Final answer:
A gas-evolution reaction is one that produces a gas, and among the options provided, the neutralization of an acid with a base, specifically a carbonate base, results in the evolution of carbon dioxide (CO2) gas, thereby fitting the definition of a gas-evolution reaction, the correct option is 2).
Step-by-step explanation:
A gas-evolution reaction is a chemical process that produces a gas. When determining which of the provided options is a gas-evolution reaction, we must identify the reaction in which gas is one of the products.
1) The combustion of methane (CH4) with oxygen (O2) forms carbon dioxide (CO2) and water (H2O), both in gaseous states under combustion conditions. However, this is more commonly characterized as a combustion reaction, as explicitly stated in the given examples.
2) The decomposition of water (2 H2O) into hydrogen (H2) and oxygen (O2) gases is classified as an electrolytic decomposition reaction and does indeed result in the evolution of gases.
3) The oxidation of iron refers to a reaction typically resulting in a solid, such as rust, and does not involve the release of gas under standard conditions.
4) Neutralization of an acid with a base often produces a salt and water, and sometimes a gas, depending on the specific reactants. For example, when a carbonate is used as the base, carbon dioxide (CO2) gas is released, as with the reaction of nitric acid and sodium carbonate.
Among the options given, number 4) the neutralization of an acid with a base, specifically when the base is a carbonate, fits the criteria for a gas-evolution reaction as it releases carbon dioxide (CO2) gas.