IB Chemistry Topic R3.1.14 Acid-base Indicators and Ph Changes Revision Notes (HL)

IB Chemistry Topic R3.1.14 Acid-base Indicators and Ph Changes Revision Notes (HL) | RevisionDojo

Written by official IB examiners

Why Do Indicators Change Color?

Analogy

You're conducting a titration, carefully adding a base to an acidic solution drop by drop.
Suddenly, the solution shifts from clear to pink: an unmistakable signal that the reaction has reached its endpoint.
But have you ever wondered why this dramatic color change occurs? What’s happening at the molecular level to produce such a visible transformation?
The answer lies in the chemistry of acid–base indicators, compounds specifically designed to reveal subtle changes in pH.

Acid–Base Indicators: Weak Acids with a Colorful Twist

Definition

Acid–base indicators

Acid–base indicators are weak acids that exist in equilibrium between their protonated form (HInd) and deprotonated form (Ind⁻).

It can be expressed by: $$
\text{HInd (aq)} \rightleftharpoons \text{H}^+ \text{(aq)} + \text{Ind}^- \text{(aq)}
$$
What makes these compounds unique is that HInd and Ind⁻ have different colors. For instance:
1. HInd might be red.
2. Ind⁻ might be blue.

Note

The observed color of the solution depends on the relative concentrations of HInd and Ind⁻, which are influenced by the solution’s pH.

At low pH (acidic conditions), the equilibrium shifts toward HInd, coloring the solution red.
At high pH (basic conditions), Ind⁻ dominates, turning the solution blue.

Tip

Indicators are not "magic".
They follow the same equilibrium principles as other weak acids.

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Tip

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Why Do Indicators Change Color?

Analogy

You're conducting a titration, carefully adding a base to an acidic solution drop by drop.
Suddenly, the solution shifts from clear to pink: an unmistakable signal that the reaction has reached its endpoint.
But have you ever wondered why this dramatic color change occurs? What’s happening at the molecular level to produce such a visible transformation?
The answer lies in the chemistry of acid–base indicators, compounds specifically designed to reveal subtle changes in pH.

Acid–Base Indicators: Weak Acids with a Colorful Twist

Definition

Acid–base indicators

Acid–base indicators are weak acids that exist in equilibrium between their protonated form (HInd) and deprotonated form (Ind⁻).

It can be expressed by: $$
\text{HInd (aq)} \rightleftharpoons \text{H}^+ \text{(aq)} + \text{Ind}^- \text{(aq)}
$$
What makes these compounds unique is that HInd and Ind⁻ have different colors. For instance:
1. HInd might be red.
2. Ind⁻ might be blue.

Note

The observed color of the solution depends on the relative concentrations of HInd and Ind⁻, which are influenced by the solution’s pH.

At low pH (acidic conditions), the equilibrium shifts toward HInd, coloring the solution red.
At high pH (basic conditions), Ind⁻ dominates, turning the solution blue.

Tip

Indicators are not "magic".
They follow the same equilibrium principles as other weak acids.

Definition

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Duis aute irure dolor in reprehenderit

Note

Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam quis nostrud exercitation.

Excepteur sint occaecat cupidatat non proident

Tip

Lorem ipsum dolor sit amet, consectetur adipiscing elit.
Sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.
Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris.
Duis aute irure dolor in reprehenderit in voluptate velit esse cillum.

Structure 1. Models of the particulate nature of matter5 subtopics

Structure 2. Models of bonding and structure4 subtopics

Structure 3. Classification of matter2 subtopics

Reactivity 1. What drives chemical reactions?4 subtopics

Reactivity 2. How much, how fast and how far?3 subtopics

Reactivity 3. What are the mechanisms of chemical change?4 subtopics

IB Chemistry R3.1.14 Acid-base Indicators and Ph Changes Notes

Structure 1. Models of the particulate nature of matter5 subtopics

Structure 2. Models of bonding and structure4 subtopics

Structure 3. Classification of matter2 subtopics

Reactivity 1. What drives chemical reactions?4 subtopics

Reactivity 2. How much, how fast and how far?3 subtopics

Reactivity 3. What are the mechanisms of chemical change?4 subtopics

Why Do Indicators Change Color?

Acid–Base Indicators: Weak Acids with a Colorful Twist

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Why Do Indicators Change Color?

Acid–Base Indicators: Weak Acids with a Colorful Twist

anim nostrud sit dolore minim proident quis fugiat velit et eiusmod nulla quis nulla mollit dolor sunt culpa aliqua

Duis aute irure dolor in reprehenderit

Excepteur sint occaecat cupidatat non proident

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