C3.3 Explain acid and base formation from metal and non-metal oxides

Ontario Grade 11 Chemistry

Chemical Reactions

Ontario Grade 11 Chemistry — study topic C3.3

Some oxides change water into an acidic or basic solution. For example, carbon dioxide dissolving in water can produce an acidic solution, while calcium oxide reacts with water to form a basic substance. The key idea is that the type of element in the oxide helps predict the product: many non-metal oxides form acids with water, while many metal oxides form bases. These are useful patterns, not a claim that every oxide reacts with water.

What you will learn

Describe the observable pattern when some oxides react with water.
Explain how non-metal oxides can form acids and metal oxides can form bases.
Use chemical formulas and balanced equations to represent these reactions.

1. Prerequisite bridge: elements, oxides, acids, and bases

An element is a pure substance made of one kind of atom. Metals and non-metals are two broad groups of elements. The periodic table helps you identify them: metals are mostly on the left and in the centre, while non-metals are mostly on the right.

An oxide is a compound made of oxygen and another element. Carbon dioxide, for example, contains carbon and oxygen. Calcium oxide contains calcium and oxygen. A formula shows which atoms are in a substance and how many of each are present.

An acid is a substance that forms hydrogen ions in water. A base is a substance that forms hydroxide ions in water. The formula for a hydrogen ion is

\mathrm{H^+}

, and the formula for a hydroxide ion is

\mathrm{OH^-}

. At this level, these definitions help connect the products of oxide reactions to acidic and basic solutions.

Water is written as

\mathrm{H_2O}

. In reaction equations, the state label

(aq)

means dissolved in water,

(s)

means solid, and

(l)

means liquid. These labels describe physical state; they do not change the substance's formula.

An oxide contains oxygen and another element.
Acids form hydrogen ions in water; bases form hydroxide ions in water.
State labels show whether a substance is solid, liquid, or dissolved in water.

2. Observable pattern and particle-level model

An acidic solution and a basic solution have different effects on indicators. An indicator is a substance that changes colour depending on whether a solution is acidic or basic. This observable change can be evidence that a reaction with an oxide has produced a different kind of solution. It does not, by itself, show the particles present.

At the particle level, the key change is that an oxide combines with water and produces a substance with acidic or basic properties. For a non-metal oxide, the product is often an acid. For a metal oxide, the product is often a base that contains hydroxide. The reaction equation shows the atoms rearranged into the product; atoms are not created or destroyed.

For example, carbon dioxide is a non-metal oxide. It reacts with water to form carbonic acid. The equation is balanced: there is one carbon atom, two hydrogen atoms, and three oxygen atoms on each side.

Calcium oxide is a metal oxide. It reacts with water to form calcium hydroxide, a base. The formula has two hydroxide groups because the calcium ion has a charge of

2+

and each hydroxide ion has a charge of

1-

. The charges balance in the neutral compound.

\mathrm{CO_2(g) + H_2O(l) \rightarrow H_2CO_3(aq)}

Many non-metal oxides form acids when they react with water.
Many metal oxides form bases when they react with water.
A balanced equation has the same number of each kind of atom on both sides.

3. Writing and interpreting oxide reactions

To predict a product, first identify whether the element joined to oxygen is a metal or a non-metal. Then use the course-level pattern: non-metal oxide plus water can form an acid; metal oxide plus water can form a hydroxide base. Check the product formula using the formulas and ion charges you have learned.

Sulfur dioxide is a non-metal oxide. With water, it forms sulfurous acid. Sulfur trioxide is another non-metal oxide; with water, it forms sulfuric acid. These examples show that different oxides can produce different acids. Do not assume that all non-metal oxides form the same acid.

Calcium oxide plus water forms calcium hydroxide. In the formula, the hydroxide group is written in parentheses when more than one is needed. The subscript outside the parentheses applies to the whole group. Parentheses are not needed when there is just one hydroxide group.

The patterns are useful for prediction, but they have limits. Some oxides do not react with water in this simple way. So, use the pattern for oxides known to form acids or bases with water, rather than stating that every oxide does so. Include state labels when they are known and relevant.

\mathrm{SO_2(g) + H_2O(l) \rightarrow H_2SO_3(aq)}

Identify the metal or non-metal before predicting the product.
Use correct compound formulas and then check that the equation is balanced.
The oxide-plus-water pattern does not apply to every oxide.

4. Checking the model and its limits

A chemical equation is a compact model of a reaction. The substances before the arrow are reactants; the substances after it are products. The arrow means that the reactants form the products. A formula gives the identity of a substance, while a coefficient placed in front of a formula counts whole particles or formula units.

