Acids, Bases and Salts | Class 10 | Activity 2.6 with Solution

Activity 2.6  
  • Take about 2 mL of dilute NaOH solution in a test tube and add two drops of phenolphthalein solution.
  • What is the colour of the solution?
✅ Answer: The solution becomes pink colour.
  • Add dilute HCl solution to the above solution drop by drop.
  • Is there any colour change for the reaction mixture?
✅ Answer: The pink colour changes and disappears.
  • Why did the colour of phenolphthalein change after the addition of an acid?
✅ Answer: Phenolphthalein is a weak acid. It gives pink colour in a strong base. When an acid is added, it reacts with the base and neutralisation reaction occurs. So pink colour disappears. 
  • Now add a few drops of NaOH to the above mixture.
  • Does the pink colour of phenolphthalein reappear?
✅ Answer: Yes.
  • Why do you think this has happened?
✅ Answer: 

It is due to the increase in amount of base (NaOH) than acid.

The effect of a base is nullified by an acid and vice-versa.

NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l)


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