Air Quality and Climate Change Exam Questions

(a) Nitrogen and oxygen [1]

(b) Bubble air through / pass air into limewater (calcium hydroxide solution) [1]. The limewater turns milky/cloudy, showing carbon dioxide is present [1]

(c) Photosynthesis [1]

Examiner note: The limewater test is the standard test for CO2 and is expected to be recalled precisely. “Goes cloudy” is acceptable; “turns white” is not — the precipitate (CaCO3) makes the solution appear milky. The equation is: Ca(OH)2 + CO2 → CaCO3 + H2O.

Any four from:

• Carbon dioxide — contributes to global warming / enhanced greenhouse effect / climate change [1]
• Carbon monoxide — toxic gas that binds to haemoglobin in blood, reducing oxygen transport [1]
• Sulfur dioxide — causes acid rain which damages buildings, kills aquatic life, and harms vegetation [1]
• Nitrogen oxides (NOx) — cause acid rain and photochemical smog / respiratory problems [1]
• Particulates / soot — cause respiratory diseases / reduce air quality / global dimming [1]

Examiner note: Carbon monoxide is produced by incomplete combustion (insufficient oxygen), not complete combustion. Sulfur dioxide comes from sulfur impurities in fossil fuels. Nitrogen oxides form at high temperatures in car engines when nitrogen and oxygen from the air react.

Formation:

• Fossil fuels containing sulfur impurities are burned, producing sulfur dioxide (SO2) [1]
• Sulfur dioxide dissolves in rainwater and reacts with water and oxygen to form sulfuric acid (H2SO4) [1]

Effects (any two):

• Damages/erodes limestone and marble buildings and statues [1]
• Acidifies lakes and rivers, killing fish and other aquatic organisms [1]
• Damages leaves and roots of trees and crops [1]
• Corrodes metal structures [1]

Examiner note: Nitrogen oxides also contribute to acid rain by forming nitric acid. For full marks on the formation, mention the specific acid formed. Simply “sulfur dioxide causes acid rain” without mentioning how the acid forms is worth only 1 mark.

• Short-wavelength radiation from the Sun passes through the atmosphere and warms the Earth’s surface [1]
• The warm Earth emits longer-wavelength infrared radiation back towards space [1]
• Greenhouse gases (CO2, methane, water vapour) absorb and re-emit this infrared radiation, trapping heat in the atmosphere [1]
• Increased levels of greenhouse gases (from burning fossil fuels, deforestation, agriculture) mean more infrared radiation is trapped, increasing the average global temperature [1]

Examiner note: The greenhouse effect itself is natural and essential for life. The “enhanced” greenhouse effect refers to the increase caused by human activities. Candidates who describe the greenhouse effect as “the Sun’s rays being trapped” lose marks — it is the infrared radiation emitted by the Earth that is absorbed, not the incoming sunlight.

(a) Carbon monoxide is produced by incomplete combustion of hydrocarbons / when there is insufficient oxygen for complete combustion [1]

(b) Carbon monoxide binds to haemoglobin in red blood cells [1]. This prevents haemoglobin from carrying oxygen around the body, which can cause death [1]

Examiner note: Carbon monoxide is colourless and odourless, making it especially dangerous because it cannot be detected without a sensor. The complete combustion of a hydrocarbon produces CO2 and H2O; incomplete combustion produces CO (and/or C) and H2O.

• Methane (CH4): produced by livestock farming / rice paddies / decomposition of waste in landfill / extraction of natural gas [1+1]
• Water vapour: increased by evaporation from warming oceans / irrigation [1]

OR Nitrous oxide (N2O): produced by use of nitrogen-based fertilisers / burning fossil fuels [1+1]

Examiner note: Methane is a far more potent greenhouse gas than CO2 per molecule but is present in much lower concentration. Candidates often forget that methane from agriculture is a significant contributor to climate change.

(a) Carbon monoxide [1] and nitrogen monoxide / nitrogen oxides [1]

(b) Word equation: carbon monoxide + nitrogen monoxide → carbon dioxide + nitrogen [1] Symbol equation: 2CO + 2NO → 2CO2 + N2 [1]

Examiner note: The catalytic converter uses a platinum/rhodium catalyst to convert two harmful gases into two harmless ones. The catalyst is a transition metal on a honeycomb structure (large surface area). Unburned hydrocarbons are also oxidised to CO2 and H2O in the converter.

Any two strategies, with an explanation for each:

• Use renewable energy sources (solar, wind, hydroelectric) instead of burning fossil fuels — these do not produce CO2 during energy generation [1+1]
• Plant more trees / reduce deforestation — trees absorb CO2 during photosynthesis [1+1]
• Carbon capture and storage (CCS) — CO2 produced by power stations is captured and stored underground rather than released into the atmosphere [1+1]
• Improve energy efficiency / reduce energy consumption — less fossil fuel burned means less CO2 produced [1+1]

Examiner note: For “discuss” questions, give a strategy and explain how it works. “Use less fossil fuels” alone is only 1 mark. Linking it to the reason (less CO2 released) completes the answer for 2 marks.