Precipitation Reaction: Examples, Properties and Uses

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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.


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 AgNOand 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 BaClis:

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

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

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.


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.

Organic Compounds Combustion Reaction

Precipitation Reaction: Properties

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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.

CBSE NCERT Solution PDF

Read More: Carbon and Its Compounds Revision Notes


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. 

Sample Questions

Ques. Explain the precipitation reaction. Give an example to support your answer. (3 marks)

Ans. Precipitation reaction takes place when in an insoluble solution the cations and anions combine together in order to form the precipitate. The precipitate is an ionic solid which is insoluble and is formed from a solution. 

Example: When we mix silver nitrate and sodium chloride we get:

\(AgNo_3(aq) + AaCl(aq)\rightarrow AgCl(s)\downarrow +NaNO_3(aq)\)

Due to the solid (AgCl) which is insoluble in water the precipitate forms.

Ques. Write the net ionic equation for the potential double displacement reactions. Make sure to include the states of matter and balance the equations. (4 marks)
\(Fe(NO_3)_3(aq) + NaOH(aq) \rightarrow\)
\(Al_2(SO_4)_3(aq) + BaCl_2(aq) \rightarrow\)

Ans. 1. In the case of the first reaction, the products are \(Fe(OH)_3 \) and \(NaNO_3\)\(NaNO_3 \) is soluble because all the nitrates are soluble according to rule 2 of solubility rules but \(Fe(OH)_3 \) is insoluble since hydroxides are considered to be insoluble according to rule 6 and Fe isn’t an exception. The following ionic bond takes place after dissociation.

\(Fe^{3+}_{(aq)} + NO_{3(aq)} + Na^+_{(aq)} \rightarrow Fe(OH)_{3(s)} + Na^+_{(aq)} + NO_{3(aq)}\)

After the cancellation, there is an absence of the spectator ions from the net ionic equation.

\(Fe^{3+}_{(aq)} + OH_{(aq)} \rightarrow Fe(OH)_{3(s)}\)

  1. After the double displacement reaction takes place \(AlCl_3\) and \(BaSO_4\) are the products formed. \(BaSO_4\) is insoluble but \(AlCl_3\) is soluble since it has chloride in it. \(BaSO_4\) has sulphate and \(Ba^{2+}\) ion makes it insoluble. The ionic equation after balancing forms to become the following:

 \(2Al^{3+}_{(aq)} + 6Cl^-_{(aq)} + 3Ba^{2+}_{(aq)} + 3SO^2_{4(aq)} \rightarrow 2Al^{3+}_{(aq)} + 6Cl_{(aq)} + 3BaSO_{4(s)}\)

After the cancellation, there is an absence of the spectator ions from the net ionic equation.

\(Ba^{2+}_{(aq)} + SO^{2-}_{4(aq)} \rightarrow BaSO_{4(s)}\)

Ques. Give two exceptions each for solubility in a precipitation reaction: chloride, sulfates, hydroxides, and carbonates. (4 marks)

Ans. The exceptions are:

Chloride – AgCl, PbCl2

Sulfates – BaSOand PbSO4

Hydroxides – KOH and NaOH

Carbonates – K2CO3 and Na2CO3

Ques. What happens if we mix barium chloride with magnesium sulfate? (2 marks)

Ans. When barium chloride is mixed with magnesium sulfate we obtain Barium sulfate which forms the precipitate along with magnesium chloride. 

Balanced Equation – \(MgSO_4 + BaCl_2 \rightarrow BaSO_4 + MgCl_2\)

Ques. Complete the following: (3 marks)
Sodium hydroxide + ________ → Sodium sulfate + Magnesium hydroxide
\(CaCl_{2(aq)} + --- \rightarrow CaSO_{2(s)} + NaCl_{(aq)} \)
\(Cu(NO_3)_{2(aq)} + NaOH_{(aq)}\rightarrow ------ + -------\)

Ans. 1. Sodium hydroxide + Magnesium sulfate → Sodium sulfate + Magnesium hydroxide

  1. \(CaCl_{2(aq)} + Na_2So_4 \rightarrow CaSO_{2(s)} + NaCl_{(aq)} \)
  2. \(Cu(NO_3)_{2(aq)} + 2NaOH_{(aq)}\rightarrow Cu(OH)_{2(aq)} + 2NaNO_{3(aq)}\)

Previous Year Questions

Ques. Identify the type of reactions taking place in each of the following cases and write the balanced chemical equation for the reactions.
(a) Zinc reacts with silver nitrate to produce zinc nitrate and silver.
(b) Potassium iodide reacts with lead nitrate to produce potassium nitrate and lead iodide. (2019)

Ans. The type of reactions taking place in each of the following cases are:

  1. It is a displacement reaction.

Zn(s) + 2AgNO3(aq) Zn(NO3)2(aq) + 2Ag(s)

  1. The given reaction is a double displacement reaction.

2KI(aq) + Pb(NO3)2(aq) 2KNO3(aq) + PbI2(s)

