The two main source of most of the chemical substances are:

  1. 1. Animal and plants
  2. 2. Mineral and rocks.

The chemical substances obtained from animal and plant (living and non-living) are called organic compounds whereas those obtained   from minerals and rocks are called inorganic compounds. These compound present in the food stuff as well as in a number of other materials that we use in everyday life like washing soda, lime etc.

The earliest classification of organic compounds, including a large number of organic compound, was on the basis of their taste. On the basis, the compounds were classified into the following three categories :

  1. I. Acid
  2. II. Bases and
  3. III. Salts.

Acids are substances which have sour taste. The term has been derived from the latin word, acidus, which means sour. A few examples of acidic substances used in everydaylife including lemon juice, tomatoes, vinger etc.

Bases are the substance which have bitter in taste. A few example of substances having basic natureincluding washing soda, baking soda etc.

Salts are the compound formed from acid and bases. A few example of the salts include sodium chloride, copper sulphate, silver nitrate, calcium carbonate etc. salts having tastes similar to that of common salt are called salty. However , some salt have very unpleasant tastes and even may be poisonous.


There are many substances which show one colour in the acidic medium and a different colour in the basic medium. Such substances are called as acid-base indicators.

Depending upon the property of the indicators, we have the following two types of acid-bases indicators:

  1. Indicators showing different colours in acidic and basic medium.
  2. Indicators that giving different colour in acidic and basic medium (called olfactory indicators).
  3. a) Indicators showing different colours in acidic and basic medium

There are the most commomly used acid-base indicators. The three well known indicators belonging to this category are litmus, Phenolphthalien and Methul orange. Litmus is natural indicators(found in nature in plants) whereas Phenolphthalien and methyl orange are synthetic indicators (synthesized in the laboratory or industry).

  1. b) Indicators that giving different colour in acidic and basic medium (called olfactory indicators).

There are some substances which gives one type of odour in acidic medium and a different odour in the basic medium. Hence, they can be used to test whether the given substances is acidic or basic. Such indicators are called olfactory indicators. A few of these are :

  • Onion odoured cloth strips
  • Vanilla essence
  • Clove oil.


  1. a. Sour taste.
  2. b. Action on litmus paper : acid turn blue litmus solution read.
  3. c. Action on methyl orange : Methyl orange turn read when 1-2 drop of its solution are added to the solution of an acidic substances.
  4. d. Corrosive nature : Most of the acids are corrosive in nature. They produce a burning sensation on the skin and holes in the clothes on which they fall.
  5. e. Reaction with metal : Acid react with reactive metal like Zinc, magnesium etc. generally evaluate hydrogen gas.
  6. f. Reaction with metal carbonate and metal hydrogen carbonate (bicarbonates) : to gives out carbon dioxide with efferscence and forming the corresponding salt and water .
  7. g. Conduction of electricity : The solution of acids in water conduct electricity.


  1. a. Bitter in taste.
  2. b. Slippery or soapy touch.
  3. c. Action on litmus paper : they turn red litmus blue.
  4. d. Action on phenolphthalein : They turn phenolphthalein solution pink.
  5. e. Corrosive nature.
  6. f. Conduction of electricity.
  7. g. Reaction with acid .
  8. h. Reaction with metals.




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