Question

a test tube contains solution of NaOH and phenolphthalein why the colour of solution change when HCL is added to it

Answer 1

we know that any which is a base and phenolphthalein doesn't change any colour with the base when HCL is added to the solution containing any Na0H and phenolphthalein the colour changes as it turns neutral

Answer 2

Since, phenolphthalein is an artificial indicator, it gives pink colour in a basic solution and remains colourless in neutral and acidic solution. 

When HCl is added to a solution of NaOH and phenolphthalein, its nature changes because of neutralisation and the resultant solution becomes neutral. Therefore the color of solution changes from pink to colorless.