Name: predicting precipitation in aqueous reactions
Uploaded: 2020-09-03T13:38:26.000Z
Duration: 3 min 10 s
Channel: JoVE

K12 Standards

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NC K-12 Science Standard Course of Study

9-12

Physical Science

Matter and its Interactions

PS.PSc.2.4

Precipitation Reactions

Ionic solids may be water-soluble or water-insoluble. Water-soluble ionic solids dissolve by breaking up &#8212; or dissociating &#8212; into ions. Water molecules then surround the dissociated ions to form an aqueous solution. Water-insoluble ionic solids stay undissolved or undissociated in aqueous solutions. 

If mixing aqueous solutions of two different ionic compounds results in a chemical reaction that produces an insoluble ionic solid &#8212; called the precipitate &#8212; it is a precipitation reaction. The precipitate stays undissolved and can be separated from the solution by filtration. 

Sodium chloride and silver nitrate are water-soluble ionic compounds. An aqueous solution of sodium chloride contains sodium ions and chloride ions in their dissociated forms. Similarly, an aqueous solution of silver nitrate contains dissociated silver ions and nitrate ions. 

When the two solutions are mixed, the resulting solution contains all four ions: the sodium and silver cations as well as the chloride and nitrate anions. 

Since oppositely charged ions are attracted to each other, the cation from each reactant can pair with the anion from the other reactant to form new ionic products. Thus, sodium cations pair

precipitation reactions

In a precipitation reaction, aqueous solutions of soluble salts react to give an insoluble ionic compound &#8211; the precipitate. The reaction occurs when oppositely charged ions in solution overcome their attraction for water and bind to each other, forming a precipitate that separates out from the solution. Since such reactions involve the exchange of ions between ionic compounds in aqueous solution, they are also referred to as double displacement, double replacement, exchange reactions, or metathesis reactions (Greek for &#8220;to transpose&#8221;). A precipitation reaction is used as an analysis technique to identify metal ions in a compound and gravimetric methods for determining the composition of matter. 
 
The extent to which a substance may be dissolved in water, or any solvent, is quantitatively expressed as its solubility, defined as the maximum concentration of a substance that can be achieved under specified conditions. Substances with relatively large solubilities are said to be soluble. A substance will precipitate when solution conditions are such that its concentration exceeds its solubility. Substances with relatively low solubilities are said to be insoluble, and these are the substances that readily precipitate from solution.

For example, precipitation is observed when solutions of potassium iodide and lead nitrate are mixed, resulting in the formation of solid lead iodide:

<img alt="Chemical reaction: 2KI + Pb(NO3)2 &#8594; PbI2 + 2KNO3; precipitation reaction equation." src="/files/core/CDNSource/lessons/1027/1027_Equations-01.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq1" />

This observation is consistent with the solubility guidelines: The only insoluble compound among all those involved is lead iodide, one of the exceptions to the general solubility of iodide salts.

The net ionic equation representing this reaction is:

<img alt="Chemical reaction equation, Pb&#178;&#8314;(aq) + 2I&#8315;(aq) &#8594; PbI&#8322;(s), precipitation process." src="/files/core/CDNSource/lessons/1027/1027_Equations-02.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq2" />

The solubility guidelines may be used to predict whether a precipitation reaction will occur when solutions of soluble ionic compounds are mixed together. One merely needs to identify all the ions present in the solution and then consider if possible cation-anion pairing could result in an insoluble compound. 

For example, mixing solutions of silver nitrate and sodium fluoride will yield a solution containing Ag+, NO3&#8722;, Na+, and F&#8722; ions. Aside from the two ionic compounds originally present in the solutions, AgNO3 and NaF, two additional ionic compounds may be derived from this collection of ions: NaNO3 and AgF. The solubility guidelines indicate all nitrate salts are soluble but that AgF is one of the exceptions to the general solubility of fluoride salts. A precipitation reaction, therefore, is predicted to occur, as described by the following equations:

<img alt="Molecular equation diagram, NaF(aq) + AgNO3(aq) &#8594; AgF(s) + NaNO3(aq), chemical reaction." src="/files/core/CDNSource/lessons/1027/1027_Equations-03.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq3" />

<img alt="Net ionic equation: Ag&#60;sup&#62;+&#60;/sup&#62;(aq) + F&#60;sup&#62;-&#60;/sup&#62;(aq) &#8594; AgF(s); chemical reaction representation." src="/files/core/CDNSource/lessons/1027/1027_Equations-04.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq4" />

This text is adapted from <a href="https://openstax.org/books/chemistry-2e/pages/4-2-classifying-chemical-reactions">OpenStax Chemistry 2e, Section 4.2: Classifying Chemical Reactions.</a>

In a precipitation reaction, aqueous solutions of soluble salts react to give an insoluble ionic compound &ndash; the precipitate. The reaction occurs ...

