Class-10 (Chemistry) Science chapter-Acids,Bases and salts

~INDICATORS TO TEST ACIDS AND BASES 

Some substances change their colour (or odour) on addition of an aqueous solution of an acid or base. These substances are called (acid-base indicators) .Using these substances, it is possible to identify acids and bases. Acid-base indicators are generally coloured organic substances. Well known three indicators are Litmus, phenolphthalein and methyl orange. Litmus is a natural indicator (i.e., found in nature in plants) while phenolphthalein and methyl orange are synthetic indicators (i.e., synthesised in the laboratory). Let us discuss them, one by one.

1) Litmus : Litmus is a natural indicator. It is extracted from lichen a plant belonging to the division Thallophyta. The colour of Litmus solution is purple and it is neither acidic nor basic. It is neutral. In the acidic solution, it turns red while in the basic solution, it turns blue. Most often, instead of using the litmus solution as such, two types of Litmus solutions are used known as blue litmus solution and red litmus solution. Blue litmus solution is formed by making the Purple litmus extract alkaline while red  litmus solution is formed by acidifying the Purple Litmus extract.

Litmus paper : Instead of blue litmus solution or red litmus solution, thin paper strips soaked in these  solutions and then dried are also used sometimes. These paper strips are known as Litmus Paper. 

It should be remembered that:

i) if the blue litmus solution changes red, then the substance is acidic, and 

ii) if the red litmus solution changes to Blue, then the substance is basic .

Vinegar, orange juice, lemon juice, tamarind juice all turn blue litmus solution to red. Thus, they are all acidic substances. Whereas, baking soda solution, washing soda solution, Neem extract, bitter gourd (loki) extract, cucumber extract all turn red Litmus solution into blue. Thus, they all are basic (alkaline) substances.

Turmeric (haldi) is a common ingredient in Indian kitchens. Turmeric can also be used to test basic solution. Turmeric contains a yellow substance which turns red in a basic solution. For example, while eating food, if the curry falls on the white cloth, a yellow strain is produced on the cloth. The yellow strain becomes reddish-brown as soap solution is basic in nature. On washing with excess of water, it turns yellow again since soap is washed away. 

The extract of red cabbage leaves can also be used as an indicator. The colour of red cabbage leaves extract is red. When it is put in an basic solution, it turns green. However, the red colour of red cabbage leaves extract does not change in an acidic solution.

Coloured petals of some flowers like Hydrangea, petunia and Geranium change their colours when added to acidic or basic (alkaline) solution. For example, blue coloured Hydrangea petals turn pink in the presence of basic (alkaline) solutions. Petunia, on the other hand, turns reddish-purple in an acidic solution and violet in a basic solution; while Geranium changes to orange-red in an acidic solution and blue in a basic solution. 

The colours of some natural indicators are summed up in the given table 2.3 : 

                   

         To be Continued........