Question

For an experiment, a beaker is filled with sodium hydroxide (NaOH) solution and phenolphthalein indicator is added to it. A student starts adding hydrochloric acid (HCl) to this mixture slowly. Select the correct statements regarding this experiment.

(I) Heat is evolved in this reaction.
(II) The colour of the mixture in the beaker is pink in the beginning.
(III) It represents neutralisation reaction.
(IV) No new substance is formed in the reaction.
(V) On addition of HCl slowly, the solution in the beaker will become colourless after some time.

• A. I, III, IV and V
• B. I, II, III and IV
• C. I, II, III and V
• D. II, III and V

Answer 1

The correct option is C I, II, III and V

$\bullet$ Neutralisation is the reaction of an acid and a base to neutralise or nullify the effects of each other.
$\bullet$ Heat is evolved during the neutralisation reaction. Therefore, statement I is correct.
$\bullet$ The colour of the phenolphthalein-sodium hydroxide mixture in the beaker is pink in the beginning as phenolphthalein shows pink colour in basic solutions and sodium hydroxide is a basic solution. So statement II is also correct.
$\bullet$ The reaction of hydrochloric acid and sodium hydroxide base, is indeed an example of a neutralisation reaction. Therefore, statement III is also correct.
$\bullet$ In a neutralisation reaction, new substances such as salt and water are formed. Therefore, statement IV is incorrect.
$\bullet$ On the addition of HCl slowly, when the amount of HCl and NaOH becomes equal, the resulting solution becomes neutral. Further addition of HCl makes the resulting solution acidic. Phenolphthalein is colourless for both neutral as well as acidic solutions. Therefore, after some time, the solution becomes colourless. So statement V is also correct.
$\bullet$ Therefore, option C is correct.