Which of the following pairs consists of two chemical compounds?

• A. Na and Cl2
• B. He and Ar
• C. H2O and CO2
• D. O2 and N2

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the difference between elements and compounds. Elements are pure substances consisting of only one type of atom, while compounds are substances formed when two or more different elements chemically combine in fixed ratios.
2. Step 2: Identify each species in the given pairs. For example, Na (sodium) and Cl2 (chlorine gas) are both elements, with Cl2 being a diatomic molecule of the element chlorine.
3. Step 3: Analyze the pair He and Ar. Both helium and argon are noble gases and exist as single atoms, so they are elements, not compounds.
4. Step 4: Look at the pair H2O and CO2. Water (H2O) is a compound made of hydrogen and oxygen atoms chemically bonded, and carbon dioxide (CO2) is a compound made of carbon and oxygen atoms chemically bonded. Both are compounds.
5. Step 5: Examine the pair O2 and N2. Both oxygen and nitrogen exist as diatomic molecules of the same element, so they are elements, not compounds.