Question

student took a half-filled test tube with dilute hydrochloric acid and a few drops of phenolphthalein in the solution. As he put a few drops of sodium hydroxide into the solution, it turns to light pink. Further, he put a few extra drops of the acid and observes that the colour disappears. What caused the colour to disappear from the solution?

Answer 1

Answer:

THIS HAPPENS BECAUSE INITIALLY THE NaOH NEUTRALIZES THE HCl BUT WHEN EXCESS OF IT IN PUT IN THE TEST TUBE, THE SOLUTION BECOMES BASIC AND THE LIGHT PINK COLOUR PRODUCED IS NEUTRALIZED AND IT DISAPPEARS

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Answer 2

The colour from the solution disappear because

• Phenolphthalein is an example of acid-base indicator. It's synthetic acid-base indicator which is used to determine the end point in acid-base titration.
• Phenolphthalein when tested with acids remains colourless while when it's tested with bases it turns to pink colour.
• As the student performs the experiment, the transfer of few drops of NaOH to solution turns into pink because phenolphthalein turns to pink with bases. After the addition of acid it turns colourless as phenolphthalein turns colourless with acids.