EPFO APFC 2023 General Studies · Question 55 of 120

Correct — A, (a) carbon dioxide. Ocean acidification is the sustained fall in the pH of sea water caused by the ocean's absorption of carbon dioxide from the atmosphere, and it is the direct chemical consequence of the same emissions that drive climate change — which is why it is often called the other carbon dioxide problem. The chemistry is short. The ocean takes up something of the order of a quarter to a third of the carbon dioxide released by human activity each year. Once dissolved, the gas reacts with water to make carbonic acid, which promptly gives up a hydrogen ion: carbon dioxide plus water gives carbonic acid, and carbonic acid dissociates into a hydrogen ion and a bicarbonate ion. It is those extra hydrogen ions that lower the pH. No other common atmospheric gas is both abundant enough and soluble enough to do this at a global scale. The numbers are worth carrying. Surface ocean pH has fallen from about 8·2 before the industrial era to about 8·1 now. That looks trivial until one remembers that pH is a logarithmic scale, so a fall of one tenth of a unit is an increase of roughly thirty per cent in the concentration of hydrogen ions. It is also worth being precise about the word: the ocean remains alkaline, comfortably above pH 7, and acidification names the direction of the change rather than a crossing into acid. The damage is done through carbonate chemistry rather than through corrosiveness. The additional hydrogen ions combine with carbonate ions to form bicarbonate, so the concentration of carbonate available in sea water falls. Carbonate is the raw material from which corals, molluscs, pteropods, foraminifera and coccolithophores build their calcium carbonate skeletons and shells, so calcification becomes harder and, where the water is sufficiently undersaturated, existing shells begin to dissolve. Coral reefs, which are both a fishery habitat and a coastal defence, are among the most exposed, and the reduction of ocean acidification is written into the Sustainable Development Goal on life below water.

• (b)carbon monoxide — Carbon monoxide is a genuine pollutant, but it cannot acidify the ocean because it barely dissolves in water and does not react with it to give an acid. It is a product of the incomplete combustion of carbon fuels — a vehicle engine burning fuel in too little air, a coal fire, a badly ventilated stove — and its danger is physiological rather than chemical: it binds to haemoglobin far more strongly than oxygen does, forming carboxyhaemoglobin and starving the tissues of oxygen, which is why it is fatal in enclosed spaces and why it is colourless and odourless enough to give no warning. The option is in the set because the name differs from the correct answer by a single word, and the discriminating fact is solubility and reactivity in water, not the presence of carbon in the formula.
• (c)hydrogen sulfide — Hydrogen sulfide is genuinely acidic in water — it is a weak acid, and it is genuinely present in the sea, produced by sulphate-reducing bacteria in oxygen-poor sediments and released at hydrothermal vents — which makes this the most respectable distractor in the set. What it is not is the main cause of ocean acidification, because it occurs in trace quantities and in particular places rather than throughout the surface ocean. Its consequences are local and mostly a matter of toxicity and of oxygen depletion: it is the gas of rotten eggs, it can kill fish where anoxic bottom water wells up, and it is a hazard in sewers and in tanneries. The word main in the stem is what settles the choice, and the booklet prints the name with an f, as sulfide.
• (d)chlorofluorocarbon — Chlorofluorocarbons belong to a different environmental problem. They are stable, non-toxic, non-flammable compounds that were used as refrigerants, aerosol propellants and foam-blowing agents, and their stability is exactly the trouble: they survive long enough in the troposphere to reach the stratosphere, where ultraviolet light breaks them up and releases chlorine atoms that catalytically destroy ozone. That is the ozone-layer problem, addressed by the Montreal Protocol of 1987 and its later amendments. They are also potent greenhouse gases, molecule for molecule far stronger than carbon dioxide. What they do not do is dissolve in sea water and release hydrogen ions, so they have no part in acidifying the ocean. Keeping the ozone hole, the greenhouse effect and ocean acidification apart, along with the gas responsible for each, is a standing requirement in this paper's environment questions.

The ocean is the largest single sink for the carbon dioxide added to the atmosphere by burning fossil fuels, cement manufacture and land-use change, and that service has a chemical price. Dissolved carbon dioxide reacts with sea water to form carbonic acid, which dissociates and releases hydrogen ions, lowering the pH — a process that has been measured directly at ocean time-series stations for decades and reconstructed for the industrial era from the carbon budget. Surface pH has fallen from roughly 8·2 to roughly 8·1, which on a logarithmic scale is about a thirty per cent rise in hydrogen ion concentration. The biological consequence works through the carbonate system rather than through acid attack. Sea water carries carbon in three linked forms — dissolved carbon dioxide, bicarbonate and carbonate — and adding hydrogen ions pushes the balance away from carbonate, because the added hydrogen combines with carbonate to make bicarbonate. Carbonate is what calcifying organisms take up to build shells and skeletons of calcium carbonate, so as its concentration falls, the saturation state of the water with respect to aragonite and calcite falls too, calcification becomes more costly in energy, and below saturation existing structures dissolve. Corals, oysters, mussels, pteropods, foraminifera and coccolithophores are all affected, which reaches human beings through fisheries, aquaculture, reef tourism and the coastal protection that reefs provide. Two related problems must be kept distinct from this one: acid rain, which is caused by sulphur dioxide and oxides of nitrogen and acts on lakes, soils, forests and buildings; and stratospheric ozone depletion, caused by chlorofluorocarbons and controlled by the Montreal Protocol.

