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Phenolphthalein, an acid-base indicator used to test the pH of a solution, turns pink due to the presence of a weak base. The indicator itself is a weak, colorless acid that forms pink anions when it dissolves in water. Although the anions are pink, the solution remains colorless in the presence of an acid.
(b) Solution A gives pink colour when a drop of phenolphthalein indicator is added, therefore A is a base. Solution B gives red colour when a drop of methyl orange is added to it, therefore B is an acid. Hence, solution A will have less concentration of hydrogen ion than B.
Phenolphthalein remains colourless in neutral or acidic solution but turns out magenta or pink in basic solutions. When it is added to NaOH, the solution has the colour of pink since sodium hydroxide is a strong base.
When a few drops of phenolphthalein are added to a dilute solution of sodium hydroxide, a pink colour is produced. On adding excess of HCl, the solution will become acidic and the solution will change colour from pink to colourless.
A few drops of phenolphthalein indicator are added. (b) The solution containing sodium hydroxide in an unknown concentration is added until a faint pink color is visible for a few seconds….
|Indicator||Color in acid||Color in base|
Ans: When ammonia gas is passed into water containing few drops of phenolphthalein, the water turns into pink colour due to the formation of alkali (ammonium hydroxide).
Hint: In a neutral solution, the phenolphthalein would give a light pink colour. When phenolphthalein is added to the base the colour of the solution intensifies and if it is added to an acidic solution the colour will fade away and become colourless.
Explanation: When the sodium hydroxide, which is a base, is mixed with phenolphthalein, it turns pink. This happens because the base is neutralized in an acid solution, which causes the indicator to become colorless. All of the base is neutralized by the citric acid.
Adding an acid increases the concentration of H3O+ ions in the solution. Adding a base decreases the concentration of H3O+ ions in the solution. An acid and a base are like chemical opposites. If an acid is added to a basic solution, the solution becomes less basic and moves toward the middle of the pH scale.
Answer. Phenolphthalein is a weak acid, which is why when it is added in a strong base, it gives pink color and the color gets intense as the PH of the base increases. By adding HCL in the solution, the pink color of phenolphthalein will disappear. This is also called Acid- Base Titration.
Why did the colour of the solution change after adding the HCl solution? The pink colour of the solution of NaOH to which phenolphthalene is added be-comes colourless after adding dilute HCl because HCl is an acid and phenolphthalene becomes colourless in acid medium.