C2.8 Compare solutions formed by metal and non-metal oxides

Ontario Grade 11 Chemistry

Chemical Reactions

How oxide type helps predict whether water becomes acidic or basic

Some oxides react with water in ways that produce solutions with different properties. A solution made using a metal oxide is often basic, while a solution made using a non-metal oxide is often acidic. In this lesson, you will connect those observations to the particles present in solution and to balanced chemical equations. The patterns are useful, but they do not mean every metal oxide or non-metal oxide behaves in exactly the same way.

What you will learn

Describe the usual difference between solutions formed by metal oxides and non-metal oxides.
Use reaction equations to show how selected oxides form acidic or basic solutions in water.
Compare the particles and indicator evidence in the resulting solutions.
Recognize that these are common patterns, not rules that apply to every oxide.

1. Start with the observable properties

A solution is a uniform mixture in which one or more substances are spread through water. You may already know that an indicator can help identify whether a solution is acidic or basic. For example, blue litmus turns red in an acidic solution, while red litmus turns blue in a basic solution. An indicator shows a property of the solution; it does not identify every substance in it.

When some metal oxides are added to water, the resulting solution is basic. Some non-metal oxides react with water to form acidic solutions. The contrast is a pattern: metal oxide to basic solution, and non-metal oxide to acidic solution. The word “often” matters. The behaviour depends on the particular oxide and whether it reacts with water or forms a solution.

Acidic and basic describe properties of a solution.
Indicators provide observable evidence of those properties.
The metal/non-metal pattern is common, not universal.

2. Connect the pattern to particles

Before using the equations, recall two familiar solution clues. In this course model, an acidic solution contains hydrogen ions, written

\mathrm{H^+(aq)}

. A basic solution contains hydroxide ions, written

\mathrm{OH^-(aq)}

. The symbol

(aq)

means that the particle is in water. These ions help explain why the two kinds of solutions have different indicator results.

A common metal oxide is sodium oxide. It reacts with water to form sodium hydroxide. Sodium hydroxide separates into sodium ions and hydroxide ions in water. The hydroxide ions account for the basic property of the solution.

A common non-metal oxide is sulfur trioxide. It reacts with water to form sulfuric acid. The acid solution contains hydrogen ions, which account for its acidic property. The substances formed in these examples are different, so their solutions contain different characteristic ions.

\mathrm{NaOH(aq) \rightarrow Na^+(aq) + OH^-(aq)}

Hydroxide ions are associated with basic solutions.
Hydrogen ions are associated with acidic solutions.
The dissolved particles explain the observed solution properties.

3. Show the changes with equations

A chemical equation represents a chemical change. The formulas on the left show the starting substances; those on the right show the products. A balanced equation has the same number of each kind of atom on both sides. State symbols give information about the substances:

(s)

means solid,

(l)

means liquid, and

(aq)

means dissolved in water.

For sodium oxide, one formula unit contains two sodium atoms. The product therefore needs two units of sodium hydroxide. The resulting equation is balanced: it has two sodium atoms, two hydrogen atoms, and two oxygen atoms on each side. The sodium hydroxide is shown as aqueous because it dissolves in water.

For sulfur trioxide, the atoms are already balanced when one water molecule forms one sulfuric acid unit. Sulfuric acid is shown as aqueous in this representation. Together, these equations show the comparison: this metal oxide produces a basic solution, while this non-metal oxide produces an acidic solution.

Do not infer a solution property from an oxide’s name alone. First identify the particular oxide and the product formed with water. Then connect the product and its dissolved particles to the solution property.

\mathrm{Na_2O(s) + H_2O(l) \rightarrow 2NaOH(aq)}

Balance atoms when writing a reaction equation.
Use state symbols when they clarify what is present.
For the examples here, sodium oxide forms a basic solution and sulfur trioxide forms an acidic solution.

4. A careful comparison

A useful comparison has three parts: identify the oxide as a metal or non-metal oxide, state what happens when it reacts with water, and describe the solution formed. For the examples in this lesson, the metal oxide forms a hydroxide solution containing hydroxide ions. The non-metal oxide forms an acid solution containing hydrogen ions.

This comparison is about solutions formed by oxides and water. It is not a claim that an oxide must dissolve completely, or that every oxide reacts with water in the same way. If a question names a particular oxide, use its reaction and the course-level information provided to decide what solution forms. Do not treat “metal oxide” or “non-metal oxide” as enough information to predict every possible case.

