So, fill in: Na2CO3+2HCl ═ 2NaCl+H2O+CO2 ↑; Sodium chloride.
Experimental inquiry
(1) (1) substance x is litmus solution in acid-base indicator, because hydrochloric acid can make litmus solution red, but hydrochloric acid cannot make phenolphthalein solution discolor.
Therefore: litmus.
(2) A small amount of magnesium powder was added to the waste liquid, and no bubbles were observed, so it was confirmed that there must be no hydrochloric acid in the waste liquid.
Therefore, no bubbles are generated.
(2) Select pH test paper to measure = l0 of waste liquid, and ensure that waste liquid must contain Na2CO3. ..
So, fill in: pH test paper.
(3) Scheme 1: When a proper amount of Ca(NO3)2 solution is added, calcium nitrate reacts with sodium carbonate to generate calcium carbonate precipitate and sodium nitrate. After filtration, evaporation and crystallization, sodium carbonate can be removed, but a new impurity-sodium nitrate is brought in, so this scheme is not feasible.
Therefore, fill in: No; Brought a new impurity, sodium nitrate.
Scheme 2: excessive dilute hydrochloric acid is added, and the dilute hydrochloric acid reacts with sodium carbonate to generate sodium chloride, water and carbon dioxide. After evaporation and crystallization, excess hydrogen chloride evaporates to obtain pure sodium chloride, so this scheme is feasible.
Therefore, fill in: dilute hydrochloric acid.