Answer: precipitate.
Step-by-step explanation:
1) When the product of the reaction between the chemicals in the solution is insoluble, then it cannot remain in the solution and it will precipiate, forming a solid substance.
2) That happens, for example, when you have two solutions with different solutes, mix them together, the solutes can react chemically with each other, yielding a product that is insoluble.
3) A typical example of such kind of precipitation reactions is athe neutralization of an acid and a base.
Acid + Base = salt + water
If the salt is insoluble in water then it will form a a precipitate (solid), if the product is soluble in water then it remains as an aqueos ions in the solution.