No amount calculation is needed for this comparison. The key evidence is the balanced reaction and the identity of the particles in the resulting solution. Keeping the equation balanced also prevents an incorrect particle count from weakening the comparison.

Compare specific examples using reaction products and solution particles.
Do not claim that every oxide follows the same pattern.
Balanced equations support accurate particle-level comparisons.

Worked example

Compare two oxide solutions

Sodium oxide and sulfur trioxide each react with water. Compare the resulting solutions using balanced equations and the ions associated with each solution.

Identify the oxide types
Sodium is a metal, so sodium oxide is a metal oxide. Sulfur is a non-metal, so sulfur trioxide is a non-metal oxide.
Write the water reactions
Sodium oxide forms sodium hydroxide in water. The coefficient 2 is needed to conserve the two sodium atoms. Sulfur trioxide forms sulfuric acid; the atoms are balanced with one water molecule.
$\mathrm{Na_2O(s) + H_2O(l) \rightarrow 2NaOH(aq)}$
Connect products to solution properties
Aqueous sodium hydroxide separates into sodium ions and hydroxide ions. The hydroxide ions are associated with a basic solution. Sulfuric acid in water is associated with hydrogen ions, so the sulfuric acid solution is acidic.
$\mathrm{SO_3(g) + H_2O(l) \rightarrow H_2SO_4(aq)}$

Answer: The metal oxide forms a basic sodium hydroxide solution containing

\mathrm{OH^-(aq)}

. The non-metal oxide forms an acidic sulfuric acid solution containing

\mathrm{H^+(aq)}

.

Check: The sodium oxide equation has two sodium, two oxygen, and two hydrogen atoms on each side. The sulfur trioxide equation has one sulfur, four oxygen, and two hydrogen atoms on each side.

Common mistakes and how to avoid them

Saying every metal oxide forms a basic solution and every non-metal oxide forms an acidic solution.

Correction: Describe these as common patterns. Check the particular oxide and its reaction with water.

Calling sodium hydroxide an acid because it is dissolved in water.

Correction: Sodium hydroxide produces hydroxide ions in water, so the solution is basic.

Leaving out a coefficient when balancing the sodium oxide equation.

Correction: Count each type of atom on both sides. Two sodium atoms require two sodium hydroxide units.

Confusing a solution’s indicator result with the identity of the oxide.

Correction: An indicator shows whether a solution is acidic or basic. Use the reaction equation to identify the substances that formed it.

Lesson summary

Some metal oxides react with water to form basic solutions; some non-metal oxides react with water to form acidic solutions.
Hydroxide ions are associated with basic solutions, while hydrogen ions are associated with acidic solutions.
Use a balanced reaction and the particles in solution to compare specific examples.
The pattern is common but does not apply automatically to every oxide.

Check your understanding

Question 1

Which ions are associated with the basic solution formed when sodium oxide reacts with water?

$\mathrm{H^+(aq)}$
$\mathrm{OH^-(aq)}$
$\mathrm{Na^+(s)}$
$\mathrm{O^{2-}(aq)}$

Show answer and explanation

\mathrm{OH^-(aq)}

Sodium oxide forms sodium hydroxide in water, and aqueous sodium hydroxide contains hydroxide ions.

Question 2

Which statement best compares the two examples in this lesson?

Both solutions are acidic because both starting substances are oxides.
The sodium oxide solution is basic, and the sulfur trioxide solution is acidic.
The sodium oxide solution is acidic, and the sulfur trioxide solution is basic.
Neither oxide reacts with water.

Show answer and explanation

The sodium oxide solution is basic, and the sulfur trioxide solution is acidic.

Sodium oxide forms sodium hydroxide, a basic solution. Sulfur trioxide forms sulfuric acid, an acidic solution.

Key terms

Aqueous: Dissolved in water; shown by the state symbol $(aq)$ .
Basic solution: A solution associated with hydroxide ions, $\mathrm{OH^-}$ .
Acidic solution: A solution associated with hydrogen ions, $\mathrm{H^+}$ .
Indicator: A substance that changes colour to give evidence about a solution’s properties.
Metal oxide: A compound made from oxygen and a metal.
Non-metal oxide: A compound made from oxygen and a non-metal.

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Published by DoAssignment. This AI-assisted lesson follows Ontario Grade 11 Chemistry (SCH3U), expectation C2.8. It is a study resource, not an official curriculum publication.

Before publication, the draft is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability, and then requires administrator approval. Errors can still occur, so corrections are welcomed.

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C2.8 · Compare solutions formed by metal and non-metal oxides