Thermal decomposition of hydrogen halides

Steps:

• Hydrogen halides decompose on heating: 2HX → H₂ + X₂, where X is halogen.
• Bond dissociation energy decreases down the group (H-F > H-Cl > H-Br > H-I), so HI decomposes more readily than HBr at the same temperature.
• I₂ vapor is purple; Br₂ vapor is brown.
• Thus, HI shows purple vapor; HBr shows no visible change.

Why D is correct:

• HI decomposes to H₂ and purple I₂ vapor (from 2HI → H₂ + I₂), while HBr's stronger H-Br bond prevents noticeable decomposition.

Why the others are wrong:

• A: HBr does not decompose to brown Br₂ vapor; HI would show change.
• B: HI produces purple I₂, not brown vapor.
• C: HBr neither decomposes nor produces purple vapor; HI yields purple, not brown.

Final answer: D