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On the basis of chemical properties or taste, the compounds were classified into three groups –

(a) Acid
(b) Base
(c) Salt

  • Acid (latin word, acidus = sour) are substances which are sour in taste. e.g. lemon juice, tomatoes, vinegar etc.
  • Bases are bitter in taste. e.g. washing soda, baking soda etc.
  • Salts have taste similar to that common salt, are called salty. They are made from acids and bases. e.g. copper sulphate, silver nitrate, calcium carbonate, sodium chloride etc.

Read More –

Chemical Reactions and Equations

We all know, it is very dangerous to taste any chemical substance. Then,

How can chemical substances be distinguished without tasting?   

There are many substances which show different colours and odours with acids and bases. Such subst-ances are called acid – base indicators. Depending upon the properties, indicators can be classified as –

(A) Indicators showing different colours.

(B)  Indicators showing different odours (Olfactory indicators)

(A)  Indicators showing different colours

            Such indicators are dyes which show a specific colour for acids and bases respectively. The common indicators to test acids and bases are –

(i) Natural indicator – such as litmus

(ii) Synthetic indicator – such as phenolphthalein , methyl orange.

(i) Natural indicators

They are found in nature. For e.g. litmus extract from the plant lichen. It can be used in the form of litmus solution or litmus paper. Litmus solution itself is neutral and purple in colour. When it is used with acidic solution, it gives red colour and with basic or alkaline solution, it gives blue colour. Other examples are-











In general, instead of using neutral litmus solution as such, two types of solutions are used, called blue litmus solution and red litmus solution.  Blue litmus solution is obtained by making the purple litmus extract alkaline whereas red litmus solution is obtained by acidifying the purple litmus extract.

An acid (e.g. vinegar, lemon juice, orange juice, tamarind juice etc.) turns blue litmus into red and a base / alkali (e.g. baking soda solution,  washing soda solution, bitter gourd extract, cucumber extract etc.) turns red litmus into blue.

♦ Water soluble bases are called alkalies.

♦ Other commonly used natural indicators are turmeric, red cabbage leaves extract, coloured petals of some flowers like Hydrangea, Petunia, Germanium etc.

(ii) Synthetic Indicators –

Such indicators are manufactured synthetically in laboratories or in industries. Phenolphthalein is colourless and methyl orange is orange coloured in neutral solution. Phenolphthalein remains colourless whereas methyl orange turns into red colour in acidic solution. In the alkaline solution, phenolphthalein turns into pink colour while methyl orange change colour from orange to yellow.


(B) Indicators showing different odours (Olfactory indicators)
Not only by colours, change in odour also shows the nature of the solution (acid or base). Such indicators which give different odour or smell in acidic and basic solutions are called olfactory indicators. (Olfactory = related to the sense of smell)




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