E3.4 Predict precipitates using a solubility table

Ontario Grade 11 Chemistry

Solutions and Solubility

Use the ions in two aqueous solutions to decide whether a solid forms

When two clear solutions are mixed, the mixture may stay clear or become cloudy as a solid appears. That solid is called a precipitate. In this lesson, you will use a solubility table to predict whether one forms. First recall that an ionic compound contains positive ions and negative ions. Its formula has a total charge of zero. In an aqueous solution, dissolved ions are separated and surrounded by water particles. A solubility table tells you which combinations of ions remain dissolved and which form a solid.

What you will learn

Describe a precipitate using observable and particle-level evidence.
Use a solubility table to decide whether an ionic compound is soluble or insoluble in water.
Predict the products when two aqueous ionic compounds exchange ions, and identify any precipitate.
Write a balanced chemical equation with correct formulas and state symbols.

1. From what you observe to particles

A precipitate is an insoluble solid that forms when solutions are mixed. Insoluble means the solubility table classifies the compound as not dissolving enough to remain in solution. Cloudiness or a solid settling out can be observable evidence of a precipitate. If a mixture remains clear, that observation alone does not establish what happened. The solubility table gives a prediction based on the possible products.

Before mixing, an aqueous ionic compound is represented as ions dispersed in water. Aqueous means dissolved in water and is shown by the state symbol

(aq)

. When solutions are combined, positive ions can pair with negative ions from the other solution. If a new pairing is insoluble, its ions form a solid. That solid is written with the state symbol

(s)

. Ions that stay dissolved are written as

(aq)

.

State symbols show a substance's state or solution form. The symbols

(s)

,

(l)

, and

(g)

mean solid, liquid, and gas. For this expectation, the main decision is whether a possible ionic product is dissolved in water or forms a solid.

(aq)

A precipitate is an insoluble solid formed from solutions.
A solubility table predicts whether an ionic compound dissolves in water.
The precipitate, if present, is written as a product with state symbol $(s)$ .

2. Read the solubility table

A solubility table lists ion combinations and classifies their compounds as soluble, insoluble, or sometimes slightly soluble in water. Use the exact table provided in class. Tables may show exceptions in different ways. Follow both the main rule and the exceptions printed in your table.

Common course-level patterns can help you read the table. Compounds containing nitrate ions are soluble. Compounds containing Group 1 metal ions or ammonium ions are generally soluble. Many compounds containing chloride ions are soluble, but the table lists exceptions such as silver and lead compounds. Many sulfates are soluble, with exceptions listed in the table. Carbonates and phosphates are generally insoluble unless paired with Group 1 or ammonium ions. Hydroxides are generally insoluble, with exceptions shown in the course table.

These patterns do not replace checking the table. Do not assume that every compound containing a familiar ion is soluble. Find the relevant ion combination and check for an exception. If your table says a compound is slightly soluble, use the table's classification for the prediction. For this lesson, identify a precipitate when the table classifies a product as insoluble.

Check the relevant ion combination and any exceptions in the supplied table.
Soluble products are written $(aq)$ ; insoluble products are written $(s)$ .
Use the table's classification for slightly soluble compounds.

3. Predict products and identify the solid

When two ionic solutions are mixed, list the ions in the starting compounds. Then consider new positive-negative pairings: the positive ion from the first compound pairs with the negative ion from the second, and the other positive and negative ions pair together. This is often called a double-displacement pattern. It is a way to organize possible products; the solubility table determines whether either product forms a precipitate.

Write each product formula so its positive and negative charges balance to a total charge of zero. Do not automatically carry the starting compound's subscripts into a new product. For example, a product made from a two-plus ion and a one-minus ion needs two one-minus ions for each two-plus ion. Use parentheses when more than one copy of a polyatomic ion is needed.

Check each product with the solubility table. If one product is insoluble, label it

(s)

. If the other is soluble, label it

(aq)

. Finally, balance the full equation so each kind of atom appears in equal numbers on both sides. If both possible products are soluble, the table predicts no precipitate. If one product is insoluble, that product is the precipitate.

Pair positive and negative ions in new combinations to form possible products.
Make each product formula electrically neutral before checking solubility.
Use the table to assign states, then balance the equation.

4. Check formulas, states, and balance

The symbols in a chemical equation represent substances and their relative amounts. A coefficient placed in front of a formula changes the amount represented. A subscript is part of the substance's formula and must not be changed to balance an equation. Balance by adjusting coefficients only.

State symbols are part of the prediction. Writing an insoluble product as

(aq)

says it stays dissolved, not that it forms a precipitate. Check every product formula, its table entry, and its state symbol. Then check that the equation has equal numbers of each kind of atom on both sides.

