[SPM Chemistry MRSM 2019 Question] How to solve this question?

R is hinted to be an antiseptic in drinking water, hence it must be chlorine/chloride related. It must belong to Group 17.

S is in Group 17, which can Bromine/Iodine/Fluorine.

I) Reacts with water to produce acid - (HBr, HF, HI) this is correct.
II) Reacts with iron to produce brown solid (FeBr3) - this is correct.
III) Reacts with NaOH to produce alkaline solution - NaBr or NaI are salts, hence neutral. Wrong
IV) Reacts with oxygen to produce black solid - This is wrong, as Group 17 elements cant react with oxygen from Group 16 to produce a solid

Therefore, answer should be A. I and II

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Ohh I seee.Thank youuu very much !:blush::+1::+1: