① This experiment is to verify the purity of sodium hydroxide. Dropping concentrated hydrochloric acid will produce bubbles, and the quality of sodium hydroxide will increase, indicating that carbon dioxide has been absorbed, so the answers are: Na2CO3 and NaOH；;

Sodium carbonate will react with calcium hydroxide, and sodium hydroxide will not exist with sodium carbonate, so the answer is: Ca(OH)2.

② The device C needs to remove water vapor, otherwise it will affect the experimental data, so the answer is concentrated sulfuric acid.

③ The measured purity of caustic soda is less than the theoretical value, indicating that the quality of sodium carbonate has been improved. Concentrated hydrochloric acid is volatile and will react with water to generate hydrochloric acid, and then it will react with sodium bicarbonate to generate carbon dioxide. The glass tube at the right end of sodium hydroxide is communicated with air, and carbon dioxide in the air will also react with sodium hydroxide, so the answer is: the hydrogen chloride gas volatilized in device A reacts with sodium bicarbonate in device B to generate carbon dioxide gas;

Carbon dioxide in the air enters the sodium hydroxide solution in equipment D.

It is enough to reduce the volatilization of hydrogen chloride, so the answer is: use concentrated hydrochloric acid instead of dilute hydrochloric acid.