When does neutralization occur in a chemical reaction?

Neutralization occurs when an acid reacts with a base to form a salt and water. This process is characterized by the chemical reaction between hydrogen ions (H⁺) from the acid and hydroxide ions (OH⁻) from the base, which combine to form water (H₂O). The remaining ions from the acid and the base then form the salt.

This concept is foundational in acid-base chemistry. The products of the neutralization reaction—salt and water—are indicative of the reaction's completion. The formation of water is particularly significant, as it demonstrates the neutralizing effect of a base on an acid.

In the context of the other options: two acids reacting does not constitute neutralization, as there is no base to balance the pH. An acid reacting with an alkali, resulting in gas, does not describe the typical neutralization process, which primarily involves the formation of water. Additionally, while metals may react with acids, this leads to the formation of a salt and hydrogen gas, rather than the straightforward salt and water produced in a neutralization reaction. Thus, the defining characteristics of neutralization are encapsulated in the reaction of an acid with a base to yield salt and water.