Balance the chemical equation: H2 + O2 → H2O.

• A. 2H2 + O2 → 2H2O
• B. H2 + O2 → H2O
• C. 2H2O + O2 → 2H2
• D. H2 + O2 → 2H2O

Answer: (A)

This question tests balancing chemical equations by conserving atoms. Each element must appear in equal amounts on both sides of the arrow, so you adjust coefficients until the counts match.

The correct balanced form uses two H2 molecules reacting with one O2 molecule to produce two H2O molecules. Counting atoms shows balance: left has 4 hydrogen atoms and 2 oxygen atoms, while right has 4 hydrogen atoms and 2 oxygen atoms (two water molecules each containing two hydrogens and one oxygen). The other forms fail because they either leave hydrogen or oxygen unbalanced (for example, keeping one H2 and one O2 gives too many or too few hydrogens on the product side, and other options either increase hydrogen without matching oxygen or leave oxygen unbalanced).