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Needed a Document for indicators? Related: What are Indicators and F...
INDICATOR: A substance which detects the acidic or basic nature of another substance by change in colour is called acid-base indicator. It is not advisable to taste any substance in laboratory because it can be harmful. Hence, acid-base indicators are used to check if a given substance is acid or base.

Types of Indicator:
Indicators can be divided into two types.

Natural Indicator: Indicators that are obtained from naturally occurring substance are called NATURAL INDICATORS. Example: litmus, turmeric, China rose, etc.

Synthetic Indicator: Indicators that are made in laboratory are called SYNTHETIC INDICATORS. Example: phenolphthalein, methyl orange, etc.

NATURAL INDICATORS:
Litmus: Litmus is extracted from Lichens. Lichen is a composite organism. Lichens consist of fungi and algae living in symbiotic relationship.

Litmus is a purple coloured liquid in distilled water. Litmus comes in the form of strips of two colours. One is called blue litmus paper and another is called red litmus paper. Litmus liquid and litmus paper are used to detect the acidic or basic nature of a substance.

Colour of litmus paper in acid: Blue litmus paper turns into red when dipped in acidic solution.

Colour of litmus paper in base: Red litmus paper turns into blue when dipped in basic solution.

Turmeric: Turmeric is also used as natural indicator. Turmeric is of yellow colour. Turmeric paper turns into red when it is dipped into basic solution. Turmeric paper does not change its colour with acid.

China Rose: China rose is another natural indicator. China rose solution gives dark pink (magenta) colour with acid and green colour with base.

Acid Rain: Carbon dioxide, sulphur dioxide and nitrogen dioxide which are released from vehicles and chimneys mix with droplets of rain and turn the rain water acidic. When this acidic rain water falls over earth, it is known as acid rain. Acid rain damages the buildings and is harmful for plants and animals.
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Needed a Document for indicators? Related: What are Indicators and Few Common Indicators?, Acids, Base and Salts, Class 7 Science
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