It is not possible to taste each and every substance to identify its chemical nature and also, it may be dangerous to touch each and every substance.

       To overcome this problem, special types of substances called indicators are used to get to know the chemical nature of substances.

      An indicator is a substance which indicates the nature of particular solution whether acidic, basic or neutral.  Hence they indicate the change in nature of the solution from acidic to basic and vice versa. Indicators are basically coloured organic substances.
     (a) Different types of indicators :
     (i) Litmus : Litmus is a purple dye which is extracted from a plant ‘lichen’. It can also be applied on paper in the form of strips and is available as blue and red strips. A blue litmus strip, when dipped in an acid solution acquires red colour. Similarly a red           strip when dipped in a base solution becomes blue.

    To test the chemical nature of a few substances.
    •    Collect lemon juice, lime water,  tap water, washing soda solution, milk of magnesia, and sugar solution in separate test tubes.
    •    With the help of dropper, one by one, put a drop of each solution on separate red and blue litmus papers.
    •    Record your observations in Table.

(ii) Phenolphthalein : It is also an organic dye. In neutral or acidic solution, it remains colourless while in the basic solution, the colour of indicator changes to pink.

(iii) Methyl Orange : Methyl orange is an orange coloured dye and keeps this colour in the neutral or basic medium. In the acidic medium the colour of indicator becomes red.

(iv) Red Cabbage Juice : It is purple in colour in neutral medium and turns red or pink in the acidic medium. In the basic or alkaline medium, its colour changes to green.

(v) Turmeric juice :  It is yellow in colour and remains as such in the neutral and acidic medium. In the basic medium its colour becomes reddish or deep brown.

(vi) China Rose :  Extract of china rose (Gudhal) petals is of pink colour. It will change into dark pink (magenta) in acidic solution and green in basic solution.    

          To test the nature of different sbstances using temeric as an indicator.
      •    Make turmeric paste in a bowl
      •    Now leave it to dry for 15-20 minutes.
      •    cut thin strips of yellow turmeric paper
      •    Collect lemon juice, lime water, tap water, washing soda solution, milk of magnesia and sugar solution in separate test tubes
      •    With the help of dropper, one by one, put a drop of each solution on the thin strip of yellow turmeric paper.
      •    Record your observation in Table

      To observer the effect of various indicators on acidic and basic solution
      •    Collect 5 mL each of dilute sulphuric acid, dilute sodium hydroxide, dilute hydrochloric acid, dilute potassium hydroxide, dilute nitric acid, dilute ammonium hydroxide and dilute calcium hydroxide in separate test tubes.

      •    One  by one test the acidic or basic nature of each of the sample solutions with blue litmus paper, red litmus paper, phenolphthalein, methyl orange, China rose and turmeric indicators.
      •    Record your observation in Table

Natural indicators around us

Some examples of natural indicators are red cabbage, turmeric, grape juice, turnip skin, curry powder, cherries, beetroots, onion, tomato, etc. Some flowers like hydrangeas can determine the acidity or basicity of the soil.