1) The answer is: chemical reaction is balanced.
Chemical reaction: CH₄ + O₂ → CO₂ + H₂O.
Oxidation reaction: C⁻⁴ → C⁺⁴ + 8e⁻.
Reduction reaction: O₂ + 4e⁻ → 2O⁻² /×2; 2O₂ + 8e⁻ → 4O⁻².
Balanced chemical reaction: CH₄ + 2O₂ → CO₂ + 2H₂O.
Carbon change oxidation number from -4 to +4 (oxidation) and oxygen change oxidation number from 0 to -2 (reduction).
2) The answer is: chemical reaction is balanced.
Balanced chemical reaction: CaCl₂ + 2AgNO₃ → Ca(NO₃)₂ + 2AgCl.
Number of atoms must be the same on left and right side of balanced chemical reaction according to the law of conservation of mass or principle of mass conservation.
There are two chlorine atoms (Cl), one calcium atom (Ca), six oxygen atoms (O), one nitrogen atoms (N) and two silver atoms (Ag) in this balanced chemical reaction.
3) The answer is: chemical reaction is unbalanced.
Chemical reaction: 3Fe₃O₄ + 4Al → 9Fe + 4Al₂O₃.
Number of aluminium atoms (Al) is unbalanced. There are four aluminium atoms on the left and eight aluminium atoms on right side of chemical reaction.
Balanced chemical reaction: 3Fe₃O₄ + 8Al → 9Fe + 4Al₂O₃.
According to principle of mass conservation, number of atoms must be equal on both side of chemical reaction.
