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JEE Advanced levelSingle correct optionDifficulty 3 of 3ev145

A stagnant lake has zero dissolved oxygen at the bottom, black mud, and a smell of rotten eggs. The gas responsible and its origin are:

A $SO X_{2}$ , formed by aerobic oxidation of sulfur compounds
B $O X_{3}$ , formed from dissolved oxygen
C $H X_{2} S$ , formed by anaerobic bacteria reducing sulfur compounds
D $CO$ , formed by complete aerobic respiration

Answer: C

Once the oxygen is exhausted by the high BOD, anaerobic bacteria take over. They reduce sulfate and sulfur-containing proteins to

H X_{2} S

, which smells of rotten eggs and precipitates black metal sulfides in the mud. Aerobic processes give

CO X_{2}

and sulfate, not

H X_{2} S

.

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