Answer
Because a compound never changes the numbers of atoms that it started with, the initial amount of hydrogen will only react with a specific amount of oxygen. The excess oxygen will remain unreacted because the chemical reaction is over and the oxygen has nothing to react with.
Work Step by Step
Because a compound never changes the numbers of atoms that it started with, the initial amount of hydrogen will only react with a specific amount of oxygen. The excess oxygen will remain unreacted because the chemical reaction is over and the oxygen has nothing to react with.
