ACT Science: Intermolecular Forces and Their Effects on Properties

Intermolecular forces are attractions between molecules. Strongest to weakest: (1) Ionic bonds (not technically intermolecular, but between ions). Example: NaCl. (2) Hydrogen bonds (between H and highly electronegative atoms like O, N, F). Example: Water molecules bonding to each other. (3) Dipole-dipole interactions (between polar molecules). Example: HCl molecules attracting. (4) Van der Waals/London dispersion forces (weakest; between all molecules). Example: Nonpolar gases like N2 and O2. On the ACT, you don't need to calculate strength, just recognize which type applies and predict how it affects boiling point, solubility, or phase. Stronger intermolecular forces lead to higher boiling points, greater solubility in polar solvents, and solids at room temperature.

Example: Water boils at 100°C due to hydrogen bonding. Methane boils at -161°C due to only van der Waals forces. Same idea, different forces, different properties.

Error 1: Assuming polar always means strong intermolecular forces. Polar molecules have dipole-dipole forces, but hydrogen bonding is stronger. Error 2: Forgetting that more molecules (higher molecular weight) often means stronger van der Waals forces, which can affect boiling point even without special interactions. Error 3: Confusing intramolecular forces (bonds within molecules, like O-H in water) with intermolecular forces (between molecules). The question will specify which. Error 4: Thinking that ionic compounds dissolve in nonpolar solvents. They don't (ionic bonds are too strong; nonpolar solvents can't break them). Always ask: "What forces hold these molecules or ions together?" Then predict the property based on force strength.

Quick checklist: (1) Is it a molecule or an ion? (2) Is the molecule polar or nonpolar? (3) Does it contain H bonded to O, N, or F (hydrogen bonding)? (4) What property does the question ask about? (5) Predict based on force strength.

Substance 1: H2O. Forces: Hydrogen bonding (O-H). Boiling point: 100°C (high). Substance 2: CH4. Forces: Van der Waals. Boiling point: -161°C (low). Substance 3: HCl. Forces: Dipole-dipole and van der Waals. Boiling point: -85°C (low-medium). Substance 4: NaCl. Forces: Ionic bonds. Boiling point: 1413°C (very high, it's a solid). Substance 5: C6H14 (hexane). Forces: Van der Waals (nonpolar). Boiling point: 69°C (low but higher than methane due to larger molecular weight). For each substance, write the dominant intermolecular force and predict whether it would be a solid, liquid, or gas at room temperature (25°C).

Daily drill: Pick five random molecules and identify their intermolecular forces. Predict boiling point and solubility in water. Check against reference data to verify. After five days, you'll spot patterns and answer these questions instantly.

About 1-2 questions per ACT Science section test intermolecular forces or properties tied to them. These questions often look intimidating but reduce to simple reasoning: "Stronger forces = higher boiling point/melting point, greater solubility in polar solvents, etc." If you memorize the four force types and their relative strengths, you can predict properties without advanced chemistry. This is a high-ROI skill: moderate memorization, straightforward logic, consistent points.

Spend one week on this topic and you'll unlock free points on otherwise tricky chemistry passages.