Ques. On heating blue colored powder of copper (II) nitrate in a boiling tube, black copper oxide, O2and a brown gas X is formed.
(a) Identify the type of reaction and gas X.(b) Write the balanced chemical equation of the reaction.
(c) Write the pH range of the aqueous solution of the gas X. (2019 Outside Delhi)

Ans. The answer to the above-mentioned questions are:

  1. The type of reaction taking place is decomposition reaction and the gas formed is Nitrogen dioxide (NO2) referred to as X.
  2. The balanced chemical equation of the reaction,
Balanced Chemical Equation
  1. The non-metal oxides are acidic in nature, hence, the aqueous solution of this particular gas would be acidic too and the pH would be less than 7.
Ques. Decomposition reactions require energy either in the form of heat or light or electricity for breaking down the reactants. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light, and electricity. (2018)

Ans. Decomposition reactions where energy is supplied in the form of heat, light, and electricity are:

  • Heat: When heat is applied to zinc carbonate it breaks down into zinc oxide and carbon dioxide.

ZnCO3ZnO + CO2

  • Light: When silver chloride is brought into contact with sunlight it breaks down into silver and chlorine. The colour of silver chloride is white which when broken into silver and chlorine the colour of silver changes to grey.

2AgCl 2Ag + Cl2

  • Electricity: When electricity is present in water, it breaks down into hydrogen and oxygen.

2H2O 2H2+ O2(g)

Ques. Identify the type of chemical reaction in the following statement and define each of them:
Digestion of food in our body.
Rusting of iron.
Heating of manganese dioxide with aluminium powder.
Blue colour of copper sulphate solution disappears when iron filings are added to it.
Dilute hydrochloric acid is added to the sodium hydroxide solution to form sodium chloride and water. (2016)

Ans. a. Decomposition Reaction: The decomposition reaction is a process under which a particular compound is fragmented into further simpler substances.

  1. Oxidation: Under the oxidation procedure the oxygen gets added or electrons get subtracted.
  2. Displacement Reaction: The displacement reaction is the process in which the less reactive metals get displaced by the more reactive metals from its salt solution.
  3. Displacement Reaction: The displacement reaction is the process in which the less reactive metals get displaced by the more reactive metals from its salt solution.
  4. Double Displacement Reaction: Under this reaction, two different compounds react by interchanging an ion and forming two new compounds.

Ques. Define chemical reaction. State four observations that help to determine whether a chemical reaction has taken place or not. Write one example of each observation with a balanced chemical equation. (2015)

Ans . The term chemical reaction refers to the reaction which denotes a chemical change by altering the molecular or ionic structure of a compound.

The four observations which help to determine whether a chemical reaction has taken place or not with one example each are:

  1. Change in the colour of the substance:

Cu(s) + 2AgNO3(aq) = Cu(NO3)(aq) + 2Ag(s)

2AgNO3(aq) is colourless

NO3is blue in colour

  1. Formation of gas:

CaCO3(s) (heat)CaO(s) + CO2(g)

  1. Precipitate formation:

Pb(NO3)2(aq) + 2KI PbI2(s) + 2KNO3(aq)

PbI2forms yellow precipitate

  1. Temperature change:

CaO(s) + H2O(l) Ca(OH)2+ Heat

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Decomposition Reaction Displacement Reaction

CBSE CLASS XII Related Questions

1.

The rate constant for the decomposition of hydrocarbons is 2.418 x 10-5 s-1 at 546 K. If the energy of activation is 179.9 kJ/mol, what will be the value of pre-exponential factor.

      2.

      Write equations of the following reactions: 
      (i)Friedel-Crafts reaction–alkylation of anisole.
      (ii)Nitration of anisole.

      (iii)Bromination of anisole in ethanoic acid medium.
      (iv)Friedel-Craft’s acetylation of anisole.

       

          3.

          Write down the electronic configuration of:
          (i) Cr3+ (iii) Cu+ (v) Co2+ (vii) Mn2+ 
          (ii) Pm3+ (iv) Ce4+ (vi) Lu2+ (viii) Th4+

              4.
              Using the standard electrode potentials given in Table 3.1, predict if the reaction between the following is feasible: 
              (i) Fe3+ (aq) and I- (aq) 
              (ii) Ag+ (aq) and Cu(s) 
              (iii) Fe3+(aq) and Br-(aq) 
              (iv) Ag(s) and Fe3+(aq) 
              (v) Br2 (aq) and Fe2+(aq).

                  5.

                  Draw the structures of optical isomers of: 
                  (i) \([Cr(C_2O_4)_3]^{3–}\)
                  (ii) \([PtCl_2(en)_2]^{2+}\)
                  (iii) \([Cr(NH_3)2Cl_2(en)]^{+}\)

                      6.
                      A solution of Ni(NO3)2 is electrolysed between platinum electrodes using a current of 5 amperes for 20 minutes. What mass of Ni is deposited at the cathode?

                          CBSE CLASS XII Previous Year Papers

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