Double Replacement

Reactions - Reaction Types

Double-displacement reactions describe reactions in which two or more new products are formed from two molecular reactants. In double-displacement reactions, reactants exchange ions or elements to form products. At least one of the products will either separate from the reaction mixture (usually as a gas or solid) or form a stable covalent compound.


Explore the discoverer's biography, including general facts about his life and anecdotes regarding how he made this particular discovery. Also see other significant scientific discoveries built largely on this concept and other real-world applications in history that may not still be relevant.

Use/Application through History

Soap was synthesized by the Egyptians, Romans, and Chinese using potassium hydroxide (KOH). Each ancient civilization used unique procedures to synthesize KOH. The ancient Chinese made KOH using a double-replacement reaction.

Ancient Chinese people heated seashells and then mixed them with plant ash in water. The procedure is highlighted below.

$\text{CaCO}_3 \to \text{CO}_2 + \text{CaO}$

$\text{CaO} + \text{H}_2\text{O} \to \text{Ca(OH)}_2$

$\text{K}_2\text{CO}_3 + \text{Ca(OH)}_2 \to \text{CaCO}_3 + \text{2KOH}$

(From unknown writer, Kao Gong Ji, ancient Chinese technology cyclopedia, as early as around 732 BCE)


Concept Definition

Study the primary definition of this concept, broken into general, basic, and advanced English definitions. Also see the mathematical definition and any requisite background information, such as conditions or previous definitions.


The common types of double-displacement reactions can be put into three calagories:

1. Precipitation reactions. Precipitation reactions occur when the exchange of ions results in an insoluble salt. Precipitation reactions can be predicted using a solubility table.


${\color{Blue}\text{CuSO}_4_{(aq)} + \text{BaCl}_2_{(aq)} \to \text{BaSO}_4_{(s)} + \text{CuCl}_2_{(aq)}}$

2. Gas formation. Gas is released when an insoluble gas is formed during the reaction. Most binary covalent gas compounds are sufficiently insoluble (excluding HCl(g) and NH3(g)).


${\color{Blue}\text{ZnS}_{(s)} + 2\text{HCl}_{(aq)} \to \text{ZnCl}_{(aq)} + \text{H}_2\text{S}_{(g)}}$

${\color{Blue}\text{CaCO}_3_{(s)} + 2\text{HCl}_{(aq)} \to \text{CaCl}_2_{(aq)} + \text{CO}_2_{(g)} + \text{H}_2\text{O}_{(l)}}$

3. Double replacement reactions forming weak electrolytes.

  1. Acid-Base Neutralization:
  2. In double replacement reactions between acids and bases, hydrogen ions from acids and hydronium ions from bases combine together to form water molecules. The anions from the acids and cations from the bases combine to form salts. If the salts are soluble in water, they will form ions in the water solution; if the salts are insoluble in water, they will appear as a precipitate.


    ${\color{Blue}\text{NaOH}_{(aq)} + \text{HCl}_{(aq)} \to \text{H}_2\text{O}_{(l)} + \text{NaCl}_{(aq)}}$
    ${\color{Blue}\text{H}_2\text{SO}_4_{(aq)} + \text{Ba(OH)}_2_{(aq)} \to \text{BaSO}_4_{(s)} + 2\text{H}_2\text{O}_{(l)}}$

  3. Weak electrolytes with no water as a product:
  4. Examples:

    ${\color{Blue}\text{NaCH}_3\text{COO}_{(aq)} + \text{HCl}_{(aq)} \to \text{NaCl}_{(aq)} + \text{CH}_3\text{COOH}_{(l)}}$

Background Information

Basic Solubility Rule Table4, 5




Alkali metals (IA)


No exception



No exception

$\text{NO}_3_^{-} , \text{ClO}_3_^{-}, \text{ClO}_4_^{-}$, $\text{CH}_3\text{COO}^{-} \text{or C}_2\text{H}_3\text{O}_2_^{-}$


No exception

Halides: $\text{ Cl}^{-}, \text{Br}^{-}, \text{I}^{-}$


Salts of $\text{Ag}^{+}, \text{Pb}^{2+}, \text{Hg}_2_^{2+}$



Insoluble for $\text{Ba}^{2+}, \text{Sr}^{2+}, \text{Pb}^{2+}, \text{Hg}_2_^{2+}$,

and slightly soluble for $\text{Ca}^{2+}, \text{Hg}^{2+}, \text{Ag}^{+}$



Alkali metals (IA), $\text{Ba}^{2+}, \text{Ca}^{2+}, \text{Sr}^{2+}, \text{NH}_4_^{+}$



