What Are Indicators?

Indicators are substances that change colour depending on the acidity or alkalinity of a solution.

They are commonly used in chemistry laboratories to identify whether a substance is acidic, neutral, or alkaline.

Indicators work because their molecules change structure in different pH conditions, which causes a visible colour change.

Why Indicators Are Important

Indicators help chemists:

• Identify acids and alkalis
• Measure pH changes in reactions
• Detect the end point of titrations
• Perform neutralisation experiments

Without indicators, it would be difficult to observe pH changes in solutions.

Litmus Indicator

Litmus is one of the most commonly used indicators in chemistry.

It is often used in the form of litmus paper.

Colour Changes of Litmus

Solution TypeLitmus ColourAcidic solutionRedNeutral solutionPurpleAlkaline solutionBlue

Example:

Hydrochloric acid turns blue litmus paper red.

Methyl Orange

Methyl orange is another common laboratory indicator used to detect acidic and alkaline solutions.

Colour Changes of Methyl Orange

Solution TypeColourAcidic solutionRedNeutral solutionOrangeAlkaline solutionYellow

Methyl orange is often used in acid–base titration experiments.

Phenolphthalein

Phenolphthalein is an indicator commonly used in titrations involving strong acids and strong bases.

Colour Changes of Phenolphthalein

Solution TypeColourAcidic solutionColourlessNeutral solutionColourlessAlkaline solutionPink

Phenolphthalein is particularly useful for detecting alkaline solutions.

Universal Indicator

A universal indicator is a mixture of several indicators that shows a wide range of colours across the pH scale.

It provides a more accurate estimate of the pH value of a solution.

Universal Indicator Colours

pH ValueColour0–3Red4–6Orange / Yellow7Green8–11Blue12–14Purple

Universal indicator is widely used to estimate pH values quickly.

Indicators and the pH Scale

Indicators help scientists measure how acidic or alkaline a solution is.

The pH scale ranges from:

• 0–6 → Acidic
• 7 → Neutral
• 8–14 → Alkaline

Different indicators change colour at different pH ranges.

Everyday Uses of Indicators

Indicators are used in many practical situations, including:

• Laboratory experiments
• Water quality testing
• Food chemistry
• Soil testing in agriculture

These applications help monitor chemical conditions in various environments.

Exam Tip (5070)

Students are often asked to:

• Identify colour changes of indicators
• Choose the correct indicator for an experiment
• Explain how indicators help detect acids and bases

Example exam question:

What colour does phenolphthalein turn in an alkaline solution?

Answer:

Pink

Practice Question

What colour does methyl orange turn in an alkaline solution?

Answer

Methyl orange turns yellow in an alkaline solution.

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