Take about 2 mL of dilute NaOH solution in a test tube and addtwo drops of phenolphthalein solution.n What is the colour of the solution?n Add dilute HCl solution to the above solution drop by drop.n Is there any colour change for the reaction mixture?n Why did the colour of phenolphthalein change after the additionof an acid?n Now add a few drops of NaOH to the above mixture.n Does the pink colour of phenolphthalein reappear?n Why do you think this has happened?
Question
Take about 2 mL of dilute NaOH solution in a test tube and addtwo drops of phenolphthalein solution.n What is the colour of the solution?n Add dilute HCl solution to the above solution drop by drop.n Is there any colour change for the reaction mixture?n Why did the colour of phenolphthalein change after the additionof an acid?n Now add a few drops of NaOH to the above mixture.n Does the pink colour of phenolphthalein reappear?n Why do you think this has happened?
Solution
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When you add phenolphthalein to a solution of dilute NaOH, the solution turns pink. This is because phenolphthalein is a pH indicator that turns pink in basic solutions.
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When you add dilute HCl to the solution, the pink color disappears and the solution becomes colorless. This is because the HCl neutralizes the NaOH, making the solution acidic. Phenolphthalein is colorless in acidic solutions.
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The color of phenolphthalein changes after the addition of an acid because it is a pH indicator. In basic or alkaline solutions, phenolphthalein is pink. In neutral or acidic solutions, it is colorless.
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If you add more NaOH to the solution after the HCl has been added, the pink color will reappear. This is because the additional NaOH makes the solution basic again, and phenolphthalein turns pink in basic solutions.
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The reappearance of the pink color indicates that the solution has become basic again. This is because phenolphthalein changes color based on the pH of the solution it is in. When the solution is acidic or neutral, phenolphthalein is colorless. When the solution is basic, phenolphthalein is pink.
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