Acid-Base Indicators

Natural Indicators

Colors from Nature

Indicators are substances that change color in the presence of an acid or base.
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Key Points

  1. Litmus: From lichens. Acid=Red, Base=Blue.
  2. Turmeric: Stays yellow in acid, turns red in base.
  3. China Rose: Turns magenta in acid, green in base.
  4. Red Cabbage: Turns red in acid, green in base.

Synthetic Indicators

Lab Precision

Man-made indicators like Phenolphthalein and Methyl Orange provide sharp color transitions.
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Key Points

  1. Phenolphthalein: Colorless in acid, bright pink in base.
  2. Methyl Orange: Red in acid, yellow in base.
  3. These are critical for precise titrations in chemistry labs.

Olfactory Indicators

Testing by Smell

Some substances change their odor (smell) in acidic or basic media. These are called olfactory indicators.
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Key Points

  1. Used for testing without visual color changes.
  2. Example: Onion and Vanilla essence.
  3. Onion loses its characteristic smell in basic solutions (like NaOHNaOH).
  4. Vanilla loses its pleasant scent in a basic medium.

Olympiad Fact

Olfactory indicators are particularly useful for students who are visually impaired to perform chemistry experiments.

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