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
If the two substances differ greatly in size, dispersion usually wins regardless of polarity. If they are similar in size, polarity or hydrogen bonding decides. Say which comparison you are making, otherwise the argument can be reversed by a counterexample.
Properties that rise with IMF strength
Stronger IMFs raise boiling point, melting point, viscosity, surface tension and heat of vaporisation, and lower vapour pressure. Vapour pressure is the one that moves the other way, since fewer molecules can escape the surface.
Volatile means high vapour pressure, which means weak intermolecular forces.
Like dissolves like
Solubility is best when the solute and the solvent form similar interactions. Ethanol mixes with water in all proportions because both hydrogen bond; hexane does not dissolve in water because breaking water hydrogen bonds is not repaid by dispersion with hexane.
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.
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.
Practice this skill
No practice questions are available for this topic yet. You can still practice the whole unit.
What you should now know
A full justification identifies the relevant IMFs in both substances, compares them on a controlled basis such as size or polarity, and states the property consequence. The same reasoning chain works for boiling point, vapour pressure, viscosity, surface tension and solubility.
- 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
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