Environment questions in this paper are single-cause identification items, and the Commission builds the option set from substances that a candidate associates with pollution generally rather than with this process specifically. That is the pattern to prepare for: not a syllabus of environmental science, but a clear one-to-one map between each named problem and the substance that causes it — carbon dioxide with global warming and ocean acidification, sulphur dioxide and nitrogen oxides with acid rain, chlorofluorocarbons with ozone depletion, methane and nitrous oxide with the greenhouse effect, particulate matter with respiratory illness, and so on. Half the environment items in a general studies paper fall to that single table. The paper carries a companion question later in the science block on the industrial effluent that lowers the pH of river water, which is worth studying alongside this one because it shows the same idea — acidification of a water body — arising from an entirely different cause and at an entirely different scale. Note the printing: the stem is an incomplete sentence completed by each option, with no question mark, and the booklet spells sulfide with an f.

• Ocean acidification is the fall in sea water pH caused by the ocean absorbing carbon dioxide from the atmosphere; the ocean takes up of the order of a quarter to a third of annual human carbon dioxide emissions.
• Dissolved carbon dioxide forms carbonic acid, which dissociates into a hydrogen ion and a bicarbonate ion; the released hydrogen ions are what lower the pH.
• Surface ocean pH has fallen from about 8·2 in pre-industrial times to about 8·1, and because pH is logarithmic that is roughly a thirty per cent increase in hydrogen ion concentration.
• The ocean remains alkaline throughout; acidification describes the direction of change, not a move below pH 7.
• The added hydrogen ions convert carbonate into bicarbonate, lowering the carbonate saturation of sea water and making it harder for corals, molluscs, pteropods and coccolithophores to build calcium carbonate shells and skeletons.
• Acid rain is a different problem, caused by sulphur dioxide and oxides of nitrogen; ozone depletion is another, caused by chlorofluorocarbons and controlled under the Montreal Protocol of 1987.
• Carbon monoxide is a product of incomplete combustion whose hazard is toxicity through binding to haemoglobin; it is nearly insoluble in water and forms no acid in it.

• Choosing carbon monoxide because it contains carbon; the property that matters is solubility in water and the formation of an acid, which carbon monoxide does not have
• Reading acidification as meaning the ocean has become an acid; it remains alkaline, and the term names a shift in that direction
• Treating the change of one tenth of a pH unit as negligible, when the logarithmic scale makes it about a thirty per cent rise in hydrogen ion concentration
• Attributing ocean acidification to acid rain; sulphur and nitrogen oxides matter locally in coastal waters but are not the global driver
• Confusing the three separate atmospheric problems and their agents — the greenhouse effect, ozone depletion and acidification
• Assuming the damage is caused by acid dissolving shells directly; the main mechanism is the reduction of the carbonate ions from which shells are built

Environment items in this paper name a phenomenon and ask for its cause, or name a substance and ask for its effect, always with four one-line options. The distractors are other well-known pollutants rather than obscure ones, so the discriminating knowledge is the pairing rather than the chemistry. Build the table of problem against agent, add the treaty or programme attached to each where one exists, and this whole family becomes a set of one-step lookups.

No directly related past PYQ was found.

• practice — not a real PYQ

  The falling concentration of carbonate ions in sea water caused by ocean acidification most directly affects

  • (a)the rate at which corals and shellfish build their calcium carbonate skeletons
  • (b)the salinity of surface waters
  • (c)the depth of the thermocline
  • (d)the speed of ocean currents

  Answer(a) the rate at which corals and shellfish build their calcium carbonate skeletons — the extra hydrogen ions combine with carbonate to form bicarbonate, so less carbonate is available to calcifying organisms, calcification becomes more costly and, in undersaturated water, existing shells dissolve. Salinity, the thermocline and current speeds are governed by other processes altogether.

• practice — not a real PYQ

  Acid rain is caused mainly by the atmospheric emission of

  • (a)carbon dioxide and methane
  • (b)sulphur dioxide and oxides of nitrogen
  • (c)chlorofluorocarbons and halons
  • (d)carbon monoxide and ozone

  Answer(b) sulphur dioxide and oxides of nitrogen — these are oxidised in the atmosphere to sulphuric and nitric acid and brought down in precipitation, damaging lakes, soils, forests and limestone buildings. Carbon dioxide drives warming and ocean acidification, chlorofluorocarbons and halons deplete stratospheric ozone, and carbon monoxide is a toxic product of incomplete combustion.