4.9 Chemistry of the atmosphere — coverage pack

Answer

• $48\,\text{cm}^3$

Method: Oxygen is approximately 20% of air, so its volume is $0.20\times240=48\,\text{cm}^3$.

Answer

• $2.0\,\text{dm}^3\text{ nitrogen};\ 0.50\,\text{dm}^3\text{ oxygen}$

Method: Apply each approximate percentage to the total: $0.80\times2.5=2.0\,\text{dm}^3\text{ nitrogen};\ 0.20\times2.5=0.50\,\text{dm}^3\text{ oxygen}$.

Answer

• $19.6\%$ oxygen
• This is close to the accepted approximate value of 20%.

Method: Calculate the volume loss and then its percentage of the starting volume: $480-386=94\,\text{cm}^3;\ (94/480)\times100=19.58\ldots\%=19.6\%$. This differs from 20% by only $0.4$ percentage points, so it is close to the accepted approximation.

Answer

• Carbon dioxide

Method: Recall the volcanic-atmosphere model: the early atmosphere is thought to have contained mainly carbon dioxide.

Answer

• Volcanic activity released gases, including carbon dioxide and water vapour.
• These gases formed the early atmosphere.
• As the Earth cooled, water vapour condensed to form oceans.

Method: Give the events in causal order: volcanic gases were released, accumulated as an atmosphere, and then the falling temperature allowed gaseous water to condense into liquid oceans.

Answer

• Modern volcanic gases support the idea that early volcanoes supplied carbon dioxide and water vapour.
• The evidence is indirect and conditions on the early Earth may have differed from conditions today.
• Very little direct evidence survives over 4.6 billion years.
• The evidence supports a theory but does not establish an exact composition with certainty.

Method: Separate support from certainty. The observation gives a plausible mechanism, but extrapolating modern activity across an immense time interval introduces uncertainty, so the word 'exact' is not justified.

Answer

• Photosynthesis

Method: Algae use light energy to make glucose and release oxygen; this process is photosynthesis.

Answer

• Photosynthetic organisms take in carbon dioxide.
• They use it with water to make glucose.
• They release oxygen, so carbon dioxide decreases while oxygen increases.

Method: Use the word equation for photosynthesis as the reasoning chain: carbon dioxide is a reactant and oxygen is a product, so more photosynthesis removes more of the former and produces more of the latter.

Answer

• The final oxygen percentage is 43 times greater.
• Algae release oxygen by photosynthesis, so their spread could cause the increase.

Method: Calculate the scale factor: $8.6/0.20=43$. The direction of the change agrees with the known product of photosynthesis, although the data alone show consistency rather than proving that algae were the only cause.

Answer

• Photosynthesis

Method: Plants consume carbon dioxide as a reactant in photosynthesis, so this process lowers its atmospheric proportion.

Answer

• Carbon dioxide dissolved in oceans and formed carbonate sediments that became sedimentary rocks such as limestone.
• Photosynthetic organisms took in carbon dioxide, and some buried remains eventually formed coal, crude oil or natural gas.

Method: Follow carbon atoms rather than saying the gas vanished. One pathway is atmosphere to ocean to carbonate rock; the other is atmosphere to living material to buried organic matter and then fossil fuels.

Answer

• Carbon dioxide dissolved in oceans and carbonates precipitated as sediments that formed limestone.
• Plants removed carbon dioxide by photosynthesis; buried plant material formed coal over millions of years.
• Carbon taken into marine organisms was buried and formed crude oil and natural gas over millions of years.
• Each route stored carbon outside the atmosphere for a long time.

Method: Distinguish the inorganic carbonate route from the two organic routes. Then connect all three stores to the same atmospheric outcome: carbon is locked in rock or fuel instead of remaining as carbon dioxide gas.

Answer

• Any two from water vapour, carbon dioxide and methane.

Method: Select two gases from the specified set: water vapour, carbon dioxide and methane.

Answer

• Short-wavelength radiation from the Sun passes through the atmosphere and warms the surface.
• The warm surface emits longer-wavelength infrared radiation.
• Greenhouse gases absorb and re-emit some of this outgoing radiation.

Method: Follow the energy transfer in order: incoming short wavelength, surface absorption and warming, outgoing longer wavelength, then interaction with greenhouse-gas molecules.

Answer

• Incoming solar radiation is mainly shorter wavelength.
• The warmed surface emits longer-wavelength infrared radiation.
• Additional carbon dioxide absorbs and re-emits more of this outgoing infrared radiation.
• Energy escapes more slowly, so the surface warms until energy transfers balance again.

Method: The key distinction is wavelength, not a solid 'blanket'. Carbon dioxide interacts with part of the outgoing long-wavelength radiation, so increasing its concentration changes the rate of energy loss while incoming sunlight can continue to reach the surface.

Answer

• Carbon dioxide: for example, burning a fossil fuel or deforestation.
• Methane: for example, livestock farming, rice cultivation or sending organic waste to landfill.

