Explain Arrhenius acids and bases

Ontario Grade 11 Chemistry

Solutions and Solubility

Identifying acids and bases by the ions they produce in water

An indicator can change colour when it is placed in an acidic or basic solution. The colour change is something we can observe, but it does not show the particles in the solution. The Arrhenius model explains acidic and basic behaviour by focusing on ions formed in water. An ion is an atom or group of atoms with an electrical charge. A solution is aqueous when a substance is dissolved in water. This lesson links the visible observation to the particle model and then to chemical equations.

What you will learn

Define an Arrhenius acid and an Arrhenius base.
Connect an indicator colour change with the particle model for acidic and basic solutions.
Represent the formation of characteristic ions with balanced chemical equations.
Classify a substance using the ions it produces in water.

1. Prerequisite bridge: ions in water

In earlier chemistry, you learned that some substances form ions when they dissolve in water. Water surrounds the dissolved particles, allowing ions to move through the solution. The state symbol (aq) means aqueous, or dissolved in water.

An ion has an electrical charge. A hydrogen ion is written

\mathrm{H^+}

. A hydroxide ion is written

\mathrm{OH^-}

. The superscript shows the charge. In a formula such as

\mathrm{Ca(OH)_2}

, the subscript 2 outside the brackets applies to the whole hydroxide group. It represents two hydroxide groups.

An indicator is a substance that changes colour in response to the chemical conditions of a solution. Its colour provides observable evidence. The Arrhenius model gives a particle-level explanation for acidic and basic behaviour.

Aqueous means dissolved in water.
Ions are charged particles.
An indicator colour is observable evidence; it does not directly show the ions.

2. The Arrhenius definitions

An Arrhenius acid is a substance that produces hydrogen ions,

\mathrm{H^+}

, when dissolved in water. Hydrochloric acid is an example. In aqueous solution, it forms hydrogen ions and chloride ions.

An Arrhenius base is a substance that produces hydroxide ions,

\mathrm{OH^-}

, when dissolved in water. Sodium hydroxide is an example. In water, it forms sodium ions and hydroxide ions.

The definitions focus on what a substance produces in water. Do not decide from a name or from one part of a formula alone. Instead, identify the ions formed in aqueous solution. In this model, hydrogen ions identify an acid and hydroxide ions identify a base.

\text{acid} \rightarrow \mathrm{H^+(aq)} \qquad \text{base} \rightarrow \mathrm{OH^-(aq)}

An Arrhenius acid produces $\mathrm{H^+}$ in water.
An Arrhenius base produces $\mathrm{OH^-}$ in water.
The defining feature is the ion produced in aqueous solution.

3. Representing ions with equations

A chemical equation represents a change in substances and particles. For hydrochloric acid, the equation shows hydrogen ions and chloride ions forming in water. The number of each type of atom must be the same on both sides. The total electrical charge must also be the same.

Ionic compounds can be shown separating into their ions. For calcium hydroxide, the formula contains one calcium ion and two hydroxide groups. The product equation must therefore show one calcium ion and two hydroxide ions. The charges balance: the calcium ion has a charge of

+2

, and the two hydroxide ions have a combined charge of

-2

.

State symbols add information about the substances. The symbol (aq) means dissolved in water, and (s) means solid. Use coefficients to show how many particles are represented. Do not change subscripts in a chemical formula to balance an equation.

\mathrm{HCl(aq) \rightarrow H^+(aq) + Cl^-(aq)}

Check that atom counts match on both sides.
Check that total charge matches on both sides.
A coefficient counts particles or groups; a subscript is part of a formula.

4. Using the definitions to classify substances

To classify a substance with the Arrhenius model, examine its aqueous ion representation. If it produces

\mathrm{H^+}

, it fits the acid definition. If it produces

\mathrm{OH^-}

, it fits the base definition.

A formula may contain hydrogen without the information needed to identify it as an Arrhenius acid. A formula may contain oxygen without showing that it produces hydroxide ions. Use the ions formed in water, not a single element in the formula, as your evidence.

When an equation is given, read it carefully. Count groups as well as individual atoms, and include every product ion. A balanced equation makes the particle representation consistent with the atoms and charges in the formulas.

