A is one of the main components of baking powder, and it is sodium bicarbonate; B is a common condiment and is produced by the reaction of A and hydrochloric acid, so it is sodium chloride; E is often used to improve acidic soil, so it is hydrated lime calcium hydroxide, which is produced by C reaction, so C is calcium oxide and D is carbon dioxide; G is calcium carbonate, the main component of scale, carbon dioxide reacts with sodium hydroxide to produce F soda ash, and E and F are calcium carbonate precipitation and sodium hydroxide produced by the reaction of calcium hydroxide and sodium carbonate, so:

(1)A is sodium bicarbonate, and the chemical formula is; NaHCO3；

(2) Because carbon dioxide can make the clarified limewater turbid, it means that carbon dioxide gas can make the clarified limewater turbid.

(3) Reaction ④ is the reaction of calcium hydroxide and sodium carbonate to produce calcium carbonate precipitation and sodium hydroxide, and the reaction equation is: Ca (OH) 2+Na2CO3 ═ CaCO3 ↓+2NaOH; This reaction is a double decomposition reaction in which two compounds exchange components to obtain two new compounds.

so the answers are: (1) nahco 3; (2) introducing gas into clarified limewater, and the limewater becomes turbid, indicating that it is carbon dioxide gas; （3）Ca（OH）2+Na2CO3═CaCO3↓+2NaOH； Double decomposition reaction.