Students in an activity group are conducting exploratory experiments on acid-base neutralization reaction. Experimental process: Xiao Nan's experiment: dilute H2SO4 in a test tube.

Exchange discussion: sulfuric acid reacts with sodium hydroxide to generate sodium sulfate and water, and hydrochloric acid can react with sodium hydroxide to generate sodium chloride and water, so the solution must contain sodium chloride and sodium sulfate. In the same solution, hydrochloric acid and sodium hydroxide solution cannot coexist, so fill in: propane, NaOH and HCl react (or they do not coexist);

Reflection and evaluation: Because the colorless phenolphthalein test solution can not change color, it may be acidic or neutral, so fill in: a; Phenolphthalein does not change color, that is, it can be neutral or acidic.