International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

Which of the following represents the overall equation for the decomposition of water?

1. 2H₂(g) + O₂(g) -> 2H₂O(l)

2. 2H₂O(l) -> 2H₂(g) + O₂(g)

3. H₂O(l) + O₂(g) -> H₂(g)

4. H₂(g) + O₂(g) -> H₂O(l)

The correct answer is: 2H₂O(l) -> 2H₂(g) + O₂(g)

The overall equation for the decomposition of water describes the process in which water molecules break down into their constituent elements, hydrogen and oxygen. In this case, the correct representation is that two molecules of liquid water decompose into two molecules of hydrogen gas and one molecule of oxygen gas. This reflects the chemical reaction where water is broken down, emphasizing the reactants on the left side of the equation and the products on the right side. The stoichiometry indicates that for every two molecules of water decomposed, two molecules of hydrogen gas and one molecule of oxygen gas are produced, which aligns with the principles of conservation of mass in chemical reactions. This equation succinctly captures the essence of the decomposition process without altering the number of atoms involved, satisfying the requirement for balanced chemical reactions. Other choices either represent synthesis reactions, incorrect amounts of substances, or involve a combination that does not adequately communicate the breakdown of water into its elemental gases.