Alkali metals (IA) and Alkali earth metals(IIA), $\text{NH}_4_^{+}$

$\text{SO}_3_^{2-}, \text{CO}_3_^{2}, \text{PO}_4_^{3-}$


Alkali metals (IA) , $\text{NH}_4_^{+}$

$ \text{CrO}_4_^{2-}$


Alkali metals (IA) , $\text{NH}_4_^{+}, \text{Ca}^{2+}, \text{Mg}^{2+}$



Learn important vocabulary for this concept, including words that might appear in assessments (tests, quizzes, homework, etc.) that indicate the use of this concept.

 Important Vocabulary

Term Context
Double Replacement Reactions
  • Predict the product and calculate the mass of solid formed by the double replacement reaction between 10.0 mL of 0.4M Ca(OH)2(aq) and 5.0mL of 0.6M NasCO3(aq). (Ans: 0.30 g CaCO3(s)).
Neutralization Reaction
  • Neutralization reactions are performed by adding acids into bases or vise versa.
  • CaCO3(s) is the precipitate formed by bubbling CO2 gas into a Ca(OH)2 solution.
Weak Electrolyte
  • Examples of weak electrolytes include: HC2H3O2(or CH3COOH, acetic acid), H2CO3(carbonic acid), and NH3(ammonia). They can be products of double replacement reactions.



Browse relevant videos from the Journal of Chemical Education's (JCE) Chemistry Comes Alive! library and other video sources.

Sodium Iodide with Mercury(II) Chloride

Reaction between sodium iodide (very pale yellow) and mercury(II) chloride (colorless), produces an mercury(II) iodide (orange) precipitate.

Chemical Equation:

$\text{HgCl}_2_{(aq)} + 2\text{NaI}_{(aq)} \to \text{HgI}_2_{(s)} + 2\text{NaCl}_{(aq)}$


Cadmium Nitrate and Sodium Sulfide

Water Gas with Burning Charcoal

Potassium Iodine with Mercury(II) Nitrat

Sodium Iodide and Lead(II) Nitrate

Adding very pale yellow sodium iodide to colorless lead(II) nitrate produces a yellow precipitate.

Solution A: 0.5 M sodium iodide (very pale yellow) 
Solution B: 0.2 M lead(II) nitrate (colorless) 
Precipitate: (yellow) 
Pb(NO3)2(aq) + 2 NaI(aq) --> PbI2(s) + 2 NaNO3(aq)

Sodium Sulfide and Copper(II) Nitrate

Sodium sulfide and copper(II) nitrate

Sodium Sulfide and Lead(II) Nitrate

Sodium sulfide and lead(II) nitrate


Computer Animations

Experience computer simulators or animations that illustrate the concept discussed here. Many simulators or animations come with worksheets for use in class.


Classroom Demonstations

Investigate lab procedures suitable for live classroom demonstrations or guided student exploration.

Students requiring adaptations to gain the full benefit of a demonstration may find a worksheet with guided observations useful. Alternatively, a teacher may wish to use a worksheet with guided observations to model what observations all students should be making during a demonstration.

The Demonstration Observation Worksheet is available in

  • PDF [ready to print]
  • Word 2007/2008 DOCX [free to edit/adapt further]


Double Replacement Demo


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Double Replacement Demo

Double Replacement Demo

Credit: American Chemical Society

Author: Yan Luo

This demonstration aims to illustrate the type of the double replacement reactions that can happen due to the formation of insoluable product (precipitation). All the reactions in this demonstration that have physical changes in this demonstration are due to formation of precipitates (no weak electrolyte or gas will be form).


To show that formation of precipitates is a type of double replacement reaction.

It can also be used to test students' ability to predict a reaction and their familiarity with the solubility rules. To do this, the teacher can let the students know what the reactants are before the experiment and ask the students to predict whether or not there will be any observable physical changes.

This demonstration can also be turned into a lab-competition. To do this, the instructor can let the students know the five types of solution for the lab without matching the names with the specific solution. A clean microplate (with ~12 wells) can be given to each pair or group of students with the expectation that the microplate not be washed before completion of the lab. The students would be asked to match the five solutions with their name tags and the pair of group of students that match all solutions solutions correctly using the minimum number of wells in the minimum amount of time will be the winner of the lab section.

