Acidity order: Carboxylic acids (pKa ~5) > Phenols (pKa ~10) > Water (pKa ~15.7) > Alcohols (pKa ~16-18). Phenol is more acidic than alcohol because the phenoxide ion (C6H5O-) is resonance-stabilised — the negative charge delocalises into the benzene ring. Alcohol alkoxide (RO-) has no such stabilisation. Substituent effects on phenol acidity: EWG (NO2, CN, COCH3) increase acidity by stabilising phenoxide (especially at para > ortho due to resonance). EDG (CH3, OCH3, NH2) decrease acidity by destabilising phenoxide. Order: p-nitrophenol (7.15) > p-chlorophenol (9.38) > phenol (9.98) > p-cresol (10.26) > p-methoxyphenol (10.20). Intramolecular H-bonding in o-nitrophenol makes it more volatile (lower b.p.) and less soluble in water than p-nitrophenol.