Precipitation Reaction

Table Of Contents

What is Precipitation Reaction?
How to Balance Precipitation Reactions
Examples of Precipitation Reaction
Precipitation Reaction Examples in Real Life
Practical Uses of Precipitation Reaction

What is Precipitation Reaction?

A precipitation reaction forms an insoluble salt when two soluble salts combine. The resulting solution is visibly cloudy due to the formation of small aggregation of the insoluble salt. This insoluble salt falls out of the solution and is filtered to form the precipitate. Precipitation reactions can help determine the presence of various ions in solution, most of which are formed from transition metals. This reaction is a double-replacement reaction ^[1-7].

General Equation

The general equation of the precipitation chemical reaction is given by:

AB + CD → AD + BC

Predicting Precipitation Reactions

The fundamental rule used to tell whether a precipitate will form or not is the solubility rule. According to this rule, if an ion is soluble, then it remains in its aqueous ion form. If an ion is insoluble, it forms a solid with an ion from the other reactant. If all the ions in a reaction are shown to be soluble, then no precipitation reaction occurs. The following table shows the solubility of various compounds. This table can be used to find and identify a precipitate ^[1,2].

Compounds soluble in water	Exception
All compounds of Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, and NH₄⁺	None
All compounds of NO₃⁻ and C₂H₃O₂⁻	None
Compounds of Cl⁻, Br⁻, and I⁻	Ag⁺, Hg₂²⁺, and Pb²⁺
Compounds of SO₄^2-	Hg₂²⁺, Pb²⁺, Sr²⁺, and Ba²⁺
Compounds insoluble in water	Exception
Compounds of CO₃²⁻ and PO₄³⁻	Compounds of Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, and NH₄⁺
Compounds of OH⁻	Compounds of Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, NH₄⁺, Sr²⁺, and Ba²⁺

How to Balance Precipitation Reactions

Precipitation reaction can be balanced by ensuring that an equal number of atoms are present on both sides of the equation. Consider the following example:

Iron (III) chloride (FeCl₃) and sodium hydroxide (NaOH) react to form sodium chloride (NaCl) and insoluble iron (III) hydroxide (Fe(OH)₃).

FeCl₃ (aq.) + NaOH (aq.) → NaCl (aq.) + Fe(OH)₃ (s/ppt.)

The equation is unbalanced. In order to balance the equation, we look at the Fe, Na, Cl, and OH^–. We notice that there are three Cl atoms on the left-hand side of the equation and only one on the right. So, we multiply the compound NaCl by 3 and obtain the following:

FeCl₃ (aq.) + NaOH (aq.) → 3 NaCl (aq.) + Fe(OH)₃ (s/ppt.)

Next, we notice that Na is unbalanced, as there are three atoms on the right and only one on the left. So, we multiply NaOH by 3 and obtain:

FeCl₃ (aq.) + 3 NaOH (aq.) → 3 NaCl (aq.) + Fe(OH)₃ (s/ppt.)

Examples of Precipitation Reaction

Here are some examples of precipitation reactions between salts of metals dissolved in aqueous solution ^[1-7].

Lead nitrate (Pb(NO₃)₂) and potassium iodide (KI) react to form potassium nitrate (KNO₃) and insoluble lead iodide (PbI₂).

Pb(NO₃)₂ (aq.) + 2 KI (aq.) → 2 KNO₃ (aq.) + PbI₂ (s/ppt.)

Barium chloride (BaCl₂) reacts with sodium sulfate (Na₂SO₄) to form sodium chloride (NaCl) and insoluble barium sulfate (BaSO₄).

BaCl₂ (aq.) + Na₂SO₄ (aq.) → 2 NaCl (aq.) + BaSO₄ (s/ppt.)

Mixing solutions of calcium chloride (CaCl₂) and sodium carbonate (Na₂CO₃) forms the precipitated calcium carbonate (CaCO₃) along with sodium chloride (NaCl)

CaCl₂ (aq.) + Na₂CO₃ (aq.) → 2 NaCl (aq.) + CaCO₃ (s/ppt.)

In the chemical reaction between potassium chloride (KCl) and silver nitrate (AgNO₃), silver chloride (AgCl) is precipitated out, and potassium nitrate (KNO₃) remains in solution.

KCl (aq.) + AgNO₃ (aq.) → KNO₃ (aq.) + AgCl (s/ppt.)