Education

JoVE Core

Chemistry

Chemical Quantities and Aqueous Reactions

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KEY TERMS AND CONCEPTS

predicting precipitation in aqueous reactions

Precipitation reactions happen when two aqueous solutions of soluble salts are mixed and an insoluble ionic compound forms. That solid is called a precipitate. The ions in solution exchange partners, so these reactions are also called double displacement, double replacement, exchange, or metathesis reactions.

These reactions are useful in chemistry for identifying metal ions in a compound. They also support gravimetric methods, which measure composition by the mass of a substance. Whether a precipitate forms depends on solubility, which is the maximum amount of a substance that can dissolve under set conditions.

A substance with high solubility is called soluble. A substance with low solubility is called insoluble, and it is more likely to fall out of solution. Precipitation occurs when the concentration of a dissolved substance becomes greater than its solubility.

A common example is potassium iodide mixed with lead nitrate. The reaction produces solid lead iodide. This matches solubility guidelines because lead iodide is one of the exceptions to the usual solubility of iodide salts.

Solubility guidelines can also help predict whether a precipitation reaction will happen before the solutions are mixed. First, identify all ions present in solution. Then check whether a possible cation-anion pairing can form an insoluble compound.

For example, mixing silver nitrate with sodium fluoride gives Ag+, NO3-, Na+, and F- ions in solution. These ions can form sodium nitrate and silver fluoride in addition to the original salts. Nitrate salts are soluble, but silver fluoride is an exception to the usual solubility of fluoride salts, so a precipitate is predicted to form.

Predicting Precipitation in Aqueous Reactions

predicting-precipitation-in-aqueous-reactions

reaction stoichiometry

Using Balanced Equations to Predict Amounts

Balanced chemical equations show how much of each substance reacts and forms in a chemical change. The formulas identify the reactants and products, which helps classify the reaction. The coefficients show the relative numbers of each substance, which lets chemists measure the relationships between amounts consumed and produced. These quantitative relationships are called stoichiometry. The word comes from the Greek words stoicheion, meaning element, and metron, meaning measure. Stoichiometry...

limiting reactant

Limiting Reactant in Water Formation

Limiting reactants control how much product forms in a chemical reaction. In a balanced equation, the listed amounts of reactants and products are called stoichiometric amounts. In real reactions, the reactants are not always present in those exact proportions. The limiting reactant is the reactant that gets used up first. Once it is gone, the reaction stops making more product. The other reactant is called the excess reactant. Chemists often use extra amounts of one reactant to make sure the...

reaction yield

Calculating Percent Yield in Reactions

Percent yield shows how much product a chemical reaction actually makes compared with how much it should make. The expected amount is called the theoretical yield. It comes from the balanced chemical equation and the stoichiometry, or mole ratios, in that equation. The theoretical yield assumes that the limiting reactant is completely converted into the desired product. The product collected after a reaction is the actual yield. In many cases, the actual yield is lower than the theoretical...

general properties of solutions

Solvents, Solutes, and Solution Types

Solutions are common mixtures found in everyday life. Ocean water, air, and gasoline are all examples of solutions. These mixtures contain two or more types of atoms or molecules in varying amounts. A solution is a homogeneous mixture, which means its composition is uniform throughout. This is different from a heterogeneous mixture, which has a non-uniform composition. In a homogeneous mixture, the components are evenly spread out and thoroughly mixed. Every solution has a solvent and a...

solution concentration and dilution

Molarity and Dilution in Solutions

Molarity and dilution are key ideas for describing solution concentration. Concentration means the relative amount of one component in a solution. In many solutions, one component is present in much larger amount. That component is the solvent, which acts as the medium that holds the other parts dissolved or dispersed. Water is a very common solvent, so aqueous solutions are common on Earth. A solute is the component present at a much lower concentration than the solvent. Chemists often...

electrolyte and nonelectrolyte solutions

Conductivity in Electrolyte Solutions

Electrolyte solutions conduct electricity because they contain mobile ions. The amount of current depends on how many ions are present in the aqueous solution. A solution with more dissolved ions conducts better, while a solution with few or no ions conducts poorly. Strong electrolytes dissociate completely in water. This complete dissociation is shown with a single forward arrow. Water-soluble ionic compounds such as sodium chloride split into sodium cations and chloride anions in solution.

