Question

a double replacement reaction can be best described as a reaction in which
a substitution takes place.
two atoms of a compound are lost.
ions are exchanged between two compounds.
electrons are exchanged between two atoms.

Explanation:

To determine the correct description of a double replacement reaction, we analyze each option:

• "A substitution takes place" is more characteristic of single - replacement reactions, not double - replacement. In single - replacement, one element replaces another in a compound.
• "Two atoms of a compound are lost" does not match the definition of a double - replacement reaction. Double - replacement is about ion exchange, not atom loss.
• "Ions are exchanged between two compounds" is the correct definition of a double - replacement reaction. For example, in the reaction $\ce{AgNO_{3} + NaCl -> AgCl\downarrow+ NaNO_{3}}$, the $\ce{Ag+}$ and $\ce{Na+}$ ions (and also $\ce{NO_3^-}$ and $\ce{Cl^-}$ ions) are exchanged between the two compounds.
• "Electrons are exchanged between two atoms" describes oxidation - reduction (redox) reactions, where there is a transfer of electrons, not double - replacement.

Answer:

ions are exchanged between two compounds.