Final answer:
A gas-evolution reaction is a chemical reaction where a gas is released as a product, such as in the reaction of nitric acid with sodium carbonate, which produces sodium nitrate, carbon dioxide, and water.
Step-by-step explanation:
A gas-evolution reaction is a type of chemical reaction where a gas is produced as a product. This happens when certain reactants undergo a chemical transformation, resulting in the release of a gas. A commonly taught example in a chemistry class is the reaction between an acid and a carbonate. For instance, when nitric acid reacts with sodium carbonate, the products are sodium nitrate, carbon dioxide, and water, represented by the following chemical equation:
Na2CO3(s) + 2HNO3(aq) → 2NaNO3(aq) + CO2(g) + H2O(l)
This reaction demonstrates the production of a gas, carbon dioxide (CO2), which bubbles out of the solution, alongside the formation of a salt (sodium nitrate) and water. Gas-evolving reactions play an important role in many industrial and biological processes, such as the water-gas shift reaction, where carbon monoxide and water vapor are converted to carbon dioxide and hydrogen:
CO(g) + H2O(g) → CO2(g) + H2(g)
These reactions differ from precipitation reactions in that, instead of forming a solid precipitate, they produce a gas as one of the products.