Final answer:
In a double-replacement reaction, two ionic compounds exchange anions to form new compounds, potentially a solid precipitate if one product is insoluble in water. Solubility rules help determine the state of the products. The occurrence of solid precipitates or molecular compounds like water indicates a chemical reaction.
Step-by-step explanation:
A double-replacement reaction occurs when two ionic compounds in aqueous solution exchange anions, leading to the formation of two new compounds. To determine if a reaction has occurred, one of the new compounds must be insoluble in water, which generally manifests as a solid precipitate, a gas, or a molecular compound such as water. For example, when combining aqueous solutions of calcium nitrate, Ca(NO3)2, and potassium bromide, KBr, a precipitation reaction may occur if the resulting product is insoluble.
In order to ascertain whether a product is solid or aqueous, you can refer to solubility rules or consult a solubility chart that indicates which compounds are soluble or insoluble in water. For example, most nitrates are soluble, so if a product is a nitrate it is likely to remain in aqueous form. If a new compound is formed that is not soluble, it will precipitate out of solution as a solid. For instance, when hydrochloric acid, HCl(aq), reacts with sodium hydroxide, NaOH(aq), the products are sodium chloride, NaCl(aq), and water, H₂O, where water is formed as a molecular compound.