Question

To a solution of sodium hydroxide in a test tube two drops of phenolphthalein are added. (i) State the colour change observed. (ii) If dil HCl is added drop wise to the solution, what will be the colour change? (iii) On adding few drops of NaOH solution to the above mixture the colour of the solution reappears. Why?

Answer 1

do you know about “Phenolphthalein” ?
actually, “ Phenolphthalein” is a basic indicator which turns into pink or magenta in basic medium whereas it remains colourless in both acidic and neutral medium.

now, Let's come to the point !

(i) sodium hydroxide, NaOH is a strong basic in nature. so, when few drops of phenolphthalein is added to sodium hydroxide, solution turns deep pink.

(ii) As you know, hydrogen chloride, HCl is acidic in nature . so, if dil HCl is added drop wise to the solution of NaOH, then the color of solution gets fade and if we keep on adding HCl, the solution becomes colourless. as HCl being strong acid will neutralize strong base NaOH ]

(iii) if few drops of NaOH is added to again to the solution, solution becomes basic in nature and hence, pink colour of Phenolphthalein reappears.

Answer 2

Answer:

When few drops of phenolphthalein are added to dilute a dilute solution of sodium hydroxide a pink colour. The colour of dilute solution of Sodium hydroxide turns pink or adding Phenolphthalein as  NaOH is a base .When excess of HCl is added the final mixture becomes colourless due to  neutralisation of base with an acid.