Classify by the ions produced in water.
Read brackets and subscripts to count complete groups.
Include all ions so atoms and total charge are conserved.

Worked example

Classify calcium hydroxide from its ions

Write an equation showing calcium hydroxide forming ions in water. Use the equation to decide whether it is an Arrhenius base.

Read the formula
The formula contains one calcium atom and two hydroxide groups. Each hydroxide ion has a charge of $-1$ . The calcium ion has a charge of $+2$ , so the ions together have zero net charge.
$\mathrm{Ca(OH)_2 \rightarrow Ca^{2+} + 2OH^-}$
Show the aqueous particles
The question asks for the ions formed in water, so label the ions as aqueous. This representation includes the calcium ion as well as the two hydroxide ions.
$\mathrm{Ca(OH)_2(aq) \rightarrow Ca^{2+}(aq) + 2OH^-(aq)}$
Check atoms and charge
Each side has one calcium atom, two oxygen atoms, and two hydrogen atoms. The product charges add to zero, matching the neutral formula unit on the left.
$(+2)+2(-1)=0$
Apply the definition
The equation shows hydroxide ions among the products. A substance that produces hydroxide ions in water fits the Arrhenius definition of a base.

Answer: Calcium hydroxide is an Arrhenius base because it produces

\mathrm{OH^-}

ions in water.

Check: The equation shows one calcium ion and two hydroxide ions. Both atom counts and total charge are conserved.

Common mistakes and how to avoid them

Calling every substance with hydrogen in its formula an Arrhenius acid.

Correction: An Arrhenius acid produces

\mathrm{H^+}

in water. Check the ions formed rather than relying on the presence of hydrogen in the formula.

Calling every substance with oxygen in its formula an Arrhenius base.

Correction: An Arrhenius base produces

\mathrm{OH^-}

in water. Oxygen in a formula alone does not show that hydroxide ions are produced.

Showing only hydroxide ions when calcium hydroxide forms ions.

Correction: Include the calcium ion as well. The complete equation must conserve both atoms and total charge.

Changing a subscript to balance an equation.

Correction: Keep formulas unchanged. Use coefficients to show the number of particles, then check atoms and total charge.

Lesson summary

An Arrhenius acid produces $\mathrm{H^+}$ in water.
An Arrhenius base produces $\mathrm{OH^-}$ in water.
Aqueous equations represent the ions formed when substances are dissolved in water.
A complete ion equation conserves atoms and total charge.

Check your understanding

Question 1

Which statement defines an Arrhenius acid?

It produces $\mathrm{OH^-}$ ions in water.
It produces $\mathrm{H^+}$ ions in water.
It always contains oxygen.
It always contains two hydrogen atoms.

Show answer and explanation

It produces

\mathrm{H^+}

ions in water.

An Arrhenius acid produces hydrogen ions in aqueous solution. The other statements do not give the Arrhenius acid definition.

Question 2

A substance produces

\mathrm{OH^-}

ions when dissolved in water. How is it classified by the Arrhenius model?

As an Arrhenius acid
As an Arrhenius base
As neither, because ions cannot form in water
As an acid because hydroxide has a charge

Show answer and explanation

As an Arrhenius base

Producing hydroxide ions in water matches the Arrhenius definition of a base.

Question 3

How many hydroxide ions are represented when

\mathrm{Ca(OH)_2}

forms ions?

One
Two
Three
Zero

Show answer and explanation

Two

The subscript 2 outside the brackets applies to the whole hydroxide group, so the formula represents two hydroxide ions.

Key terms

Aqueous: Dissolved in water; shown by the state symbol $\mathrm{(aq)}$ .
Arrhenius acid: A substance that produces $\mathrm{H^+}$ ions when dissolved in water.
Arrhenius base: A substance that produces $\mathrm{OH^-}$ ions when dissolved in water.
Ion: An atom or group of atoms with an electrical charge.
Indicator: A substance that changes colour in response to the chemical conditions of a solution.

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About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 11 Chemistry (SCH3U), expectation E3.5. 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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