Question

How will you test for the gas which is liberated when hydrochloric acid reacts with an an active metal

Answer 1

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Here's the answer you are looking for

When an acid reacts with a reactive metal, the metal displaces Hydrogen from the acid and hence salt is formed and hydrogen gas is released. 

Similarly, when Hydrochloric acid reacts with an active metal, hydrogen gas is released.

Now question is How will you test for the gas which is liberated when hydrochloric acid reacts with an an active metal?

Answer is:
When you bring a burning matchstick to the mouth of the test tube while the gas is being released, then the matchstick will blow off with a loud pop sound. This indicates the presence of Hydrogen gas.


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Answer 2

When HCl reacts with any active metal then it liberate Hydrogen gas.
Suppose that the metal is Sodium,then
2HCl+2Na gives 2NaCl+H2 gas will liberate.
For testing the liberating gas , we will take the solution in any test tube and cover it with cork.We have to take it on the flame.
Hydrogen gas has the property that it burns with pop sound.So,the gas which liberate will burn with the pop sound when it comes in contact with fire or its flame.