Halogen physical states and reactivity trends

Steps:

• Recall halogens are F, Cl, Br, I; exist as X₂ molecules.
• Check states at room temp/pressure: F/Cl gases, Br liquid, I solid.
• Evaluate reactivity: decreases down group; F>Cl>Br>I.
• Assess color/volatility: color darkens, volatility decreases with increasing atomic mass.

Why A is correct:

• Bromine is a liquid at room temperature (20°C) and standard pressure, forming diatomic Br₂ molecules, unlike gaseous Cl₂ or solid I₂.

Why the others are wrong:

• B: Iodine is less reactive than bromine, so cannot displace Br⁻ from KBr(aq); displacement occurs top-down in reactivity series.
• C: Halogens lighten in color down the group (F pale yellow to I violet-black) as relative molecular mass increases.
• D: Volatility decreases with higher relative molecular mass due to stronger London forces in larger X₂ molecules.

Final answer: A