# Question #3ef7d

May 22, 2017

The difference is not completely clear-cut.

#### Explanation:

A reagent is a substance added to a system to cause a chemical reaction.

It is often dissolved in a solvent before it is added as a solution to the reaction mixture.

Chemists sometimes call a solution a reagent if it is used to test for the presence of another substance, for example, by a colour change or the formation of a precipitate.

For example, Tollens' reagent is a colourless, basic, aqueous solution containing diamminesilver ions $\left[\text{Ag"("NH"_3)_2^"+}\right]$.

In the Tollens test for aldehydes, these ions react with aldehydes to form a silver mirror on the inside of the container.

$\underbrace{\text{CH"_3"CHO(aq)")_color(red)("an aldehyde") stackrelcolor(blue)("Ag"("NH"_3)_2^"+""(aq)" color(white)(mm))(→) "CH"_3"COO"^"-""(aq)" + underbrace("Ag(s)")_color(red)("silver mirror}}$