In a combustion analysis, what substance must methane (CH_4) be combined with to undergo a complete combustion reaction?

• A. N_2 (nitrogen gas)
• B. H_2O (water)
• C. CO_2 (carbon dioxide)
• D. O_2 (oxygen gas)

Answer: (D)

Verified step by step guidance

1. Understand that a combustion reaction involves a hydrocarbon (like methane, CH_4) reacting with a substance that allows it to burn completely, producing carbon dioxide (CO_2) and water (H_2O).
2. Recall that for complete combustion, the hydrocarbon must react with oxygen gas, O_2, because oxygen supports the burning process and combines with carbon and hydrogen atoms in methane.
3. Write the general form of the complete combustion reaction for methane: \(\mathrm{CH_4 + O_2 \rightarrow CO_2 + H_2O}\).
4. Recognize that nitrogen gas (N_2) does not react in combustion but is often present in air; water (H_2O) and carbon dioxide (CO_2) are products, not reactants, in combustion.
5. Conclude that the substance methane must be combined with for complete combustion is oxygen gas, O_2.