Question

# Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?

Hint: When metals react with acids, a gas is released. This gas is the lightest gas and highly flammable. It is also used as clean fuel because of its high calorific value in space vehicles. When this gas is burned a pop sound is heard.

Complete step by step answer:
The reactivity series lists metals from the most reactive to the least reactive. A metal can displace a less reactive metal from its compounds.
As a general rule, acids react with most metals producing salt and hydrogen, unlike acid-base reactions in which case we get water.
Hence, the gas produced is hydrogen.
The word equation for this is:
$Metal + Acid \to Salt + Water$
Irrespective of the strength of acid and the nature of metal, if there is a displacement reaction with these reactants, we always obtain hydrogen gas. The rate of the reaction depends on the reactivity of the metals. Highly reactive metals like Potassium and Sodium which are placed at the top of the reactivity series reacts instantly with acids producing hydrogen. On the other hand, reactions with Zinc like metals occur at a slower rate.
Let us take the example of Magnesium and write the reaction:
$Mg + 2HCl \to MgC{l_2} + {H_2}$

We can confirm that the gas produced is hydrogen through the following test:
On introducing a burning splinter into the test tube where this reaction has taken place, we will see that the splinter extinguishes with a distinct pop sound as hydrogen is not a supporter of combustion.

Note:We should remember that all metals do not react with acids to produce hydrogen. In case of non-reactive metals which are below hydrogen in the reactivity series like gold, platinum or silver, no reaction takes place on adding acids.