Theory:

We learned that the foods we eat, the material we use contains acid or base in nature. But the question is, how can we find which is a base or acidic substances?
We can find the substance, whether it is an acid or base, through indicators with the help of colour change.
In this chapter, we will study about three major natural indicators as followed.
 
1. Litmus paper
 
2. Turmeric powder
 
3. China rose
Litmus paper:
The litmus paper is made from normal paper and treating it with natural dyes that are extracted from lichens.
 
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The litmus is the most common indicator, which obtained from the lichens. And we use it mostly in the form of paper that is litmus paper, which is available in blue, red and purple colours.

When we introduce a blue litmus paper to an acidic solution, it turns red, and if we add the basic solution to red litmus paper, it turns blue.
 
When exposed to acid, a purple or neutral litmus paper changes colour from purple to red, indicating an acid, and turns blue when exposed to alkaline (or basic) conditions.
 
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The first two beakers show the test for acid and base and the last two for the neutral solution.
  
Litmus paperTest with the acid substanceTest with the base substance
Blue Litmus paper
No changes
Red \(→\) Blue
PurpleLitmus paper
     Purple \(→\) Red
Purple \(→\) Blue
Red Litmus paper
    Blue \(→\) Red
No change
Example:
How to create an indicator to identify the nature of a substance?
 
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Step 1: Now, let us take red cabbage and cut it into pieces.
 
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Step 2: Put these pieces into hot boiling water (minimum amount) for a while.
 
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Step 3: Leave them aside for some time till the water colour changes to purple.

Step 4: After the watercolour changes, take the filter the filtrate from it and slow it to cool.

Step 5: Now, take the filter paper and dip it in the filtrate.

Step 6: After it has been soaked in it, take it out and let it dry completely.

Step 7: Now, if need, we can cut the filter paper into pieces like that of pH paper strips.
 
Note: Anthocyanin, a water-soluble pigment found in red cabbage, changes colour when it reacts with an acid or a base.
 
Lets now check lemon extract and soap solution with these prepared pH paper strips.

Result:
1. In acidic environments (lemon extract) with a pH of less than \(7\), the pigment turns red.
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2. In basic environments (soap solution) with a pH greater than \(7\), the pigment turns bluish-green.
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We can just identify only the nature of the substance and not the exact pH range of the substances, whether it is more acidic or basic.