solubility of ionic compounds

Solubility Rules for Ionic Salts

Solubility rules help predict whether ionic salts will dissolve in water. Solubility is the maximum amount of solute that can dissolve in a given amount of solvent at a set temperature and pressure. It is often measured in molarity, or moles per liter (mol/L). A compound is soluble if it dissolves in water. When a soluble salt dissolves, its ions separate and spread evenly through the solution. This physical change is called dissociation. Potassium chloride, KCl, is a simple example. Water...

chemical reactions in aqueous solutions

Aqueous Reactions and Ionic Equations

Aqueous reactions are chemical reactions that take place in water. Water is a good solvent, so it can dissolve many substances. Because of that, many chemical reactions happen in aqueous solution. In high school chemistry, these reactions are often grouped into three common patterns: precipitation, acid-base, and oxidation-reduction. A precipitation reaction happens when ions in two aqueous ionic compounds exchange partners and form an insoluble salt, called a precipitate. In an acid-base...

Precipitation reactions happen when two aqueous solutions of soluble salts are mixed and an insoluble ionic compound forms. That solid is called a precipitate. The ions in solution exchange partners, so these reactions are also called double displacement, double replacement, exchange, or metathesis reactions. These reactions are useful in chemistry for identifying metal ions in a compound. They also support gravimetric methods, which measure composition by the mass of a substance. Whether a...

oxidation-reduction reactions

Redox Reactions and Oxidation Numbers

Redox reactions are chemical changes in which electrons move between reactants, and oxidation numbers help track that change. The term oxidation once referred only to reactions with molecular oxygen, but it now covers a wider reaction class called oxidation-reduction, or redox, reactions. Some redox reactions form ionic products. A simple example is the reaction between sodium and chlorine that produces sodium chloride. To follow what happens to each reactant, chemists often write...

oxidation numbers

Assigning Oxidation States in Redox Reactions

Oxidation states help track electron transfer in redox reactions. An oxidation number, also called an oxidation state, is a hypothetical number that shows the effective charge of an atom or element. Chemists assign it by using a set of rules. In an ionic compound, oxidation numbers are based on how many electrons move between reacting species. For example, when calcium chloride forms, calcium loses two valence electrons and the two chlorine atoms each gain one electron. In CaCl2, calcium has...

acids, bases and neutralization reactions

How Acids and Bases Form Ions

Acids and bases form ions in water through acid-base reactions. These reactions move a hydrogen ion, H+, from one chemical species to another. They matter in many natural and technological processes, from chemistry inside cells and in lakes and oceans to the large-scale production of fertilizers, pharmaceuticals, and other useful substances. In water, an acid is a substance that produces hydrogen ions. This is the Arrhenius definition of an acid. The H+ ion is also called a proton. In...

synthesis and decomposition reactions

Building and Breaking in Chemical Reactions

Synthesis and decomposition are two redox reactions that show how substances can be built up or broken down. Synthesis means to make something, while decomposition means to break something apart. These reactions also involve chemical changes and energy changes. Synthesis reactions are also called combination reactions. In a synthesis reaction, two or more substances join to form one more complex substance. They are often written as A + B → AB or A + B → C. One example is the formation of...

key terms and concepts

Explore how solubility rules predict precipitation reactions, why insoluble solids form, and how examples like lead iodide and silver fluoride appear in solution.

Predicting Precipitation in Aqueous Reactions | JoVE K12

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Pennsylvania State Academic Standards

Chemical Reactions

3.2.9-12.C

3.2.9-12.G

Louisiana Student Standards for Science

High School

Matter And Its Interactions

HS-PS1-2

HS-PS1-7

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Ohio’s Learning Standards and Model Curriculum

Interactions Of Matter

Chemical reactions

Tennessee Academic Standards

Matter and Its Interactions

CHEM1.PS1.4

Arizona Science Standards

HS+C.P1

HS+C.P1U1.5

Texas Essential Knowledge and Skills

CHEM.9

CHEM.9.B

Alabama Course of Study Science

Matter and Its Interactions: Energy

9-12.C.3.3

9-12.PS.3.5.d

Florida Department of Education

SC.912.P.8

SC.912.P.8.8

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New York State Science Learning Standards

HS. Chemical Reactions

HS-PS1-4.

HS-PS1-12.

Georgia Standards of Excellence

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Balancing Chemical Equations and Reaction Outcomes

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Physical Properties of Matter

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Construct an explanation to classify the type of chemical bond that occurs (covalent, ionic, or metallic) in a given substance.

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PS.PSc.2.2

Understand types, properties, and structure of matter.

Predicting Precipitation in Aqueous Reactions

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