Gas Evolution Reactions

A gas evolution reaction is a chemical process that produces a gas, such as oxygen or carbon dioxide. In the following examples, an acid reacts with a carbonate, producing salt, carbon dioxide, and water, respectively.

Nitric acid reacts with sodium carbonate to form sodium nitrate, carbon dioxide, and water:

$2HNO_{3}(aq)+Na_2CO_{3}(aq) \rightarrow 2NaNO_{3}(aq)+CO_{2}(g)+H_{2}O(l)$

Sulfuric acid reacts with calcium carbonate to form calcium sulfate, carbon dioxide, and water:

$H_{2}SO_{4}(aq)+CaCO_{3}(aq) \rightarrow CaSO_{4}(aq)+CO_{2}(g)+H_{2}O(l)$

Hydrochloric acid reacts with calcium carbonate to form calcium chloride, carbon dioxide, and water:

$2HCl(aq)+CaCO_{3}(aq) \rightarrow CaCl_{2}(aq)+CO_{2}(g)+H_{2}O(l)$

The following setup demonstrates this type of reaction:

Reaction of acids with carbonates

In this reaction setup, lime water is poured into one of the test tubes and sealed with a stopper. A small amount of hydrochloric acid is carefully poured into the remaining test tube. A small amount of sodium carbonate is added to the acid, and the tube is sealed with a rubber stopper. The two tubes are connected. As a result of the acid-carbonate reaction, carbon dioxide is produced and the lime water turns milky.

The test tube on the right contains limewater (a solution of calcium hydroxide, Ca(OH)₂). On the left, a solution of hydrochloric acid has been added to a solution of sodium carbonate to generate $CO_2(g)$. The test tubes are sealed with rubber stoppers and connected with a delivery tube. As the reaction proceeds, the limewater on the right turns from clear to milky; this is due to the $CO_2(g)$ reacting with the aqueous calcium hydroxide to form calcium carbonate, which is only slightly soluble in water. The entire experiment is illustrated in the following video:

When this experiment is repeated with nitric or sulfuric acid instead of HCl, it yields the same results: the clear limewater turns milky, indicating the production of carbon dioxide.

Oxidation of Metals in Acidic Solution

The oxidation of a metal in acidic solution will yield a metal salt and hydrogen gas:

Acid and metal reaction

Hydrochloric acid oxidizes zinc to produce an aqueous metal salt and hydrogen gas bubbles.

Recall that oxidation refers to a loss of electrons, and reduction refers to the gain of electrons. In the above redox reaction, neutral zinc is oxidized to Zn²⁺, and the acid, H⁺, is reduced to H₂(g). The oxidation of metals by strong acids is another common example of a gas evolution reaction.