Method: Choose one activity from each gas-specific list and make the pairing explicit. One correct carbon dioxide source and one correct methane source earn the two marks.

Answer

• Carbon dioxide: burning fossil fuels and deforestation.
• Methane: livestock farming and decomposition of organic waste in landfill.

Method: Keep the gases separate. Combustion releases stored carbon and deforestation reduces carbon dioxide uptake; digestion in livestock and anaerobic decay of buried organic waste release methane.

Answer

• One town is not representative of the whole globe.
• Six years is too short to establish a long-term climate trend reliably.
• No uncertainty is reported, so the reliability and significance of the measurements cannot be judged.
• The company may have a conflict of interest or present only selected evidence.
• The claim should be compared with peer-reviewed evidence from wider and longer datasets.

Method: Test the report against five evidence-quality questions: Is the dataset broad, long, uncertainty-aware, independent and peer reviewed? It fails each check, so its conclusion is not well supported.

Answer

• For example: sea-level rise, coastal flooding, altered rainfall, more extreme weather, habitat change, changed species distribution or reduced biodiversity.

Method: Give one specific consequence rather than repeating 'the temperature increases'. Sea-level rise is one valid example.

Answer

• Higher temperatures can melt land-based ice.
• They can also cause seawater to expand.
• Both processes raise sea level, increasing coastal flood risk.

Method: Link temperature to two mechanisms and then to the hazard: land ice adds water and thermal expansion increases ocean volume, so sea level rises and flooding becomes more likely.

Answer

• Higher sea level increases the likelihood or severity of flooding and erosion.
• Salt water may change or destroy the wetland habitat.
• Rare-bird populations or their distribution may decline or shift.
• Loss of the wetland could remove natural protection and increase risk to the town.
• The size of the effect depends on the amount and rate of sea-level rise and on local protection or adaptation.

Method: Cover all three command areas. Scale concerns how large and rapid the change is; risk combines likelihood with severity; environmental implications include habitat, biodiversity and the wetland's protective function.

Answer

• The total amount of carbon dioxide and other greenhouse gases emitted over the product's full life cycle.

Method: Include both required ideas: more than carbon dioxide alone is counted, and emissions are totalled across the complete life cycle rather than only manufacture.

Answer

• For example, provide shared electric transport, but vehicles and charging infrastructure may be expensive or unavailable.
• For example, replace diesel generators with renewable electricity, but supply may be intermittent or the necessary connection may be costly.

Method: For each pair, name a change that reduces carbon dioxide or methane emissions, then attach a realistic economic, technical or social limitation to that same change.

Answer

• $\text{new footprint}=44\,\text{kg};\ \text{percentage reduction}=18.5\%$

Method: Keep every life-cycle stage in the total: $18+4+30+2=54\,\text{kg};\ 4(0.75)=3\,\text{kg};\ 30(0.60)=18\,\text{kg};\ 21+3+18+2=44\,\text{kg};\ ((54-44)/54)\times100=18.5\%$. The redesigned manufacture value is $18+3=21\,\text{kg}$.

Answer

• Carbon monoxide

Method: Too little oxygen causes incomplete combustion, which can form carbon monoxide rather than only carbon dioxide.

Answer

• Carbon dioxide from carbon.
• Water vapour from hydrogen.
• Sulfur dioxide from sulfur.

Method: Excess oxygen favours complete combustion. Match each stated fuel element to its oxidised product: carbon to carbon dioxide, hydrogen to water, and sulfur to sulfur dioxide.

Answer

• $32\,\text{g}$ of sulfur dioxide

Method: Use the sulfur percentage and the fact that each sulfur atom forms one sulfur dioxide molecule: $m(\mathrm{S})=2000(0.0080)=16\,\text{g};\ M_r(\mathrm{SO_2})=32+2(16)=64;\ m(\mathrm{SO_2})=16(64/32)=32\,\text{g}$. The product-to-sulfur mass ratio is therefore $2:1$.

Answer

• It is colourless.
• It is odourless.

Method: Select the two properties linked to detection by sight and smell. Toxicity explains the danger but not why the gas is hard to notice.

Answer

• Sulfur dioxide or oxides of nitrogen can cause respiratory problems.
• They can cause acid rain.
• Particulates can cause health problems or global dimming.

Method: Allocate one statement to each requested category: human respiratory harm, environmental acid rain, and either the health or light-reduction effect of particulates.

Answer

• Sulfur dioxide and/or oxides of nitrogen are consistent with the breathing problems.
• The same gases can cause acid rain, which is consistent with the lake becoming more acidic.
• Particulates are consistent with less sunlight reaching the ground because they cause global dimming.
• The observations indicate more than one type of pollutant rather than a single gas causing every effect.

Method: Match each observation to a specified effect: respiratory problems and acid rain point to sulfur dioxide or nitrogen oxides, while reduced incoming sunlight points to particulates. Because those effect profiles differ, infer a mixture of pollutants.