• The earliest classification of inorganic compounds including the large number of organic compounds was based on their taste. On this basis of these compounds were classified into acid, bases and salt. We use many such compounds in our daily lives, which are acids, bases and salt. We use curd, pickle, amla, soap and detergents, tamarind, toothpaste etc. We must learn to classify them based on scientific tests.  
  • Test to distinguish between acids and basesThere are many substances, which show one colour or odour in the acidic medium and a different colour or odour in the basic medium. Such substances are called acid base indicators. 

A. Indicators showing different colours in acidic and basic medium 
    a. Litmus solution as indicator is a purple coloured dye extracted from the lichen plant. It is the most commonly used indicator in the science laboratory. In the neutral solution, it has purple colour. In the acidic solution, it turns red whereas in the basic solution it turns blue. There are two types of litmus solution Blue and Red litmus solution. Red litmus solution is obtained by acidifying the purple litmus extract whereas blue litmus solution is obtained by making the purple litmus extract alkaline. 
        To test whether the given sample is acidic or basic, take few drops of distilled water in a test tube and two drops of blue litmus solution. Add few drops of sample substance that is to be tested. If the blue litmus solution changes into red colour, the substance is acidic. For instance, lemon juice, vinegar, orange juice, juice of unripe mangoes, tamarind all turn blue litmus solution to red. Thus, they are all acidic substances. 
        We may repeat the above experiment with red litmus solution. Those substances, which turn red litmus solution into blue colour, are bases. For instance, cucumber, washing soda solution, baking soda solution, bitter gourd etc. turn red litmus solution into blue. Thus, they are bases. 

    b. Synthetic indicators such as Phenolphthalein and Methyl orange  

  • Methyl orange is the compounds prepared industrially or in the laboratory. There are many such man-made substances, which can act as synthetic indicators. 
  • Phenolphthalein  is colourless in neutral solution and in acidic solution but turns pink in basic solution. 
  • Methyl orange is of orange colour in neutral solution, red in acidic solution and yellow in basic solution. Ask your teacher to help you classify various substance using these indicators.

B. Indicators giving different odours in acidic and basic medium. (Olfactory indicators)

  • Clove oil. Take two test tubes. Mark them test tube ‘A’ and test tube ‘B’. Add some hydrochloric acid in test tube ‘A’ and some Sodium Hydroxide in test tube ‘B’. Record the odour, of clove oil. Now add few drops of  clove oil in each test tube and shake it gently. Smell the sample in test tube ‘A’ and ‘B’. Record the odour of each test tube. You will notice that the odour of acid and base after addition of clove oil is different. Ask your teacher to help you demonstrate the experiment in the laboratory. 
  •  Vanilla essence.  Repeat the experiment (a). Take another two test tubes and mark them ‘A’ and ‘B’. Add some acid
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