The formation of what three classes of substances cause double displacement reactions to occur

The three substances that cause a double displacement reaction to occur are:
(i)   Ionized compounds. These are usually water.
(ii)   A gas that bubbles out a solution.
(iii)  A precipitate.

In double displacement reaction is where two new compounds are formed when two parts of two compounds switch places. For example; when sodium chloride combines with silver nitrate, the two new compounds formed are Sodium nitrate and silver chloride reason being sodium and silver switched places.

Where are they though?