Which chemical equation shows that the total mass during a chemical reaction stays the same?
A) Mg + Cl2 → MgCl2
B) NaOH + MgCl2 → NaCl + MgOH
C) 2Na + 2H2O → NaOH + H2
D) H2O + O2 → H2O

Explanation:

The law of conservation of mass is guaranteed in a chemical equation. Since the mass of the atoms do not change, that means that the number of each kind of atoms in the reactant side is equal to the number of atoms of the same kind in the product side.

The first equation is:

A) Mg + Cl₂ → MgCl₂

Number of atoms:

        atom      Reactant side        Product side

          Mg                1                              1

           Cl                 2                             2

Therefore, the table displays that there are the same number of atoms of each kind on both sides, showing that the total mass during the chemical reaction stays the same.

B) NaOH + MgCl₂ → NaCl + MgOH

This equation displays 2 atoms of Cl on the left side and 1 atom of Cl on the right side; thus, it is not showing that the total mass stays the same during the chemical reaction.

C) 2Na + 2H₂O → NaOH + H₂

Neither the sodium, nor oxygen, nor hydrogen atoms are balanced. Thus, this does not show that the total mass stays the same.

D) H₂O + O₂ → H₂O

The reactant side contains 3 oxygen atoms and the product side contains 1 atoms of oxygen; thus, this is not balanced: it does not show that the total mass stays de same during the chemical reaction.