(2) Calcium carbonate can react with hydrochloric acid, the main component of human gastric acid, to generate calcium chloride, which is soluble in water and can be absorbed by human body; So the answer is: 6;

(3) slaked lime is an alkaline substance with low price and low corrosiveness, which can neutralize acid in acidic soil; So, the answer is: 4;

(4) Because potassium nitrate contains potassium and nitrogen, it contains two nutrient elements; So the answer is: ⑤;

(5) Activated carbon can adsorb pigments and odors in water because of its adsorbability; So, the answer is: ③;

(6) Baking soda can react with organic acids produced during dough making to generate carbon dioxide gas, thus making steamed bread soft and not sour; So, the answer is: ①;

(2) (1) Nitrogen is the most abundant gas in the air; So the answer is: N2;

(2) Atoms are represented by element symbols, and the number in front of the element symbols represents the number of atoms; The symbol of Argon is: ar, so the answer is: 2Ar；;

(3) The number in front of ions indicates the number of ions, and the symbol of ferrous ions is Fe2+; So the answer is: 3Fe2+;

(4) 4) The valence of carbon element in H2CO3 is +4, and the valence should be marked directly above the element symbol; So, the answer is:+4h2co3;

(3) (1) Ethanol (C2H5OH) is fully burned in the air and reacts with oxygen in the air to generate carbon dioxide and water, so its chemical equation is: C2H5OH+3O2? Light it? . ? ? 2co 2+3H2O；

So the answer is: C2H5OH+3O2? Light it? . ? ? 2co 2+3H2O；

(2)A, ethanol is produced by fermentation with sorghum, corn and other grains as raw materials, which is a renewable energy source; So, A is wrong;

B, ethanol gasoline is a mixture of ethanol and gasoline, not pure; So, B is wrong;

C, using ethanol gasoline can effectively reduce automobile exhaust emissions and reduce air pollution; Therefore, C is correct;

D, ethanol combustion will also produce heat, so the use of ethanol gasoline can save gasoline; Therefore, d is correct;

The answer is: ab, because the incorrect one is selected from the topic.

(4) (1) steel is an alloy of iron containing 0.03% ~ 2% carbon, and it is a mixture of many substances; So the answer is: mixture;

(2) Iron corrosion is a series of complex chemical reactions caused by the interaction between iron and oxygen and moisture in the air; The method to prevent steel from rusting is one of the conditions to eliminate steel rusting. Spray painting can isolate oxygen and moisture, so it can prevent iron from rusting. The purpose of recycling scrap steel is to make full use of scrap steel and reduce the exploitation of new iron ore; So the answer is: O2 H2O? Paint (any other reasonable answer will do)? a； ?

(3) Iron consists of iron atoms.

A, matter is composed of molecules, and a molecule is composed of three atoms; So, A is wrong;

B, it is a mixture of various molecules; So, B is wrong;

C is composed of the same kind of atoms, so C is correct;

D is a molecule composed of the same atom; So, d is wrong;

So the answer is: C.