Which of the following pairs of substances will react further with oxygen to form a higher oxide?

Analyze the question: We need a pair in which both substances can react with more oxygen to form a higher oxide. This means each substance must be an oxide of an element that has a higher oxidation state available.

Concept: An oxide can add more oxygen only if its central element is not already in its highest oxidation state.

Check each option:

• A. CO₂ and H₂O: Carbon in CO₂ is already +4, its maximum. H₂O is also fully oxidised (H₂O₂ is a peroxide, not a normal "higher oxide" formed by reacting with oxygen). Neither reacts further. ✗
• B. NO and H₂O: NO reacts with oxygen: 2NO + O₂ → 2NO₂. But H₂O does not react further. ✗
• C. CO and CO₂: CO reacts: 2CO + O₂ → 2CO₂. But CO₂ is already the highest oxide of carbon. ✗
• D. SO₂ and NO: Both react further. • 2SO₂ + O₂ → 2SO₃ (in the presence of V₂O₅ catalyst, the Contact Process) • 2NO + O₂ → 2NO₂ (this happens readily in air, giving brown fumes)

Both form higher oxides. ✓

Tip: Remember the pairs "lower → higher oxide": CO → CO₂, SO₂ → SO₃, NO → NO₂. Any oxide in which the element is already at its maximum oxidation state (CO₂, H₂O, SO₃, N₂O₅) cannot take on more oxygen. In exam questions like this, look for the option where both members are "lower" oxides.