The reaction represents the law of conservation of mass (a balanced chemical equation :
2NaN₃ → 2Na + 3N₂
Conservation of mass applies to a closed system, where the masses before and after the reaction are the same, so balanced chemical equations show that mass is conserved in chemical reactions.
So we just have to look at the amount of each type of element present in the reactants and products must be the same
1. Hg + O₂ → HgO
not balanced, because element O not equal(2 in left, 1 in right)
2. 2NaN₃ → 2Na + 3N₂
balanced, because element in reactant and product are the same
Na, left = 2, right=2
N, left =2x3=6, right=3x2=6
3. C+O → CO₂
not balanced, because element O not equal(1 in left, 2 in right)
4. CH₄ + O₂ → CO₂ +H₂O
not balanced, because element O and H not equal
