Answer:
The pH of the pure water is not always neutral but actually sometimes it can be slightly acidic but not of the same extent or value as HCl obviously. However ,it is enough to result to a color change in the presence of phenolphthalein.
Step-by-step explanation:
I will try to be brief and spare the boring details .Basically ,this is caused by the reaction of water and CO2 that is naturally present in the atmosphere to produce a weak acid known as carbonic acid.
~Hope this helps:)