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Acidity comparison is one of the most frequently tested GOC topics in JEE. The master strategy: (1) Write the conjugate base for each acid, (2) Compare conjugate base stability, (3) More stable base = stronger acid. Factors stabilizing conjugate bases in priority order: resonance (carboxylate > phenoxide > alkoxide), electronegativity of the charge-bearing atom (across a period: F- > OH- > NH2- > CH3-), size/polarizability (down a group: I- > Br- > Cl- > F-), inductive effects (-I stabilizes, +I destabilizes), and s-character (sp > sp2 > sp3 for C-H acidity). Key acidity series to memorize: RCOOH > PhOH > H2O > ROH > RC≡CH > NH3 > RCH=CH2 > RCH2CH3. The ortho effect is a frequent JEE trap — ortho-substituted benzoic acids are generally more acidic than para isomers regardless of substituent nature, due to steric inhibition of resonance, intramolecular H-bonding in the conjugate base, and through-space field effects. For basicity in aqueous solution, remember that solvation effects modify the gas-phase order: (CH3)2NH > CH3NH2 > (CH3)3N > NH3 in water (secondary amine is the sweet spot between +I and solvation). Aniline is much weaker than cyclohexylamine because the N lone pair delocalizes into the ring. Steric inhibition of resonance (2,6-disubstitution) INCREASES basicity of anilines by blocking this delocalization.