Predict double-displacement products

Ontario Grade 11 Chemistry

Chemical Reactions

Use ion charges and solubility information to predict what forms when two ionic compounds exchange ions.

When two clear ionic solutions are mixed, the mixture may become cloudy. The cloudiness can come from a solid forming in the liquid. A double-displacement reaction is one way this can happen. In this lesson, you will use the ions in the reactants to predict possible products, then use a solubility table to decide whether a solid forms. The main idea is to exchange partners without changing the ions’ charges.

What you will learn

Recognize a double-displacement reaction from its reactants.
Predict product formulas by exchanging the positive ions’ partners.
Use ion charges to write correct product formulas.
Use a solubility table to determine whether a product forms a precipitate.
Write and balance a chemical equation with appropriate state symbols.

1. Review: ions, formulas, and aqueous solutions

An ion is an atom or group of atoms with an electric charge. A positive ion is a cation. A negative ion is an anion. For example, sodium ions have a charge of

+1

, while chloride ions have a charge of

-1

.

An ionic compound has a total charge of zero. Its formula shows the smallest whole-number ratio of ions that makes the total charge zero. For example, calcium ions have a

2+

charge and chloride ions have a

1-

charge, so calcium chloride is written as

\mathrm{CaCl_2}

.

The state symbol

(aq)

means a substance is dissolved in water. In an aqueous ionic solution, the ions are separate and can move through the water. The state symbol

(s)

means solid. A solid that forms from solutions is called a precipitate.

Keep each ion’s charge when predicting products.
Write each ionic compound so its total charge is zero.
Use a solubility table to check whether an ionic product is aqueous or solid.

2. The exchange pattern

In a double-displacement reaction, two ionic compounds exchange ions. The positive ions change partners: the cation from the first compound pairs with the anion from the second, and the other two ions pair together.

Think of the reactants as two pairs of partners. Exchange the partners, but do not change the identity or charge of any ion. Once you have the new pairs, use their charges to write neutral product formulas. Do not copy the subscripts from the reactants automatically. Subscripts depend on the charges of the ions in each product.

The exchange pattern predicts possible products; it does not by itself prove that a reaction occurs. Check the products with a solubility table. A common visible result is a precipitate. If both products remain dissolved, the mixture may have no net chemical change that can be represented as a reaction. At this course level, focus on predicting products and identifying whether a product is insoluble.

\mathrm{AB + CD \rightarrow AD + CB}

Exchange ion partners, not charges.
Write product formulas from ion charges.
Check product solubility after predicting the formulas.

3. Use solubility to predict the result

A solubility table tells you whether an ionic compound dissolves in water. A soluble compound is usually written

(aq)

in a reaction. An insoluble compound is written

(s)

and can form a precipitate.

Use the table supplied for your course. Learn its general patterns, but check the exceptions listed in that table. For example, many nitrate compounds are soluble, while many carbonate compounds are insoluble. These are useful patterns, not rules without exceptions.

A simple prediction has three parts: exchange the ions, write each product formula using charges, and check the products’ solubility. If an insoluble product forms from aqueous reactants, write it as a solid. Keep the other product’s state consistent with the solubility table.

Some double-displacement reactions also form water or a gas. These products must still be written with correct formulas and balanced atoms. Do not assume a gas or water forms merely because ions exchanged partners; use the reaction information and course-level rules you have been given.

Solubility determines whether an ionic product is written $(aq)$ or $(s)$ .
A precipitate is a solid product formed in solution.
The product formulas must be correct before you add state symbols.

4. Write and balance the equation

A chemical equation represents a reaction using formulas. The reactants go on the left and the products go on the right. An arrow means “forms.” A balanced equation has the same number of each kind of atom on both sides.

First, write the correct formulas for the reactants and predicted products. Then add state symbols when the information is known. Finally, balance the equation by changing coefficients, the numbers placed in front of formulas. Never change a formula’s subscripts to balance an equation. Changing a subscript changes the substance.

For double-displacement predictions, correct product formulas are essential. A balanced equation cannot fix an incorrectly written ionic formula. Check both the ion charges used in each formula and the atom counts in the final equation.

