Mg + HCl -> H_{2} + MgCl_{2} H_{2}O + CO_{2} -> H_{2}CO_{3} KClO_{3} -> KCl + O_{2} H_{2} + O_{2} -> H_{2}O
I gave my students this multiple choice question: Which of the following is a correctly written chemical equation that demonstrates the conservation of mass?

Only one answer is correct among these 4 choices:
- Mg + HCl -> H_{2} + MgCl_{2}
- H_{2}O + CO_{2} -> H_{2}CO_{3}
- KClO_{3} -> KCl + O_{2}
- H_{2} + O_{2} -> H_{2}O

Could you tell me which one is correct?

H_{2}O + CO_{2} -> H_{2}CO_{3}