Final answer:
Single-replacement reactions are redox reactions where one element replaces another in a compound, causing changes in oxidation numbers. These reactions illustrate the changing reactivity of metals, where a more reactive metal can replace a less reactive one.
Step-by-step explanation:
Single-replacement reactions are always redox reactions. A single-replacement reaction is where one element replaces another similar element in a compound, following the general form A+BC → AC+B. This is a type of oxidation-reduction reaction because it involves a change in the oxidation numbers of the elements. An element in its free state (oxidation number of zero) replaces an element that is part of a compound, leading to a change in oxidation numbers. For instance, if a metal A displaces another metal B in a compound BC, the metal B is oxidized (since it goes from being part of a compound to being a free element), and the metal A is reduced (since it goes from being a free element to part of a compound).
Examples of single-replacement reactions include the acid oxidation of certain metals and cases where a non-metal replaces another non-metal in a compound. The reactions where metals replace other metals are more common, and they underline the concept of metals having varying reactivities, where a more reactive metal will displace a less reactive metal from its compound.