Carbon (iv) oxide is added to the atmosphere when ________?

Analyze the question: We need the process that releases CO₂ into the air.

Concept: CO₂ is added to the atmosphere mainly through respiration, combustion, and decay. It is removed by photosynthesis.

Going through the options:

• A. Plants build up organic compounds: This is photosynthesis. Plants take in CO₂ and release oxygen. CO₂ is removed from the air, not added.
• B. Plants absorb mineral salts: This happens through the roots by active transport or diffusion. It has nothing to do with CO₂ release.
• C. Sugars are completely broken down in animals: This is aerobic respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy The CO₂ is breathed out into the atmosphere, so CO₂ is added.
• D. There is thunderstorm: Lightning makes nitrogen and oxygen combine to form nitrogen oxides. This is part of the nitrogen cycle and does not add CO₂.

Exam tip: Remember "photosynthesis removes CO₂, respiration adds CO₂." When a question uses the words "completely broken down" for sugars, it means complete oxidation, which ends in CO₂ and water.