- 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?
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)