No Description available
  • A spot plate of 6 or more wells
  • One dropper for each solution, five droppers in total

5 Types of solutions needed are described as following:

  • $0.1\text{M NaCl}$
  • $0.1\text{M CuSO}_4$
  • $0.1\text{M AgNO}_3$
  • $0.1\text{M Na}_3\text{PO}_4$
  • $0.1\text{M NaOH}$

Add the following two solutions into each watch-glass seperately:

  1. $0.1\text{M NaCl}$ and $0.1\text{M CuSO}_4$
  2. $0.1\text{M NaOH}$ and $0.1\text{M CuSO}_4$
  3. $0.1\text{M Na}_3\text{PO}_4$ and $0.1\text{M CuSO}_4$
  4. $0.1\text{M NaCl}$ and $0.1\text{M AgNO}_3$
  5. $0.1\text{M NaOH}$ and $0.1\text{M AgNO}_3$
  6. $0.1\text{M Na}_3\text{PO}_4$ and $0.1\text{M AgNO}_3$

No doulbe replacement reaction would happen due to formation of gas or weak electrolytes in this demonstration. All reactions would happen due to formation of precipitates and can be predicted by solubility rules

Chemcial Reactions demonstrated can be writeen as following:

  1. $\text{NaCl}_{(aq)} + \text{CuSO}_4_{(aq)} \to \text{No Reaction}$
  2. $2\text{NaOH}_{(aq)} + \text{CuSO}_4_{(aq)} \to \text{Na}_2\text{SO}_4_{(aq)} + \text{Cu(OH)}_2_{(s)}$
  3. $\text{Na}_3\text{PO}_4_{(aq)} + \text{CuSO}_4_{(aq)} \to \text{No Reaction}$
  4. $\text{NaCl}_{(aq)} + \text{AgNO}_3_{(aq)} \to \text{NaNO}_3_{(aq)} + \text{AgCl}_{(s)}$
  5. $\text{NaOH}_{(aq)} + \text{AgNO}_3_{(aq)} \to \text{NaNO}_3_{(aq)} + \text{AgOH}_{(s)}$
  6. $\text{Na}_3\text{PO}_4_{(aq)} + \text{AgNO}_3_{(aq)} \to 2\text{NaNO}_3_{(aq)} + \text{Ag}_3\text{PO}_4_{(s)} \text{(yellow)}$

Aqueous solutions should be neutralized before disposal

Solids should be disposed to solid waste container

Difficulty:Some laboratory experience required
Preparation Time:5 minutes Demonstration Time: 1 minutes
Availability of Materials:Not specified Cost of materials:$5
Last Updated:Tue 21 Jun 2011 15:37:23 EDT Viewed:155716 times viewed
Source:The Science House, North Carolina State University, The Science House

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Sample Problems

Explore sample problems from the JCE QBank and other sources.

 Sample Problems


[ Full view of question ]

In order to answer the following questions, write the net ionic equation for the reaction between equal volumes of 0.10 M solutions of the following acid and base:

KOH(aq) + HBr(aq)

What are the reaction products? Does the reaction yield an acidic, basic, or neutral aqueous solution?


[ Full view of question ]

What is the balanced net ionic equation for the following reaction:

AgNO3(aq) + HCl(aq) AgCl (s) + HNO3(aq)


[ Full view of question ]

Which of the following compounds would result in an exchange reaction if mixed in an aqueous solution with Ba(OH)2?


[ Full view of question ]

Identify each of the following statements as describing a combination, displacement, decomposition, or exchange reaction.


[ Full view of question ]

Which of the following statements concerning the properties of ionic compounds is NOT correct?


[ Full view of question ]

NiSO4(aq) and Li2CO3(aq) are combined in a beaker. Which of the following diagrams is the most appropriate representation of the contents of the beaker after any reaction occurs? (Water molecules from the aqueous solutions are not shown.)


[ Full view of question ]

What are the most likely products from the reaction of aqueous sulfuric acid and aqueous sodium hydroxide?


[ Full view of question ]

The reaction Mg + 2HCl MgCl2 + H2 is an example of a(n)


Works Cited

Review the works cited to write the researched parts of this page, such as the discover's biographical information and other areas.

 Works Cited

"" Chemistry Dictionary / Glossary! Everyone's Online Source for Chemistry Terms and Definitions. Web. 27 Oct. 2010. . (vocabulary definition)

Birk, James P. "Chemical Reactions." Chemistry: Foundations and Applications. Advameg, Inc., 2010. Web. 27 Oct. 2010. . (reaction type descriptions)

IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8.

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