(1) According to the ingredient list of "Seven Up" soda and combined with the raw materials given in the question, it can be seen that the missing substances are: citric acid, sodium citrate, water and carbon dioxide, and citric acid can be combined with small Soda reacts to produce sodium citrate, water, and carbon dioxide;

So the answer is: citric acid? Baking soda

(2) Clear lime water is used to test carbon dioxide in the laboratory, so the answer is: :

Conjecture and hypothesis verification methods and operations Observed phenomenon conclusion (expressed by chemical equation) The gas may be carbon dioxide. Pass the gas generated by soda water into the clear lime water. The clear lime water becomes turbid Ca (OH )2+CO2=CaCO3↓+H2O