(1) As can be seen from the image, the pH value is greater than 7 at first, then gradually reduced to 7, and then less than 7, indicating that the stock solution is alkaline, and then acidic solution is continuously added to reduce the pH value, that is, dilute hydrochloric acid is dripped into the sodium hydroxide solution.

(2) When the mass of the added solution is ag, the pH value of the solution is greater than 7, and the solution is alkaline, which can make the phenolphthalein test solution turn red.

(3) When the added solution is bg, the pH value of the solution is less than 7, and the solution is acidic, indicating that there is too much dilute hydrochloric acid, and the solutes in the obtained solution are HCl and NaCl. ..

(4) Sodium hydroxide solution reacts with dilute hydrochloric acid to generate sodium chloride and water, and the chemical equation of the reaction is: HCl+NaOH-NaCl+H2O.

So the answer is: (1) dilute hydrochloric acid; (2) red; (3) hydrochloric acid and sodium chloride; ; (4)HCl+NaOH═NaCl+H2O.