Worked example 1
Justify why water has a higher surface tension than ethanol.
Try it first: Count the hydrogen bonds each molecule can form.
0 of 3 steps revealed.
Types of Solids and IMFSJustification with IMFSContent level: Core 22 min
Justifying Properties with Intermolecular Forces
What you'll be able to do: Write a complete justification that links structure to intermolecular forces to an observed property.
Best after: Hydrogen Bonding, Ionic, Metallic, Covalent Network and Molecular Solids
Introduction
Naming the strongest force earns almost no credit. A justification has to connect structure to force to property in one unbroken chain.
These are recommended, not required. You can start this lesson at any time.
Learning objectives
- Build a three part justification: structure, force, property
- Choose the correct comparison variable when size and polarity conflict
- Predict vapour pressure, viscosity and surface tension from IMF strength
- Apply like dissolves like to solubility questions
Lesson
The three part chain
Step one: state the structural feature, such as an O-H group or a long carbon chain. Step two: name the intermolecular force that feature produces and compare its strength with the other substance. Step three: state the property consequence. Skipping any step leaves the argument incomplete.
Because ethanol has an O-H group, it hydrogen bonds, so more energy is needed to vaporise it and its boiling point is higher. That is the full chain.
Control the right variable
Compare intermolecular forces while controlling for the other features as well as possible. Larger, more polarizable particles can have stronger dispersion forces, which may outweigh differences in polarity, but this is not an absolute rule. Consider molecular size and shape, polarizability, permanent dipoles, and hydrogen bonding before deciding which attraction matters most.
Properties that rise with IMF strength
Stronger cohesive attractions generally raise boiling point, heat of vaporization, viscosity, and surface tension, and lower vapor pressure, when other factors are reasonably comparable. Melting point is less predictable from IMF strength alone because crystal packing and symmetry also matter. Volatile means a substance evaporates readily and has relatively high vapor pressure.
Volatile means high vapour pressure, which means weak intermolecular forces.
Like dissolves like
Dissolving requires separating solute particles and solvent molecules, then forming new solute–solvent attractions. Both the energy changes and the entropy change matter, so “like dissolves like” is a useful first clue, not a complete explanation. Water hydrates Na⁺ and Cl⁻ ions strongly, offsetting much of the energy needed to separate the crystal; NaCl dissolution is slightly endothermic near room temperature, yet mixing increases entropy and dissolution is favorable. Hexane cannot stabilize separated ions effectively, so NaCl does not dissolve appreciably in it.
Control competing factors in a property claim
A boiling-point explanation compares energy needed to separate particles, not energy to break covalent bonds inside them. If two substances have the same formula, their mass and electron count are controlled, making shape or hydrogen bonding easier to isolate. If their sizes differ greatly, stronger dispersion may outweigh a permanent dipole; avoid treating an IMF ranking as a universal rule without the comparison.
State what is held constant, what attraction differs, and how that changes the property.
Key ideas
Rule
Justification template
Structural feature, then intermolecular force comparison, then property consequence.
Rule
Vapour pressure is inverse
Stronger IMFs mean lower vapour pressure and lower volatility.
Rule
Choose the control
Similar size means polarity decides; very different size means dispersion usually decides.
Rule
Like dissolves like
Solubility is high when solute and solvent share the same dominant interaction.
Worked examples
Worked example 2
Which is more volatile, diethyl ether or 1-butanol? Both are C₄H₁₀O.
Try it first: Note the identical formula, then hunt for an O-H bond.
0 of 3 steps revealed.
Worked example 3
Explain why NaCl dissolves in water but not in hexane.
Try it first: Ask what would have to attract the ions in each solvent.
0 of 3 steps revealed.
Worked example 4
Why does water boil much higher than hydrogen sulfide H₂S despite their similar molar masses?
Try it first: Compare the attractions between intact molecules.
0 of 3 steps revealed.
Common mistakes
Answering with only the name of a force.
Why it's wrong: The link from structure to force to property is where the reasoning lives.
Check instead: Write all three steps every time.
Saying stronger IMFs give higher vapour pressure.
Why it's wrong: Stronger attraction keeps molecules in the liquid, lowering vapour pressure.
Check instead: Remember vapour pressure is the one inverse property.
Comparing polarity across substances of very different size.
Why it's wrong: Dispersion scales with electron count and can overwhelm polarity.
Check instead: State which variable is controlled before comparing.
After the lesson · Question bank
Independent practice
Work through new problems on this topic on your own. The lesson's quick checks and worked examples stay above if you need to review a step.
No questions are available for this topic yet. Explore the unit question bank.
Your next step
Lesson: Not marked completed
Marking a lesson completed records that you finished reading it. Practice establishes mastery separately. Saved on this device.
Continue through the unit’s lessons and available practice.
What you should now know
A strong property explanation links molecular structure to intermolecular attractions and then to the observed property. Compare size, polarizability, polarity, and hydrogen bonding instead of assuming one factor always wins. Stronger cohesion usually raises boiling point, viscosity, surface tension, and heat of vaporization and lowers vapor pressure; melting point also depends on crystal packing. Solubility depends on both the attractions formed and the entropy of mixing, so “like dissolves like” is a starting rule rather than a full explanation.
- Justify with structure, then force, then property
- Control for size before comparing polarity
- Vapour pressure falls as IMF strength rises
- Like dissolves like explains most solubility questions
Sources and further reading
- Chemistry 2e, Section 10.1: Intermolecular ForcesOpenStax · Rice University · 10.1View source
CC BY 4.0 License