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When a flask containing sodium thiosulfate is placed over a paper marked with ‘X’ and HCl added to it, the ‘X’ seems to disappear when looked through the flask. Why?

When a flask containing sodium thiosulfate is placed over a paper marked with ‘X’ and HCl added to it, the ‘X’ seems to disappear when looked through the flask. Why? Correct Answer Because of sulfur precipitating

When sodium thiosulfate is treated with hydrochloric acid, sodium chloride is formed along with elemental sulfur. This yellow sulfur precipitates and causes the mark ‘X’ to fade away.

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