② The density of carbon dioxide is greater than that of air, so when the bottle is filled with air, the collected carbon dioxide should enter from the A end;

(3) The density of oxygen is less than that of water, so when the bottle is filled with water, the collected oxygen should enter from the B end;

⑤ Dry gas needs to enter from long airway and be discharged from short airway, so dry CO2 gas needs to enter from end A;

⑥ Use clarified limewater to verify carbon dioxide, and the verification gas needs to enter the short airway from the long airway, so the verification carbon dioxide needs to enter from the A end;

Combined with physical knowledge, the principle of successful operation of collecting O2 by item ③ drainage method in the table is briefly described. Oxygen is insoluble in water.

In item 5, concentrated sulfuric acid can dry CO2 gas because of water absorption, which shows that the properties of the substance can determine the use of the substance.

So the answer is: ① water (or air); ②a； ? ③b； ⑤a； ⑥a； Clear limewater; Oxygen is not easily soluble in water; Nature.