Indicators for testing acids and bases

• An indicator is a ‘dye’ that shows a change in colour when brought in contact with acids and bases. This tells us whether the substance we are testing is an acid or base with respect to the change of colour.

• The three most common indicators to test for acids and bases are:
  • Litmus
  • Methyl orange
  • Phenolphthalein.

• Litmus is a natural indicator. It is a purple dye which is extracted from a type of plant called ‘lichen’.
• There are two types of litmus paper: red litmus paper and blue litmus paper.
• Blue litmus paper turns red if the substance is acidic.
• Red litmus paper turns blue if the substance is basic or alkaline.
• Litmus turns red in acidic solutions and blue in basic solutions.

• Turmeric is also a natural indicator. Turmeric (Haldi) contains a yellow dye, which turns red in basic solutions.

• The red cabbage extract obtained from red cabbage leaves is also a natural indicator. It remains red in acidic solutions but turns green on adding to basic solutions.

OLFACTORY INDICATORS:-

• The term ‘olfactory’ means ‘relating to the sense of smell’. So, olfactory indicators are those substances whose smell or odour changes in acidic or basic.
• Onion and vanilla extract are olfactory indicators.
• Onion has a characteristics smell. The smell of onion is not detected when a basic solution like sodium hydroxide solution is been added to a cloth strip treated with onions (or onion extract).
• Vanilla extract has a characteristic pleasant smell.
• The smell of vanilla extract is not detected, if a basic solution like sodium hydroxide solution is been added .Yet, an acidic solution like hydrochloric acid, does not destroy the smell of vanilla extract.

COLOUR OF INDICATOR IN ACID/BASE: