Question

The addition of dilute $HCl$ to unknown metal produced a transparent gas. What is the likely identity of this gas?
$(A)\;\;\;C{l_2}$
$(B)\;\;\;{H_2}$
$(C)\;\;\;{O_2}$
$(D)\;\;\;C{O_2}$
$(E)\;\;\;N{O_2}$

Answer 1

Hint: Hydrochloric acid is the water based, or aqueous solution of hydrogen and chlorine gas. It is the main component of gastric acid, an acid produced naturally in the human stomach to help digest food. It is a colorless solution with a pungent smell. It is classified as a strong acid.

Complete answer:
Hydrogen gas is liberated when dilute hydrochloric acid reacts with a metal. Not all metals react with $HCl$ and liberate ${H_2}$ gas because of reactivity. Those metals which are placed below the hydrogen in the reactive series cannot displace hydrogen from hydrochloric acid. Thus, only those metals can displace hydrogen atoms which are placed above the hydrogen in reactive series, for example-Zinc.
Zinc reacts easily with the acid to produce hydrogen bubbles. It is a single displacement reaction in which hydrogen is displaced by zinc metal to form hydrogen gas and a salt, zinc chloride. To produce bubbles of hydrogen, zinc reacts rapidly with the acid. Zinc chloride is formed when $Zn$ is combined with dilute hydrochloric acid and hydrogen gas is liberated, the reaction is exothermic and reaction generates heat.
$Zn + 2HCl(aq) \to ZnC{l_2} + {H_2}$
Test for hydrogen:
To test the presence of hydrogen the following test can be performed, a lighted wooden splint makes a popping sound in a test tube of hydrogen. Characteristics tests for hydrogen gas can be performed by bringing a burning candle near the test tube. On doing so, hydrogen gas burns with a squeaky pop sound. Hydrogen gas is recognized by the ‘pop’ when it burns. The ‘pop’ is the sound of a small explosion.
Hence the correct answer is option B.

Note:
Hydrogen gas is highly flammable. ${H_2}$ is unreactive . The thermodynamic basis of this low reactivity is the very strong $H - H$ bond. The pop sound on liberation of hydrogen gas is because a lot of heat is released into the gaseous mixture and the air adjacent to it expands very quickly. The gaseous mixture contains hydrogen liberated from the reaction and because of this explosion we hear the pop sound.