When checking an equation, count each kind of atom on both sides. Change coefficients if needed, not the small subscripts in a formula. Changing a subscript changes the identity of a substance. For instance, changing the number of oxygen atoms in an oxide would describe a different compound.

The product formula also matters. A metal oxide forming a base often produces a metal hydroxide, which contains

\mathrm{OH^-}

. A non-metal oxide forming an acid produces an acid with hydrogen in its formula. These are product patterns that help explain why the resulting water-based solution is called acidic or basic.

The equations in this lesson represent the overall substances formed. They do not show every detail of particles in solution. For this expectation, the important explanation is the relationship between oxide type, reaction with water, and acid or base formation.

Balance equations by changing coefficients, never by altering formulas.
A metal hydroxide contains hydroxide groups.
Use the oxide type and the product formula to explain acid or base formation.

Worked example

Predicting a base from a metal oxide

Calcium oxide reacts with water. Predict the product, write a balanced equation, and explain why the product is a base.

Classify the oxide
Calcium is a metal, so calcium oxide is a metal oxide. The course-level pattern predicts a hydroxide base when this oxide reacts with water.
Write the product formula
Calcium forms a $2+$ ion, and hydroxide has a $1-$ charge. Two hydroxide ions are needed to balance one calcium ion, so the neutral product is calcium hydroxide.
$\mathrm{Ca(OH)_2}$
Write and check the equation
Place calcium oxide and water before the arrow and calcium hydroxide after it. One calcium, two hydrogen, and two oxygen atoms appear on each side, so the equation is balanced with coefficients of one.
$\mathrm{CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(s)}$
Connect the product to a base
Calcium hydroxide contains hydroxide groups. In water, the hydroxide ions are associated with the basic properties of the solution. The product therefore fits the metal oxide to base pattern.

Answer: The product is calcium hydroxide, a base. The balanced equation is

\mathrm{CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(s)}

.

Check: The equation has one calcium atom, two oxygen atoms, and two hydrogen atoms on each side. No numerical measurement is involved, so units and significant digits do not apply.

Common mistakes and how to avoid them

Assuming every oxide reacts with water to form an acid or base.

Correction: Treat acid or base formation as a pattern for oxides that react with water in this way. Do not apply it to every oxide.

Calling a metal oxide an acid because it contains oxygen.

Correction: Classify the other element in the oxide. Many metal oxides form bases with water; many non-metal oxides form acids.

Changing a formula subscript to balance an equation.

Correction: Keep each substance's formula fixed. Balance the equation by changing coefficients in front of formulas.

Writing calcium hydroxide as

\mathrm{CaOH_2}

.

Correction: Use parentheses because the subscript two applies to the whole hydroxide group:

\mathrm{Ca(OH)_2}

.

Lesson summary

An oxide is a compound containing oxygen and another element.
Many non-metal oxides react with water to form acids.
Many metal oxides react with water to form hydroxide bases.
Use correct product formulas and balance equations by conserving each type of atom.
The pattern is not a rule for every oxide.

Check your understanding

Question 1

Which product pattern is expected when a non-metal oxide reacts with water?

It can form an acid.
It always forms a metal hydroxide.
It always forms a metal oxide.
It cannot form a new substance.

Show answer and explanation

It can form an acid.

Many non-metal oxides react with water to form acids. The word “many” matters because the pattern does not apply to every oxide.

Question 2

Which equation correctly represents calcium oxide reacting with water?

$\mathrm{CaO + H_2O \rightarrow Ca(OH)_2}$
$\mathrm{CaO + H_2O \rightarrow CaOH_2}$
$\mathrm{CaO + H_2O \rightarrow CaH_2 + O_2}$
$\mathrm{CaO + H_2O \rightarrow Ca(OH)_3}$

Show answer and explanation

\mathrm{CaO + H_2O \rightarrow Ca(OH)_2}

Calcium hydroxide has two hydroxide groups. The first equation has the correct formula and one calcium, two oxygen, and two hydrogen atoms on each side.

Key terms

Oxide: A compound made of oxygen and another element.
Acid: A substance that forms hydrogen ions in water.
Base: A substance that forms hydroxide ions in water.
Hydroxide: A group of one oxygen atom and one hydrogen atom with a charge of $1-$ , written as $\mathrm{OH^-}$ .
Indicator: A substance that changes colour to show whether a solution is acidic or basic.
Balanced equation: A reaction equation with the same number of each kind of atom on both sides.

Continue through SCH3U

View the complete SCH3U Ontario Grade 11 Chemistry curriculum and lessons

About this lesson and its review

Published by DoAssignment. This AI-assisted lesson follows Ontario Grade 11 Chemistry (SCH3U), expectation C3.3. 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.

Official curriculum reference

Report a correction or ask a question

C3.3 · Explain acid and base formation from metal and non-metal oxides