write a short note on natural indicators

Indicators are substances whose solutions change colour due to changes in pH. These are called acid-base indicators. They are usually weak acids or bases, but their conjugate base or acid forms have different colours due to differences in their absorption spectra.

Natural Indicator -- An indicator prepared from natural substances (from plants, insects etc) is known as natural indicator.

Example - Litmus, Turmeric, red cabbage, etc.

Litmus is a water-soluble dye extracted from certain lichens, turns red when it contacts something acidic and blue with alkaline materials. They are available as litmus papers.

Turmeric (curcumin) is a natural indicator used to differentiate between acidic and basic solutions. It turns from yellow at pH7.4 to red at pH8.6. Turmeric will turn pinky-red in alkali, but is yellowish in both acid and neutral solutions.

Cabbage (red) contains anthocyanins, is a natural indicator used to differentiate between acidic and basic solutions. The colour of red cabbage is enhanced with the addition of vinegar or other acid.

When cooked in aluminium pans, which cause amore alkaline environment, the colour changes to purple and blue.

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Ans. i) Indicators are defined as the acidic and basic nature of the compounds can also be indicated by some natural as well as chemical substances are known as indicators.

ii) Phenolphthalein methyl orange are two acid-base indicators.

iii) Phenolphthalein is colourless in acid solution while it turns into pink colour in base solution.

iv) Methyl orange gives orange colour with acid solution yellow colour with base solution.

1) Tamarind tartaric acid 2) Butter Butyric acid

3) Roots of plant velerum Valeric acid 4) Lemon citric acid

5) Milk Lactic acid 6) Orange citric acid

7) Red ants Formic acid 8) Honey bee Formic acid

Explain the term olfactory indicator.

Ans. Substances whose odour changes in acidic or basic medium are called olfactory indicator.

e.g. Eucalyptus (nilgiri) Oil, Onion clove extract.

What is universal indicator? Does Mg OH2 react with sodium hydroxide? If not, why?

Ans. i) Universal indicator is a mixture of several indicators which gives different colours at different pH values of the pH scale.

ii) Mg OH2 does not react with sodium hydroxide as both are bases having the same negative radical i.e. OH-

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