van der Waals Forces — Predicted 2026
AI-Predicted Question Angles for UPSC 2026
Comparative Boiling Points with Mixed IMFs
highNEET frequently tests the ability to compare boiling points of compounds where different types of intermolecular forces are at play, including LDFs, DDFs, and hydrogen bonding. A common trap involves comparing a large nonpolar molecule (strong LDFs) with a smaller polar molecule (DDFs or H-bonding). Students need to accurately assess the cumulative strength of all IMFs. For example, comparing $ ext{CH}_3 ext{OH}$ (H-bonding) with $ ext{C}_5 ext{H}_{12}$ (strong LDFs) or $ ext{H}_2 ext{S}$ (DDFs) with $ ext{SiH}_4$ (LDFs). The question might involve a series of compounds requiring a nuanced understanding of relative strengths.
Effect of Isomerism on Physical Properties
mediumQuestions comparing the boiling points or melting points of structural isomers (e.g., n-pentane vs. neopentane) are a classic way to test the understanding of how molecular shape and surface area influence the strength of London Dispersion Forces. This angle assesses a deeper understanding beyond just molar mass, requiring students to visualize or interpret molecular geometry and its impact on intermolecular contact. It's a good discriminator for students who truly grasp the concept of polarizability and contact area.
Solubility and Intermolecular Forces
mediumWhile less frequent than boiling point comparisons, questions relating solubility to intermolecular forces are important. This could involve explaining why nonpolar substances dissolve in nonpolar solvents (LDF-LDF), polar in polar (DDF-DDF, H-bond-H-bond), or the limited solubility of nonpolar gases in polar solvents (dipole-induced dipole forces). A question might present a scenario and ask to identify the primary force responsible for solubility or insolubility, requiring students to apply the 'like dissolves like' principle with a detailed understanding of the underlying IMFs.