What is the color change of litmus in acid and base?

Short Answer

Litmus is a natural indicator that shows different colours in acidic and basic solutions. In an acid, blue litmus paper turns red, showing the presence of acidity. In a base, red litmus paper turns blue, indicating a basic or alkaline solution.

This simple colour change makes litmus one of the most common indicators used in schools and laboratories. It helps quickly identify whether a substance is acidic or basic.

Detailed Explanation :

Color Change of Litmus in Acid and Base

Litmus is one of the oldest and most widely used natural indicators for testing acidity and basicity. It is extracted from lichens and has the special ability to change colour when dipped into different types of solutions. Because of its clear and quick colour change, litmus paper is commonly used in school experiments, chemical laboratories, and simple household tests.

The main function of litmus is to detect whether a solution is acidic or basic by changing its colour. The colour change happens due to the chemical reaction between the litmus dye molecules and the hydrogen (H⁺) or hydroxide ions (OH⁻) present in the solution.

Litmus in Acidic Solutions

An acidic solution contains a higher concentration of hydrogen ions (H⁺). When litmus paper comes in contact with these ions, a chemical reaction takes place that alters the structure of the litmus dye.

Result:

  • Blue litmus turns red in acidic solution.

This colour change is a clear sign of acidity. For example:

  • Lemon juice
  • Vinegar
  • Coca-cola
  • Stomach acid

All these substances will turn blue litmus paper red.

This reaction occurs because hydrogen ions modify the litmus dye so that it reflects red light instead of blue.

Litmus in Basic Solutions

A basic (or alkaline) solution contains a higher concentration of hydroxide ions (OH⁻). When litmus paper is placed in a basic solution, the OH⁻ ions interact with the litmus dye in a different way.

Result:

  • Red litmus turns blue in basic solution.

Examples of basic substances include:

  • Soap solution
  • Baking soda solution
  • Detergent
  • Lime water

All these substances turn red litmus paper blue.

This change happens because hydroxide ions shift the chemical balance inside the litmus dye, causing it to reflect blue light.

Why Litmus Changes Colour

The colour change in litmus is due to acid–base sensitivity of the dye. Litmus is made of molecules that behave like weak acids and weak bases. When they interact with H⁺ or OH⁻ ions:

  • In acidic medium, more molecules accept H⁺ ions → appear red.
  • In basic medium, more molecules lose H⁺ ions → appear blue.

This shift in molecular form is what creates the visible colour difference.

Litmus Paper Types and Their Use

There are two main types of litmus paper:

  1. Blue Litmus Paper
  • Turns red in acid
  • Stays blue in base

Useful to check if a solution is acidic.

  1. Red Litmus Paper
  • Turns blue in base
  • Stays red in acid

Useful to check if a solution is basic.

Neutral Solution

In a neutral solution (pH 7):

  • Blue litmus stays blue
  • Red litmus stays red

For example, pure water does not change the colour of either litmus paper.

Practical Uses of Litmus

Litmus is used in many real-life and laboratory situations:

  1. School Chemistry Experiments

Students use litmus to learn about acids and bases.

  1. Testing Unknown Solutions

A quick and simple test to classify liquids.

  1. Environmental Studies

Used to check acidity of rainwater or soil samples.

  1. Food Industry

Helps detect spoilage or acidity in some food items.

  1. Household Uses

People sometimes use litmus strips to test water quality.

Limitations of Litmus

Although litmus is useful, it has some limitations:

  • It cannot tell the exact pH value; it only shows acid or base.
  • It does not work well with dark-coloured solutions where the colour change is hard to see.
  • Extremely strong acids or bases may damage the paper.
  • It may not work correctly if the paper is too old or wet.

Because of these limitations, more accurate tools like universal indicator or pH meter are used when precise measurement is needed.

Conclusion

Litmus is a natural indicator that changes colour based on whether a solution is acidic or basic. In acidic solutions, blue litmus turns red, while in basic solutions, red litmus turns blue. This simple colour change helps identify the nature of a solution quickly and easily. Although litmus is not a precise pH measuring tool, it remains one of the most commonly used indicators in laboratories, schools, and everyday testing.