Question

why purified and dried hydrogen is collected over Mercury​

Answer 1

It is not collected by downward displacement of air as it forms explosive nature with air.It is collected over mercury in absence of air.

Answer 2

Purified and dried hydrogen is collected over mercury because mercury has no reaction with it. Dilute sulphuric acid cannot be replaced by concentrated acid in the preparation of hydrogen because it is a strong oxidising agent and it will produce sulphur dioxide.

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