?
A substance with a low boiling point is described as volatile — it evaporates readily. Petrol is volatile; motor oil is not, because its far longer molecules have much greater contact area.
🧩 How to compare two boiling points
Compare the number of electrons. Similar? Then dispersion forces are similar and the answer lies elsewhere.
Check polarity. Is either molecule polar? That adds dipole–dipole attractions.
Check for hydrogen bonding. Is there an O–H, N–H or F–H bond? That trumps everything else.
Check shape. For isomers, the straighter molecule has more contact area and boils higher.
Name the strongest force in each and say which needs more energy to overcome.
Three molecules of almost the same mass
This is the comparison examiners keep returning to, because it isolates one variable at a time. Mass is controlled, so any difference must come from the type of intermolecular force.
Learn this trio as a set: butane, propanone, propan-1-ol. If you can explain their order fluently you can handle almost any boiling point comparison the IB sets.
Solubility: like dissolves like
For something to dissolve, the solvent has to form attractions with the solute that are comparable to the ones it is breaking. Polar solvents can do that for polar solutes; nonpolar solvents can do it for nonpolar solutes. Mix the two kinds and neither can offer the other a good enough deal.
Why ethanol mixes with water and hexane does not
Solubility in water falls as an alcohol chain grows. Ethanol mixes with water in any proportion; hexan-1-ol barely dissolves, because the long nonpolar tail now outweighs the one polar group.
Conductivity
To conduct electricity you need charged particles that can move. Molecular covalent substances have neither free electrons nor free ions — every electron is tied into a bond and every molecule is neutral. So as a rule they do not conduct in any state.
There are two exceptions worth knowing:
Polar molecules that ionise in water. HCl is a covalent gas and does not conduct on its own, but in water it splits into H+ and Cl−, and the solution conducts well.
Graphite and graphene. Each carbon bonds to only three others, so one electron per atom is delocalised and free to drift.
Property
Nonpolar molecular
Polar molecular
Giant covalent
Melting and boiling point
low
low to moderate
very high
Volatility
high
moderate
very low
Solubility in water
insoluble
often soluble
insoluble
Solubility in hexane
soluble
often insoluble
insoluble
Conducts as a solid
no
no
only graphite and graphene
Conducts in solution
no
only if it ionises
does not dissolve
Worked examples
WE 1
Place in order of increasing boiling point: CH3CH2CH2CH3, CH3COCH3, CH3CH2CH2OH
Step 1: compare relative molecular masses58, 58 and 60 — near enough identical, so dispersion forces are similar.
Step 2: identify the strongest force in each
Butane: nonpolar → dispersion only
Propanone: polar C=O → dispersion + dipole–dipole
Propan-1-ol: has O–H → dispersion + hydrogen bondingStep 3: rank by force strengthbutane < propanone < propan-1-olstate the Mₕ values first — it shows you ruled out the size explanation
WE 2
Explain why ethanol dissolves in water but hexane does not [3]
Mark 1: describe the solvent
Water is polar and forms hydrogen bonds with itself.
Mark 2: what ethanol offers
Ethanol has an O–H group, so it can form hydrogen bonds with water and replace the ones broken.
Mark 3: what hexane cannot offer
Hexane is nonpolar with only C–H and C–C bonds. It can form only weak dispersion forces with water, not enough to replace the hydrogen bonds lost.
Like dissolves like: ethanol matches water, hexane does notthe phrase “replace the forces broken” is what turns a description into an explanation
WE 3
Substance Y melts at −114°C, does not conduct as a pure liquid, but its aqueous solution does. Suggest what Y is.
Step 1: read the melting point
Very low, so simple molecular, not giant or ionic.
Step 2: it does not conduct when pure
Confirms neutral molecules with no free electrons or ions.
Step 3: but it does in water
So it must ionise when it dissolves.
A polar covalent molecule that ionises in water, such as HClthe “only in solution” clue is the giveaway for a covalent acid every time
💡 Exam tips
Quote the Mr or electron count when comparing. It proves you have controlled for dispersion forces.
Name the strongest force in each substance and compare those, rather than talking vaguely about “stronger forces”.
For solubility questions, say what the solute can offer the solvent, not just whether it is polar.
Never write “the covalent bonds break” for a melting or boiling question.
For conductivity, always mention the need for mobile charged particles.
Watch for the word volatile — it means low boiling point, so weak intermolecular forces.
⚠ Common mix-ups
“It has weak covalent bonds.” Covalent bonds are strong. The forces between molecules are the weak part.
Assuming everything covalent is a gas. Iodine is a solid, and giant covalent structures melt above 1000°C.
Saying HCl conducts because it is covalent. It conducts because it ionises in water — pure liquid HCl does not.
Forgetting dispersion forces exist in polar molecules too. Polar molecules have both.
Ignoring shape for isomers. Same formula does not mean same boiling point; branching lowers it.
Assuming anything with oxygen dissolves in water. The nonpolar part of the molecule matters just as much.
Up next: Chromatography — a separation technique built entirely on the intermolecular forces you have just learned, and one that gives you a number to calculate.
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