There are no measured values or units to calculate for this prediction. The useful evidence is the solubility classification in the course table. In an actual mixture, visible cloudiness or a solid would be consistent with the prediction. In this lesson, the prediction comes from the table rather than from invented observations.

Balance with coefficients; never change subscripts to balance.
Use $(s)$ for an insoluble product and $(aq)$ for a soluble product.
This task uses classifications, not numerical calculations or significant figures.

Worked example

Predict a precipitate from two clear solutions

Aqueous lead(II) nitrate is mixed with aqueous potassium iodide. Predict whether a precipitate forms, identify it, and write a balanced equation.

List the ions
Lead(II) nitrate contains lead(II) ions and nitrate ions. Potassium iodide contains potassium ions and iodide ions. The charges help you form correct product formulas.
$\mathrm{Pb^{2+},\ NO_3^{-},\ K^{+},\ I^{-}}$
Pair ions to form products
Pair lead(II) with iodide, and potassium with nitrate. The lead(II) ion needs two iodide ions to make a neutral compound. Potassium and nitrate combine in a one-to-one ratio. The possible products are lead(II) iodide and potassium nitrate.
$\mathrm{PbI_2,\ KNO_3}$
Check solubility
The solubility table identifies lead(II) iodide as insoluble, so it is the precipitate. Potassium nitrate is soluble because it contains a Group 1 ion and a nitrate ion.
$\mathrm{PbI_2(s),\ KNO_3(aq)}$
Balance and label states
There are two iodide ions in each formula unit of lead(II) iodide, so use two units of potassium iodide. This also supplies two potassium ions, which form two units of potassium nitrate. The resulting equation has equal numbers of each kind of atom on both sides.
$\mathrm{Pb(NO_3)_2(aq) + 2KI(aq) \rightarrow PbI_2(s) + 2KNO_3(aq)}$

Answer: A precipitate forms. It is solid lead(II) iodide, written as

\mathrm{PbI_2(s)}

.

Check: There is one lead atom, two potassium atoms, two iodine atoms, and two nitrate groups on each side. The equation is balanced.

Common mistakes and how to avoid them

Assuming that every possible product is a precipitate.

Correction: Check each product against the solubility table. A product is a precipitate only if the table identifies it as insoluble.

Copying the original subscripts into a new product formula.

Correction: Build each formula from the ions in the new pairing and make the total charge zero.

Changing a subscript to balance the equation.

Correction: Keep each correct formula unchanged. Use coefficients to balance atom counts.

Writing an insoluble product as aqueous.

Correction: Use

(s)

for the insoluble product that forms the precipitate, and

(aq)

for a soluble product.

Lesson summary

A precipitate is an insoluble solid formed when solutions are mixed.
List the ions, pair positive and negative ions in new combinations, and write neutral product formulas.
Use the solubility table and its exceptions to decide which product is insoluble.
Assign state symbols and balance the complete equation using coefficients.

Check your understanding

Question 1

Aqueous sodium carbonate and aqueous calcium chloride are mixed. Which product is predicted to be the precipitate?

$\mathrm{CaCO_3(s)}$
$\mathrm{NaCl(s)}$
$\mathrm{CaCl_2(s)}$
No precipitate is predicted.

Show answer and explanation

\mathrm{CaCO_3(s)}

Calcium carbonate is insoluble according to the course-level solubility pattern for carbonates. Sodium chloride is soluble because it contains a Group 1 ion.

Question 2

Aqueous potassium nitrate and aqueous sodium chloride are mixed. What does the solubility table predict?

A potassium chloride precipitate forms.
A sodium nitrate precipitate forms.
Both possible products are soluble, so no precipitate is predicted.
A nitrate precipitate forms because nitrates are insoluble.

Show answer and explanation

Both possible products are soluble, so no precipitate is predicted.

The possible products are potassium chloride and sodium nitrate. Both are soluble: potassium is a Group 1 ion, and nitrate compounds are soluble.

Key terms

Aqueous: Dissolved in water; shown by the state symbol $(aq)$ .
Insoluble: Classified by the solubility table as not dissolving enough in water to remain as a dissolved solution.
Precipitate: An insoluble solid that forms from substances in solution.
Solubility table: A reference table that classifies ionic compounds as soluble, insoluble, or sometimes slightly soluble in water.
State symbol: A symbol after a formula that shows whether a substance is solid, liquid, gas, or dissolved in water.

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Published by DoAssignment. This AI-assisted lesson follows Ontario Grade 11 Chemistry (SCH3U), expectation E3.4. It is a study resource, not an official curriculum publication.

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E3.4 · Predict precipitates using a solubility table