Explain why the reactivity series must be used for single replacement reactions, but not for double replacement.

Here's what I get

Explanation:

In a single replacement reaction, one element must displace another element from its compound.

Thus, we must consider the reactivity series, because an element lower down in the series cannot replace one that is above it.

In a double replacement reaction, the cations change partners with the anions. All we need consider are the solubilities of the new cation-anion partners.