Reactions may be done in solution

Many reactions in your body and many in your environment occur between substances in solution. Chemists use reactions in solution both to prepare substances and to determine the composition of unknown substances. This is because

reactions may be faster in solution where there is better mixing of the reactants.
working with a small quantity of substance in a manageable volume of solution is easier than working with a small mass of the pure substance

Titration is an example of using a reaction in solution to determine the composition of an unknown substance.

The apparatus for doing the reaction is shown at the right.

One reactant is in solution in a conical flask and the other is in solution in the burette.
The reactant in the burette is added gradually to the reactant in the conical flask.
The addition stops when all of the reactant in the flask is consumed.
If the amount of one of the reactants is known, the amount of the other can be calculated.

The solution containing a known concentration of one reactant is referred to as the standard solution.