Question

Ashwini has been given four test-tubes containing the following:
(a) Universal indicator
(b) Distilled water
(c) Sodium hydrogencarbonate
(d) Phenolphthalein
​What will be observed by Ashwini when he adds a few drops of acetic acid to each test-tube, one by one?

Answer 1

(a) When Ashwini adds acetic acid in the universal indicator, he will observe a colour either orange or yellow, depending upon the concentration of acetic acid. Because acetic acid is a weak acid, having a pH range of 3-6, so it will change the colour of universal indicator to orange or yellow.

(b) When Ashwini adds acetic acid in distilled water, he will observe acid and water mix uniformly.

(c) When Ashwini adds acetic acid in the sodium hydrogen carbonate, he will observe effervescence due to the formation of carbon dioxide.

${\mathrm{CH}}_3\mathrm{COOH}+{\mathrm{NaHCO}}_3\to {\mathrm{CH}}_3\mathrm{COONa}+{\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}$

(b) When Ashwini adds acetic acid in the phenolphthalein, he will observe the pink colour of phenolphthalein changes to colourless, because phenolphthalein turns colourless in acids.