All the halogens combine directly with hydrogen to form halogen acids but their reactivity progressively decreases from fluorine to iodine, H₂ + X₂ 2HX (X = F, Cl, Br or I).

(i) Boiling points or volatility : In other words volatility decreases in the order : HCl > HBr > HI > HF as the boiling points increase in the order : HCl (189K) < HBr (206K) < HI (238K) < HF (292.5K).

(ii) Thermal stability : Thermal stabilityof the hydrides decrease from HF to HI i.e., HF > HCl > HBr > HI.

(iii) Acidic strength : The acidic strength of halogen acids decreases from HI to HF i.e, HI > HBr > HCl > HF.

(iv) Reducing properties: Since the stability of hydrides decreases from HF to Hl, their reducing properties increase in the order HF < HCl < HBr < HI.

(v) Dipole moments : The dipole moments of hydrogen halides decrease in the order : HF > HCI > HBr > HI as the electro negativity of the halogen atom decreases form F to I.

HX HF HCl HBr Hl