Precipitation Reaction: Examples, Properties and Uses

A precipitation reaction is a chemical reaction in an aqueous solution that takes place by the combination reaction of two ionic bonds which results in the formation of an insoluble salt called the precipitate. Since the precipitate is formed by the exchange of the ions in the aqueous solution, it is also called a double displacement reaction. Precipitation reaction yields a soluble and an insoluble substance in the aqueous solution.

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Examples of Precipitation Reaction

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Example 1: Take two aqueous salts, namely, silver nitrate (AgNO₃) and potassium chloride (KCl). The precipitation reaction between aqueous AgNO₃ and aqueous KCl is:

\(AgNO_{3(aq)} + KCl_{(aq)} \rightarrow AgCl_{(s)} + KNO_{3(aq)}\)

Ag and K are positive ions while negative ions NO₃^-and Cl^- displace the positions to form an insoluble solid precipitate of AgCl and a soluble aqueous solution of KNO₃.

The ionic equation for the precipitation reaction of aqueous AgNO₃ and aqueous KCl is:

\(Ag^+NO_3^- + K^+Cl^- \rightarrow Ag^+Cl^-_{(s)} + K^+NO_{3(aq)}^-\)

The precipitate of AgCl is white in color.

Example 2: Take two aqueous solutions of sodium sulphate (Na₂SO₄) and barium chloride (BaCl₂). The precipitation reaction between aqueous Na₂SO₄and aqueous BaCl₂is:

\(Na_2SO_{4(aq)} + BaCl_{2(aq)} \rightarrow BaSO_{4(s)} + 2NaCl_{(aq)}\)

Na and Ba are positive ions while negative ions \(SO_4^{2-}\) and \(Cl^-\) displace the positions to form an insoluble solid precipitate of BaSO₄and soluble aqueous solution of NaCl.

The ionic equation for the precipitation reaction of aqueous NaSO₄and aqueous BaCl₂ is:

\(Na^+SO_4^{2-} + Ba_2^+Cl^- \rightarrow Ba_2^+SO_{4(s)}^{2-} + 2Na^+Cl^-_{(aq)}\)

The precipitate of BaSO₄is white in color.

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Example 3: Take two aqueous solutions of Sodium hydroxide (NaOH) and Magnesium chloride (MgCl₂). The precipitation reaction between aqueous NaOH and aqueous MgCl₂is:

\(2NaOH_{(aq)} + MgCl_{2(aq)} \rightarrow Mg(OH)_{2(s)} + 2NaCl_{(aq)}\)

Na and Mg are positive ions while negative ions \(OH^-\) and \(Cl^-\) displace the positions to form an insoluble solid precipitate of Mg(OH)₂and soluble aqueous solution of NaCl.

The ionic equation for the precipitation reaction of aqueous NaOH and aqueous MgCl₂ is:

\(Na^+OH^- + Mg_2^+Cl^- \rightarrow Mg_2^+(OH)_2^- + 2Na^+Cl^-\)

The precipitate of Mg(OH)₂is white in color.

Example 4: Take two aqueous solutions of cadmium sulphate (CdSO₄) and potassium sulphide (K₂S). The precipitation reaction between aqueous CdSO₄and aqueous K₂S is:

CdSO₄(aq) + K₂S(aq) CdS(s) + K₂SO₄(aq)

Cd and K are positive ions while negative ions \(SO_4^{2-}\) and \(S^{2-}\) displace the positions to form an insoluble solid precipitate of CdS and a soluble aqueous solution of K₂SO₄.

The ionic equation for the precipitation reaction of aqueous CdSO₄ and aqueous K₂ S is:

\(Cd_2^+SO_4^{2-} + K_2^+S^{2-} \rightarrow Cd_2^+S^{2-} + K_2^+SO_4^{2-}\)

The precipitate of CdS is yellow to brownish.

Check Important Formulas for Chemical Reaction

Double displacement reactions are said to be precipitation reactions when the chemical reaction between two aqueous salts gives the products as a soluble aqueous solution and an insoluble salt precipitate.

Double displacement reaction

The solubility rules of the molecules determine whether the molecules in consideration will be soluble in water or aqueous solution. There are in total six solubility rules that have to be followed. The molecules in the precipitation reaction tend to form the solid insoluble precipitate.

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Precipitation Reaction: Uses

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Precipitation reaction helps to detect the presence of certain salts in a solution. It helps to segregate and understand the ions, both cations, and anions, in the solution. It also evaluates the soluble salts quantitatively and qualitatively. The separation of the products of the reactions is done with the help of a precipitation reaction. At a greater level, precipitation reaction has benefits in the medical and industrial sectors as well. For example, the toxic substances from water can be filtered out by precipitation reactions. It is also carried out to detect food adulteration, antigen, and antibodies to a greater sensitivity level. A precipitation reaction is also used for identifying stains of blood and semen. In wastewater treatment, precipitation reaction is used to separate heavy metals from usable water by separating the precipitate impurities.

The various properties associated with precipitation reaction and the insoluble precipitate formed are given below:

1. The chemical reaction called precipitation reaction takes place in an aqueous solution with the state as ionic.
2. For precipitation reaction, there is a dependency on various factors, for example, temperature, concentration of the solution, buffer solution, etc.
3. Precipitation reaction yields an insoluble solid precipitate in the aqueous soluble solution.
4. Double displacement reaction of the ions gives a precipitation reaction by the ionic exchange in the aqueous solution.
5. The solid precipitate of the precipitation reaction is crystalline in nature which suspends in the aqueous solution.
6. The precipitate and the aqueous solution after precipitation reaction can be separated out by various methods like filtration, centrifuging, etc.

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Read More: Carbon and Its Compounds Revision Notes

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Precipitation Reaction: Methods of Obtaining the Precipitate

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Some of the simple methods used to obtain the precipitate after the precipitation reaction are given below.

1. Filtration: In the process of filtration, the entire solution obtained after the precipitation reaction is poured into a funnel of fine pores. The solid precipitate sticks to the surface of the funnel and the soluble liquid pour down. The process has to be done repeatedly to obtain good results.
2. Centrifugation: In the process of centrifugation, the entire mixture is rotated at high speeds. The solid insoluble precipitate must be thicker as compared to the aqueous solution. Upon pouring out the fluid, the pellet, which is the compressed precipitate, is recovered through the solution. This centrifugation method is preferably more effective in comparison to the method of filtration.
3. Decantation: In the process of decantation, the solid insoluble precipitate is separated from the aqueous solution or fluid layer by the suction method. In a repeated manner, an extra amount of the soluble particles are added in order to segregate the entire mixture or solution from the insoluble solid precipitate.

Points to Remember

• A precipitation reaction is a reaction that takes place in an aqueous solution where the two ionic bonds combine and lead to the formation of an insoluble salt called the precipitate. 
• Precipitation reactions can be obtained through filtration, centrifugation, and decantation.
• It helps in the detection of salts present in a solution and differentiate the ions and understanding them.
• The precipitation reaction is also called the double displacement reaction since it forms through interchanging the ions in an aqueous solution.
• Precipitation reactions provides both soluble as well as insoluble substances present in an aqueous solution.