Solutions can undergo a chemical reaction to produce an insoluble solid. The solid is called the precipitate, appearing as sediment at the bottom of the solution or as a suspension in the solution. Precipitating solutions can produce colorful results, causing clear solutions to become opaque and making liquids change color. Precipitation is used to identify some of the chemical components of solutions, to produce valuable metals from solutions and to remove contaminants from liquids. Some of the most important industrial and chemical processes rely on precipitation.

Some precipitating reactions are among the most interesting of chemical experiments. For example, when a clear and colorless solution of silver nitrate is poured into a clear and colorless solution of sodium chloride, a white precipitate of silver chloride forms. Sodium hydroxide added to copper sulfate produces a blue copper hydroxide precipitate. Ferric nitrate added to sodium hydroxide results in a precipitate of reddish brown iron hydroxide and adding potassium chromate to lead acetate gives a yellow precipitate of lead chromate.

The distinctive colors of the precipitates make precipitate reactions useful for determining the presence of specific materials in solutions. Such reactions are a key tool for analyzing solutions to determine their chemical composition. The analyst adds a known chemical to the solution to be tested. If a specific color of powder or crystal precipitates out of the solution, the analyst knows that the corresponding metal or chemical is present.

Industry uses precipitation reactions to remove metals or metallic compounds from solutions. The goal is either to clean wastewater that is contaminated with metal ions or to retrieve metals for eventual sale. The reactions typically target metals such as:

• copper
  
• silver
  
• gold
  
• cadmium
  
• zinc
  
• lead

The industrial process introduces a new chemical into the solution and the metallic ions react with it to form a salt that precipitates out. Filtration, centrifuges or settling basins separate the precipitate from the water and further processing prepares the metallic precipitate for safe disposal or for the extraction of the valuable metals.

Whether for demonstrations, for chemical analysis or for industrial purposes, the ability to predict whether a precipitate will form when a chemical is introduced into an aqueous solution is critical. Solubility rules are guides for determining whether the salt produced by a reaction is soluble. Only insoluble salts will precipitate.

Phosphates (PO₄), carbonates (CO₃) and chromates (Cr0₄) are usually insoluble. Fluorides (F₂) and sulfides (S) are mostly insoluble. Most hydroxide salts (OH) and oxides (O) are either insoluble or only slightly soluble. The salts of the elements of the first column of the periodic table, such as sodium, potassium and lithium, are all soluble. While there are exceptions and specific chemical reactions may have to be tried out to see if a precipitate appears, these guidelines can be used for general direction. Using them provides a starting point for determining the type of reaction that will produce a precipitate.