Final answer:
In the reaction 2H₂(g) + O₂(g) → 2H₂O(l), the substance oxidized is H₂ and the oxidizing agent is O₂ as it gains electrons and is reduced.
Step-by-step explanation:
To identify the substance oxidized and the oxidizing agent in the redox reaction 2H₂(g) + O₂(g) → 2H₂O(l), we must look at the changes in oxidation numbers of the elements involved.
Hydrogen in H₂ is in the +1 oxidation state and in H₂O it remains in the +1 oxidation state, so there is no change in the oxidation state of hydrogen. Oxygen, on the other hand, goes from the 0 oxidation state in O₂ to a -2 oxidation state in H₂O. This change indicates that oxygen has been reduced.
Since hydrogen is the element that goes from an element (oxidation state 0) to a compound (oxidation state +1), it is the substance oxidized. The molecule O₂, which accepts electrons during the process and is reduced, acts as the oxidizing agent.