According to Arrhenius concept, acids are those compounds which give hydrogen ions in aqueous solution and bases are those compounds which give hydroxide ions in solution. Among halides of hydrogen intermolecular H-bonding is present. So, when we go top to bottom in halogen group, size of I− ion increases and the inter molecular H-bonding becomes weak and easily gives H+ in aqueous solution. So, it works as a strong acid. Acidity decreases in this order. HI>HBr>HCl>HF