Four oxyacids: HClO (hypochlorous, +1), HClO2 (chlorous, +3), HClO3 (chloric, +5), HClO4 (perchloric, +7). Acid strength increases with oxidation state: HClO < HClO2 < HClO3 < HClO4. Reason: more O atoms stabilise the conjugate base through delocalisation. HClO4 is one of the strongest known acids. Oxidising power decreases with increasing OS: HClO > HClO2 > HClO3 > HClO4. Reason: HClO is thermodynamically less stable, readily releases oxygen. Paradox: HClO4 is strongest acid but weakest oxidiser; HClO is weakest acid but strongest oxidiser. This inverse relationship is a key JEE concept.