There are a number of observations that indicate a chemical reaction has occurred. One is the formation of a precipitate. A precipitate is a solid formed in a chemical reaction that is different from either of the reactants.
This can occur when solutions containing ionic compounds are mixed and an insoluble product is formed. The identity of the precipitate can often be determined by examining solubility rules.
It also occurs in single displacement when one metal ion in solution is replaced by another metal ion. Notice the new solid forming on the bottom of the tube.