Practice a College Board-style short-answer question on Intermolecular Forces & Properties. Write your response, then reveal the model answer to see exactly what earns each point.
Short Answer Question · Unit 3 · Intermolecular Forces & Boiling Point
The table below shows the normal boiling points of four substances with similar molar masses.
Substance
Molar mass (g/mol)
Boiling point (°C)
Propane, C₃H₈
44.1
−42
Dimethyl ether, CH₃OCH₃
46.1
−24
Ethanol, CH₃CH₂OH
46.1
78
Formic acid, HCOOH
46.0
101
A
Identify the strongest intermolecular force present in propane, and explain why this force is the only one available in this molecule.
✓ Model answer (earns the point)
The strongest (and only) intermolecular force in propane is London dispersion forces. Propane is a nonpolar molecule (C and H have very similar electronegativities, and the molecule's shape gives no net dipole), so it has no permanent dipole to support dipole-dipole forces or hydrogen bonding — only the temporary, fluctuating dipoles responsible for London dispersion forces are possible.
Why it scores: Names London dispersion forces specifically AND explains why (nonpolar molecule, no permanent dipole) rather than just stating "it's nonpolar."
B
Explain why ethanol has a significantly higher boiling point than dimethyl ether, even though the two compounds have the same molar mass and molecular formula (isomers).
✓ Model answer (earns the point)
Ethanol has an O–H bond, allowing it to form hydrogen bonds between molecules. Dimethyl ether has its oxygen bonded only to two carbons (no O–H bond), so it can only participate in weaker dipole-dipole forces and London dispersion forces. Because hydrogen bonds are significantly stronger than dipole-dipole forces, more energy is required to separate ethanol molecules into the gas phase, giving ethanol a much higher boiling point despite identical molar mass.
Why it scores: Identifies the structural difference (O–H bond present vs. absent) AND names the specific IMFs in each compound AND connects the stronger IMF to the higher boiling point.
C
Formic acid has an even higher boiling point than ethanol, despite having a similar molar mass and also being capable of hydrogen bonding. Propose one structural reason that could explain formic acid's higher boiling point.
✓ Model answer (earns the point)
Formic acid (HCOOH) has a carboxylic acid group with two oxygen atoms capable of accepting hydrogen bonds (the C=O oxygen and the O–H oxygen), allowing each formic acid molecule to form more hydrogen bonds per molecule than ethanol, which has only one oxygen. More hydrogen bonds per molecule means more total energy is required to separate the molecules into the gas phase, raising the boiling point even further than ethanol's single-hydrogen-bond-donor structure.
Why it scores: Proposes a specific, chemically reasonable structural difference (more H-bond acceptor/donor sites) AND connects it to increased total IMF strength and thus higher boiling point. A vague answer like "more bonds" without identifying which atoms support extra hydrogen bonding would not earn full credit.
How to score points on SAQs
Always name the specific IMF, not just "intermolecular forces." "Hydrogen bonding" scores higher than "strong forces."
Connect structure to IMF to property. The strongest answers trace the full chain: molecular structure → which IMFs are present → how that explains the observed property (boiling point, solubility, etc.).
When comparing isomers or similar-mass molecules, look for structural differences. Same formula doesn't mean same IMFs — check for differences in bonding (e.g., O–H vs. C–O–C).
Answer the actual question. "Identify" needs a name. "Explain" needs a name plus a reason.
Keep it tight. 1–3 sentences per part is plenty. Long answers don't score higher; they just waste exam time.