Solution ① According to the image, with the addition of dilute hydrochloric acid, the temperature is gradually increasing. Because the reaction between hydrochloric acid and sodium hydroxide is exothermic, the temperature is two situations around point B, so point B represents two situations of the reaction, so the reaction is complete at point B, and sodium chloride and water are generated after the reaction. The chemical equation is NaOH+HCl/NaCl+H2O.

(2) Hydrochloric acid from A to B gradually reacts with sodium hydroxide, the solution changes from alkaline to neutral, and the pH value of the solution gradually decreases;

(3) In the solution from B to C, after neutralizing sodium hydroxide, hydrochloric acid is added, which is excessive, so the solute in the solution has both sodium chloride and hydrogen chloride. If you want to get solute crystals in this solution, you can only get sodium chloride crystals. Because hydrogen chloride is a gas, you can get crystals by evaporating the solvent.

So the answer is:

(1) hydrochloric acid and sodium hydroxide have just completely reacted, NaOH+HCl-NaCl+H2O;

② Reduce (or become smaller)

③ Sodium chloride and hydrochloric acid; ? b.