The green house effect is a climatic condition associated with the presence of excess

1. Analyze the question The question asks which gas, when present in excess, is mainly responsible for the greenhouse effect.

2. Explain the concept The greenhouse effect is the trapping of heat in the Earth's atmosphere. Sunlight reaches the Earth's surface and warms it. The Earth then radiates heat back as infrared radiation. Greenhouse gases absorb some of this heat and re-emit it towards the surface, which warms the planet. This is natural and necessary for life. When there is too much of these gases, it leads to global warming.

The main greenhouse gases are:

• Carbon (IV) oxide (CO₂), the most significant one linked to human activity (burning fossil fuels, deforestation, gas flaring)
• Methane (CH₄)
• Water vapour
• Nitrous oxide (N₂O)
• Chlorofluorocarbons (CFCs)

3. Solve by checking the options

• A. Carbon (II) oxide (CO): A poisonous gas that binds to haemoglobin. It is not the main greenhouse gas.
• B. Carbon (IV) oxide (CO₂): The classic greenhouse gas, and the one examiners expect. ✔
• C. Hydrogen sulphide (H₂S): Has a rotten-egg smell and is toxic. It is linked to acid rain, not the greenhouse effect.
• D. Nitrogen (II) oxide (NO): Contributes to acid rain and smog, not the greenhouse effect.
• E. Oxygen (O₂): Does not trap heat significantly.

4. Answer The correct option is B. carbon (IV) oxide.

5. Exam tip For a "greenhouse effect" question, think CO₂, and remember that CO₂ is a heat-trapping gas. For "acid rain", think SO₂, NO₂ and H₂S. For "ozone layer depletion", think CFCs. For "poisonous gas from incomplete combustion", think CO.