Ø  INDICATOR: Substances which show the acidic or basic behaviour of other substance by change in colour are known as indicator.
ü  Type of Indicator: There are many types of indicators. Some common types of indicators are
Natural Indicator, Olfactory Indicator, Synthetic Indicator, Universal Indicator

v Natural Indicator: Indicators obtained from natural sources are called natural indicators. Litmus, turmeric, red cabbage, China rose, etc. are some common natural indicators used widely to show the acidic or basic character of substances. For Eg.
·     Litmus - Litmus is obtained from lichens. The solution of litmus is purple in colour. Litmus paper comes in two colour – blue and red.
An acid turns blue litmus paper red.
A base turns red litmus paper blue.



·     Turmeric: Turmeric is another natural indicator. Turmeric is yellow in colour. Turmeric solution or paper turns reddish brown with base. Turmeric does not change colour with acid.



·      Red Cabbage: The juice of red cabbage is originally purple in colour. Juice of red cabbage turns reddish with acid and turns greenish with base.




v Olfactory Indicator: Substances which change their smell when mixed with acid or base are known as olfactory indicators. For example, onion, vanilla, clove, etc.
·     Onion: Paste or juice of onion loses its smell when added with base. It does not change its smell with acid.
·     Vanilla: The smell of vanilla vanishes with base, but it’s smell does not vanishes with an acid. Olfactory indicators are used to ensure the participation of visually impaired students in laboratory.



v Synthetic Indicator: Indicators that are synthesized in laboratory are known as synthetic indicators. For example; phenolphthalein, methyl orange, etc.
·     Phenolphthalein: It is a colourless liquid. It remains colourless with acid but turns into pink with a base.
·     Methyl orange: It is originally orange in colour. It turns into red with acid and turns into yellow with base.



v Universal Indicator: In case of a colourless liquid, the accurate pH can be obtained by adding a universal indicator.  It is a mixture of several indicators and shows different colours at different concentration of hydrogen ions in a solution. For Example: 
·     A universal indicator produces green colour in a neutral solution, pH = 7.
·     The colour changes from blue to violet as pH increases from 7 to 14.
·     The colour changes from yellow to pink and then to red as pH decreases from 7 to 1.