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What are acid–base indicators? Give examples of natural indicators.

Class 7 ScienceNCERT chapter: Exploring Substances: Acidic, Basic, and Neutralइसे हिंदी में पढ़ें

Exam-ready answer

Acid–base indicators are substances that change colour when added to an acidic or a basic solution, so they tell us whether a substance is acidic, basic or neutral. Natural indicators come from plants: litmus (from lichens), turmeric, and extracts of red rose or China rose (hibiscus) petals. Blue litmus turns red in acids, and red litmus turns blue in bases.

Explanation

Acids such as lemon juice and vinegar are sour, and bases such as soap solution feel soapy, but tasting or touching unknown substances is unsafe. So we use indicators, which show a different colour depending on whether a solution is acidic or basic. Substances that are neither acidic nor basic, such as pure water and sugar solution, are neutral and do not change the colour of indicators.

Litmus is the most common natural indicator. It is extracted from lichens and is used as a solution or as strips of red and blue litmus paper. An acid turns blue litmus red, a base turns red litmus blue, and a neutral solution changes neither. Turmeric is yellow; it turns red in a basic solution and stays yellow in acidic and neutral ones. That is why a turmeric stain on a white cloth turns red when washed with soap.

Flower petals also work as indicators. China rose (hibiscus) extract, made by soaking petals in warm water, turns dark pink (magenta) in acids and green in bases. Red rose extract also turns green in basic solutions. Some indicators are made in laboratories; phenolphthalein, for example, is colourless in acidic and neutral solutions and turns pink in basic solutions.

Points to remember

  1. Indicator: a substance that changes colour in acidic or basic solutions.
  2. Litmus (from lichens): blue litmus turns red in acid; red litmus turns blue in base; no change in a neutral solution.
  3. Turmeric: yellow turns red in a basic solution; no change in acidic or neutral solutions.
  4. China rose (hibiscus) extract: dark pink (magenta) in acid, green in base; red rose extract also turns green in base.
  5. Phenolphthalein is a synthetic indicator: colourless in acidic and neutral solutions, pink in basic solutions.

Exam tip

Show the colour changes in a table with columns for indicator, colour in acid and colour in base; make sure blue-to-red (acid) and red-to-blue (base) for litmus are not mixed up.

Questions students also ask

Why does a turmeric stain turn red when washed with soap?
Soap solution is basic. Turmeric is a natural indicator that turns red in basic substances, so when a yellow turmeric (haldi) stain on a cloth meets soap, the stain becomes red.
How can we test whether a solution is neutral?
Put a drop of the solution on both red and blue litmus paper. If neither paper changes colour, the solution is neutral. Pure water, sugar solution and common salt solution are examples of neutral solutions.
Where does litmus come from?
Litmus is a natural dye obtained from lichens, which are small plant-like organisms formed by an alga and a fungus living together. It is purple in distilled water and is commonly used as red and blue litmus paper strips.

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