The condition required for corrosion to take place is the presence of

1. Analyze the question The question asks which conditions are required for corrosion (usually rusting of iron) to take place.

2. Explain the concept Corrosion is the gradual destruction of a metal by chemical reaction with its environment. For iron, rusting needs two things together:

• Water (moisture), which acts as the electrolyte
• Oxygen (air), which acts as the oxidizing agent

The overall reaction is:

4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃·xH₂O (hydrated iron(III) oxide, or rust)

Carbon (IV) oxide is not essential. It dissolves in water to form carbonic acid, which makes the water more conductive and speeds up corrosion. But corrosion still happens without it.

3. Step-by-step solution

• Option A (water and CO₂): no oxygen, so rusting cannot occur. Eliminated.
• Option B (water, CO₂ and oxygen): this includes everything, but CO₂ is not required. It only speeds things up. Eliminated, since the question asks for the necessary condition.
• Option C (oxygen and CO₂): no water, so no electrolyte. Eliminated.
• Option D (water and oxygen): these are the two essential conditions. ✔

4. Answer The correct option is D. water and oxygen.

5. Exam tip Remember the classic test-tube experiment:

• Nail in dry air (with a drying agent): no rust
• Nail in boiled water (no dissolved oxygen, with oil on top): no rust
• Nail in water exposed to air: rusts

So rusting needs water + oxygen only. Anything else, such as salt or CO₂, just speeds it up. In JAMB, watch for the word "required" or "necessary" versus "accelerates".