Theory:

Let us start with a small activity to test acid, base and neutral solution.
You have been given three test tubes. One contains distilled water, while the other two contain acidic and basic solutions, respectively. How will you identify the contents of each test tube if you only have red litmus paper?
 
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Effect of litmus paper in different types of solution
 
Red litmus paper has no effect in acid solution. If it changes to blue, then we call the solution a base. If the colour changes to purple, then we call it a neutral solution.
Activity 1:
  
Collect the samples listed below from the science laboratory, observe the colour change with the help of the indicators provided, and tabulate the results.
  
Samples:
  • Hydrochloric acid \(HCl\)
  • Sulphuric acid \(H_2SO_4\)
  • Nitric acid \(HNO_3\)
  • Acetic acid \(CH_3COOH\)
  • Sodium hydroxide \(NaOH\)
  • Calcium hydroxide \({Ca(OH)_2}\)
  • Potassium hydroxide \(KOH\)
  • Magnesium hydroxide \({Mg(OH)_2}\)
  • Ammonium hydroxide \(NH_4OH\)
Indicators:
  • Red litmus
  • Blue litmus
  • Phenolphthalein
  • Methyl orange
Note:
Phenolphthalein solution is colourless, whereas methyl orange solution is orange in colour.
To be remember that acids and bases react differently with these indicators.
 
Sample solution
Red litmus
Blue litmus
Phenolphthalein
Methyl orange
Acids
No change
Changes to red colour
Colourless
Red
Bases
Changes to blue colour
No change
Pink
Yellow
 
Therefore, from the given samples, we can now predict the changes with the help of the above table. The change in colour of these indicators tells us whether a substance is acidic or basic in nature.
 
Sample solution
Red litmus
Blue litmus
Phenolphthalein
Methyl orange
Hydrochloric acid \(HCl\)
No change
Changes to red colour
Colourless
Red
Sulphuric acid \(H_2SO_4\)
No change
Changes to red colour
Colourless
Red
Nitric acid \(HNO_3\)
No change
Changes to red colour
Colourless
Red
Acetic acid \(CH_3COOH\)
No change
Changes to red colour
Colourless
Red
Calcium hydroxide \({Ca(OH)_2}\)
Changes to blue colour
No change
Pink
Yellow
Potassium hydroxide \(KOH\)
Changes to blue colour
No change
Pink
Yellow
Magnesium hydroxide \({Mg(OH)_2}\)
Changes to blue colour
No change
Pink
Yellow
Ammonium hydroxide \(NH_4OH\)
Changes to blue colour
No change
Pink
Yellow
Some substances' odours change when exposed to acidic or basic media. These are called olfactory indicators.
Let us put some of these indicators to the test.
  
Activity 2:
 
Olfactory indicators:
  • Onion
  • Vanilla essence
  • Clove oil
Step 1: Place some finely chopped onions in a plastic bag with some clean cloth strips.
Step 2: Tie the bag tightly and place it in the fridge overnight.
Step 3: Acids and bases can now be tested using cloth strips.
 
Examine the odour of these cloth strips. Place them on a clean surface and add a few drops of dilute \(HCl\) solution to one of the cloth strips and dilute \(NaOH\) solution on the other.
 
Observation:
 
Sample
Acid
\(HCl\) solution
Base
\(NaOH\) solution
Onion
Retains the odour
Odourless
Vanilla essence
Retains the odour
Odourless
Clove oil
Retains the odour
Odourless
 
Result: All the above olfactory indicators do not lose their odour in an acidic medium, but they lose their odour in a basic medium.
 
So now we can find whether the given substance is acid or base based on the indicators. In activity \(1\), we found the changes visually, whereas, in activity \(2\), we found the changes with the help of smell.