Question 4
Which is the balanced equation for the decomposition of hydrogen peroxide?
The right answer is the one marked above.
2H2O2(aq) → 2H2O(l) + O2(g). Four hydrogen and four oxygen on the left; four hydrogen and four oxygen (2 + 2) on the right.
The version with 2O2 on the right has six oxygen atoms there, so it is not balanced. The sides do not match in the other two either. Always count the atoms once you have written an equation.