Final answer:
In a double displacement reaction, the positive ion switches with the negative ion (anion) of another compound, often leading to the formation of a precipitate, a gas, or a different molecular compound.
Step-by-step explanation:
In a double displacement reaction, also known as a double-replacement reaction, the positive ion (cation) of one compound switches places with the positive ion of another compound. This type of chemical reaction typically occurs between substances in aqueous solution and one of the products is often a solid precipitate, a gas, or a molecular compound such as water. The reaction can be generally represented as AB + CD → AD + BC, where A and C are positively-charged cations and B and D are negatively-charged anions. So, in such reactions, the positive ion always switches with the negative ion (also known as an anion).