What characterizes a double replacement reaction?

A double replacement reaction is characterized by the exchange of partners between two compounds, resulting in the formation of two new compounds. The general form of this reaction can be represented as AB + CD --> AD + CB, where the elements or ions from both reactants switch places to form new product compounds. In this transformation, typically anions and cations swap between the reacting species.

This type of reaction often occurs in aqueous solutions where ions from the reactants reassociate into different combinations, often leading to the formation of a precipitate, water, or a gas. The breaking and forming of ionic bonds during this process is key to understanding the dynamics of double replacement reactions.

The other options describe different types of chemical reactions or processes:

• A decomposition reaction describes a single reactant breaking down into two or more products, not a double replacement.

• A reaction between an ion and a molecule resulting in a new molecule is typically indicative of a different reaction type, such as a synthesis or combination reaction, not a double replacement.

• Excess energy released as a product could be characteristic of a combustion or exothermic reaction but does not specifically define a double replacement reaction.

Therefore, option A accurately captures the essential characteristics of a double replacement reaction.