The correct option is Hydrogen.
Explanation
Electrolysis of water is the process of decomposing water molecules ($H_2O$) into hydrogen and oxygen gases by passing an electric current through the liquid. This process takes place in an electrolytic cell, typically using acidified water to increase conductivity.
Detailed Analysis
The relationship between the reactants and products during the electrolysis of water is governed by the stoichiometry of the balanced chemical equation:
$2H_2O(l) \xrightarrow{\text{Electric Current}} 2H_2(g) + O_2(g)$
- Stoichiometric Ratio: The equation indicates that the decomposition of 2 moles of water yields 2 moles of hydrogen gas ($H_2$) and 1 mole of oxygen gas ($O_2$).
- Volume Relationship: According to Avogadro's Law, at the same temperature and pressure, the volume of a gas is directly proportional to the number of moles. Since 2 moles of hydrogen are produced for every 1 mole of oxygen, the volume of hydrogen gas collected is double the volume of oxygen gas.
- Electrode Reactions:
- Cathode (Negative Electrode): Reduction occurs here. Hydrogen ions gain electrons to form hydrogen gas ($2H^+ + 2e^- \rightarrow H_2$).
- Anode (Positive Electrode): Oxidation occurs here. Water molecules lose electrons to form oxygen gas ($2H_2O \rightarrow O_2 + 4H^+ + 4e^-$).
Key Takeaway: In the electrolysis of water, hydrogen gas is collected at the cathode and oxygen gas at the anode in a volume ratio of 2:1.