Final answer:
A double-replacement reaction occurs when two elements in a compound swap places with another set of elements in a different compound.
Step-by-step explanation:
A double-replacement reaction, also known as a metathesis reaction, occurs when two elements in a compound swap places with another set of elements in a different compound. This type of reaction involves the exchange of partners between two substances. For example, consider the reaction:
AB + CD → AD + CB
In this reaction, the elements A and B from compound AB swap places with elements C and D from compound CD, forming new compounds AD and CB.