Q.vi. Which of the following occurs in liquid state at room temperature ? A. HIO$_3$
B. HBr
C. HCl
D. HF

Group 16 elements react with hydrogen to form angular, sp3-hybridised hydrides of the type H2E (H2O, H2S, H2Se, H2Te, H2Po); H2O is a colourless, odourless liquid, while the rest are colourless, foul-smelling, poisonous gases. These hydrides are all weakly acidic, with acidity increasing and thermal stability decreasing from H2O to H2Te, because the H-E bond dissociation enthalpy falls steadily down the group; all except H2O also show reducing power, increasing H2S < H2Se < H2Te. Group 17 elements react with hydrogen (H2 + X2 -> 2HX) to give the hydrogen halides HF, HCl, HBr, HI; acidic strength increases HF < HCl < HBr < HI and thermal stability decreases in the same order HF > HCl > HBr > HI, both tracking the falling H-X bond dissociation enthalpy down the group. HF stands out as the only hydrogen halide …