Question

Is precipitation always involved in a double displacement reaction?

Answer 1

Not necessarily.

Explanation:

There are a variety of spontaneous double displacement reactions. Some but not all of them involve the production of a precipitate.

For example, the neutralization reaction between hydrochloric acid `"HCl"` and sodium hydroxide `"NaOH"` counts as a double displacement reaction. However, this reaction produces no precipitate.

`"NaOH" (aq) + "HCl" (aq) color(navy)(to) "NaCl" (aq) + "H"_2"O"color(purple)((l))`

Neutralization reactions are feasible as double displacement reaction thanks to the production of water `"H"_2"O"`, a weak electrolyte that barely disassociate on its own. Meaning that as the reaction proceeds, the formation of water would act like a `"H"^(+)` / `"OH"^(-)` sink that continuously removes the two ions from the system.

Other types of reactions, such as those involving the evolution of a gas e.g., `"CO"_2` or `"NH"_3` can also be spontaneous. Here's a list of them on the English Wikipedia.