an aqueous solution of Ca(OH)2; a common indicator used to detect the presence of carbon dioxide gas
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:
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:
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.