Question

Why is that the first jar of hydrogen gas collected during it's preparation is discard

Answer 1

Hydrogen gas is almost insoluble in water,it can be collected by displacement of water using an inverted apparatus. Hydrogen is highly explosive when mixed with air. Hydrogen is explosive when mixed with air so burns with explosion near the mouth of gas jar but burns out inside the jar because the burning requires the presence of oxygen which in not present inside the jar.

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