Indicators of Acids and Bases

Indicators are substances which when added to an acid or base change colour and help decipher the nature of the substance being tested.There are three types of Indicators:

Natural Indicators : 

Indicators organically found in nature. Some common examples are :

a.) Litmus - There are two types of litmus papers 

1. Blue litmus paper - 
   Turn red in acid solution         
   Does not change colour in a basic solution
   Does not change colour in a salt solution Comment
   
   
2. Red litmus paper - 
   Turn blue in a basic solution             
   Does not change colour in an acidic solution 
   Does not change colour in a salt solution      
      

Note : Litmus is obtained from Lichen , a plant.

 b.) Turmeric- It is naturally yellow in colour. Turmeric turns red when it comes in contact with a basic solution.

Q.) You have been provided with three test tubes. One of them contains distilled water and the other two contain an acidic solution and a basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tube?

Ans.) 

• Start by dipping the red litmus paper into all three solutions. 
• The solution which turns the red litmus paper into blue will be the basic solution . 
• The red litmus paper has now turned blue.
• Dip the newly formed blue litmus paper into the remaining two solutions. 
• We know blue litmus turns red in an acidic solution. 
• The solution which turns the blue litmus paper into red is the acidic solution. The only remaining solution is the distilled water.

Intext Question - Page 18 Q1

NCERT Question

Synthetic Indicators: 

Indicators which are synthesized and created by chemical processes. Some of the commonly used Synthetic indicators are:

1.   Methyl Orange : It is orange in colour.
   • It turns dark red-orange in an acidic solution
   • It turns yellow in a basic or neutral solution
   • It turns yellow in a neutral or salt solution
     
      
2. Phenolphthalein : It is a colorless solution.

• • It turns pink in a basic solution
  • It does not change colour in an acidic solution
  • It does not ch in a neutral or salt solution  

Q.) Sometimes there is a stain of curry on white cloth. If we put soap on it,it turns red. Why?

Ans.) It is because of the presence of turmeric (natural indicator) in curry. We know turmeric turns red on contact with a base and hence the base in the soap turns the turmeric in the curry into a red colour stain.

Olfactory Indicators : 

Those Substances whose smell changes in acidic or basic solutions are called Olfactory Indicators.

Example : Onion has a characteristic sharp smell . 

In a basic medium , this smell cannot be detected , but in an acid it retains its strong smell.

Learn more about them here