Use coefficients to balance; do not alter subscripts.
Check atom counts on both sides.
Include state symbols when they are known from the solubility information.

A compact prediction checklist

Question	What to do
What ions are present?	Separate each ionic reactant into its cation and anion.
What are the products?	Exchange the anion partners and write neutral formulas.
Does a precipitate form?	Check each ionic product against the solubility table.
Is the equation ready?	Add known states and balance atoms with coefficients.

Worked example

Predicting a precipitate

Aqueous silver nitrate is mixed with aqueous sodium chloride. Predict the products, states, and balanced equation.

Identify the ions
Silver nitrate contains silver ions and nitrate ions. Sodium chloride contains sodium ions and chloride ions. Keep the charges of these ions unchanged as you exchange partners.
$\mathrm{Ag^+,\ NO_3^-,\ Na^+,\ Cl^-}$
Exchange partners
Pair silver with chloride, and sodium with nitrate. These are the possible products. Write each formula so the total charge is zero.
$\mathrm{AgCl,\ NaNO_3}$
Check solubility
Use the course solubility table. Silver chloride is insoluble, so it is written as a solid. Sodium nitrate is soluble, so it remains aqueous.
$\mathrm{AgCl(s),\ NaNO_3(aq)}$
Check balance
There is one silver, one chlorine, one sodium, one nitrogen, and three oxygen atoms on each side. The equation is already balanced with coefficients of one.
$\mathrm{AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s) + NaNO_3(aq)}$

Answer: Silver chloride forms as a precipitate, while sodium nitrate remains aqueous. The balanced equation is

\mathrm{AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s) + NaNO_3(aq)}

.

Check: The product formulas are neutral: silver and chloride combine in a one-to-one ratio, as do sodium and nitrate. Each element has the same atom count on both sides.

Common mistakes and how to avoid them

Copying reactant subscripts directly into the products.

Correction: Use the charges of the ions in each new pair to determine the product formula.

Changing an ion’s charge during the exchange.

Correction: The ions keep their charges. Only their partners change.

Assuming every predicted product is a solid.

Correction: Check each product in the solubility table before assigning its state.

Changing subscripts to balance the equation.

Correction: Keep formulas fixed and adjust coefficients to balance atom counts.

Lesson summary

A double-displacement reaction involves two ionic compounds exchanging ion partners.
Keep ion charges unchanged, then use those charges to write neutral product formulas.
Use the course solubility table to identify an insoluble product and assign its state.
Write correct formulas before balancing, and balance with coefficients rather than subscripts.

Check your understanding

Question 1

When predicting products, what changes in a double-displacement reaction?

The ions exchange partners, while their charges stay the same.
The ions exchange charges, while their partners stay the same.
The cations become anions and the anions become cations.
The subscripts of both reactants are copied into the products.

Show answer and explanation

The ions exchange partners, while their charges stay the same.

The ions keep their identities and charges. The cation from each reactant pairs with the other reactant’s anion.

Question 2

What is the correct formula for a product made from aluminium ions and chloride ions?

$\mathrm{AlCl}$
$\mathrm{AlCl_2}$
$\mathrm{AlCl_3}$
$\mathrm{Al_3Cl}$

Show answer and explanation

\mathrm{AlCl_3}

An aluminium ion has a

3+

charge and a chloride ion has a

1-

charge. Three chloride ions are needed to make the compound’s total charge zero.

Question 3

Two aqueous ionic compounds exchange ions. What should you check to decide whether one product is a precipitate?

The solubility table
The reactants’ coefficients only
Whether the product formula has a subscript
Whether the equation has an arrow

Show answer and explanation

The solubility table

A solubility table indicates whether an ionic product is soluble or insoluble in water. An insoluble product can be written as a solid.

Key terms

Ion: An atom or group of atoms with an electric charge.
Cation: A positively charged ion.
Anion: A negatively charged ion.
Aqueous: Dissolved in water; shown by the state symbol $(aq)$ .
Precipitate: An insoluble solid that forms from a solution.
Solubility table: A reference table used to determine whether an ionic compound dissolves in water.
Coefficient: A number placed in front of a chemical formula to balance an equation.

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About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 11 Chemistry (SCH3U), expectation C2.6. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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