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An indicator is a ‘dye that changes colour when it is put into acid or a base . An indicator gives different colours in acid and base .Thus, an indicator tells as whether the substance we are testing is an acid or base by change in its color. In other word, an indicator tells us whether the substance we are testing is acidic or basic by change in its color. the three common indicators to test for acids and bases are : litmus , methyl orange and phenolphthalein .
Litmus solution is a purple dye which is extracted from a type of plant called ‘lichen’. Lichen is a plant belonging to the division Thallophytic. When litmus solution is neither acidic nor basic (it is neutral), then its colour is purple. It turns red in acidic solutions and blue in basic solutions. Turmeric is also a natural indicator. Turmeric (haldi) contains a yellow dye. It turns red in basic solutions. Many times we have noticed that a yellow stain of curry on a white cloth (which is due to the presence of turmeric in curry), turns reddish-brown when soap in scrubbed on it. This is due to the fact that soap solution is basic in nature which changes the colour of turmeric n the curry stain to red-brown. This stain turns to yellow again when the cloth is rinsed with plenty of water. This is because then the basic soap gets removed with water.
The red cabbage extract (obtained from red cabbage leaves) is also a natural indicator. It is red in colour. The red cabbage extract remains red in acidic solutions but turns green on adding to basic solutions. The coloured petals of some flowers (such as Hvdrangea, Petunia and Geranium) which change colour in the presence of acids or bases also act as indicators. Fr example, the flowers of Hydrangea plant are usually blue which turn pink in the presence of a base.
The most common indicator used for testing acids and base in the laboratory is litmus . Litmus can be used in the form of litmus solution or in the form of litmus paper. It is of two types: blue litmus and red litmus.
(a) If a drop of the given solution turns blue litmus to red, then the given solution will be acidic in nature (or it will be an acid). For example, orange juice turns blue litmus to red, so orange juice is acidic in nature. That is, orange juice contains an acid.
(b) If a drop of the given solution turns red litmus to blue, then the given solution will be basic in nature (or alkaline in nature). Or it will be a base (or alkali). For example, sodium hydroxide solution (caustic soda solution) turns red litmus to blue, so sodium hydroxide solution is basic in nature (or alkaline in nature). In other words, sodium hydroxide is a base (or an alkali). Please note that a water soluble base is called an alkali.
Litmus is a natural indicator (whose neutral color is purple). It is made into blue litmus and red litmus for the sake of convenience in detecting color change when an acid or base is added to it. But methyl orange and phenolphthalein are synthetic indicators. The neutral color of methyl orange is ‘orange’. The color changes which take place in methyl orange are as follows:
(i) Methyl orange indicator gives red color in acid solution.
(ii) Methyl orange indicator gives yellow color in basic solution.
The term ‘olfactory’ means ‘ relating to the sense of smell’. Those substances whose smell changes in acidic or basic solutions are called olfactory indicators .
An olfactory indicatory usually works on the principle that when an acid or a base is added to it, then its ‘characteristic smell cannot be detected. Onion and vanilla extract are olfactory indicators.