### Theory:

In the process of neutralisation, the chemical reaction occurs when an acid and a base are combined to form a salt. Neutralisation is an exothermic reaction, which means that the reaction mixture heats up.
Similarly, let's see the reaction of phenolphthalein with acid and base.

In a beaker, diluted hydrochloric acid $$HCl$$ has been taken. And a few drops of phenolphthalein is mixed into the solution, which will act as an indicator. Here, the colour of the solution remains unchanged.

Observation: Now the Sodium hydroxide solution is added in the same beaker drop by drop continuously till the pink colour appears.

After a certain amount of $$NaOH$$ is added to the solution, if we add more $$NaOH$$ to it the colour remains the same. When we add more of $$HCl$$ to the solution the pink colour turns colourless again. Now, the solution is neutralized.

Result: Phenolphthalein gives pink colour if it is treated with basic solutions $$NaOH$$, and it remains unchanged with acidic solutions $$HCl$$.
Note:
Indicator: Phenolphthalein (Colorless)
In acid solution: The colour remains the same.
In basic solution: The colour changes to pink.