EPFO APFC 2023 General Studies · Question 87 of 120

Correct — D, (d) Lead battery manufacturing industries. A lead-acid battery works on sulphuric acid, which is its electrolyte, and the manufacturing process handles that acid in bulk: the plates are formed and cured in acid, cells are filled with it, and floors, tanks and equipment are washed down afterwards. Every one of those streams carries free sulphuric acid into the plant's wastewater, along with lead in dissolved and particulate form. Sulphuric acid is a strong mineral acid, meaning that it dissociates essentially completely in water and delivers its hydrogen ions in full, so the effluent is not merely acidic but strongly so; untreated wastewater from such plants has been measured at around pH 2, with lead of the order of ten milligrams per litre. Discharged into a river, an effluent of that strength is the most direct chemical cause of a fall in pH there is. The scale matters to appreciating the size of the effect, because pH is logarithmic: each unit down multiplies the hydrogen-ion concentration by ten, so water at pH 2 carries a hundred thousand times the hydrogen-ion concentration of neutral water at pH 7. A river resists small acid loads through the bicarbonate buffering it carries naturally, but that buffer is finite and a continuous discharge of a strong mineral acid exhausts it. The consequences run beyond the pH figure itself: fish and invertebrate eggs and larvae fail at low pH, and acid water mobilises heavy metals from sediment into solution, which in a stretch already receiving lead is the more serious harm. This is why the Central Pollution Control Board's general standard for discharge into inland surface water fixes pH between 5·5 and 9·0, and why an effluent of this kind must be neutralised — commonly with lime — and its lead precipitated before it may be released.

• (a)Alcohol distillery industries — The Commission's key does not take this option, and the row is written carefully because distillery effluent is genuinely acidic. Spent wash, the residue left after alcohol is distilled off the fermented mash, leaves the still hot, dark brown and acidic, typically around pH 3·8 to 4·4, so a discharge of it would indeed lower the pH of a receiving stream. What distinguishes it from the keyed industry is the nature and the source of the acidity. A distillery's acidity comes from organic acids produced during fermentation, which are weak acids and only partly dissociated, whereas a battery plant discharges free sulphuric acid, a strong mineral acid that dissociates completely and drives pH far lower. More to the point, acidity is not what makes spent wash notorious. Its defining burden is organic: a biochemical oxygen demand of the order of 45,000 to 60,000 milligrams per litre and a chemical oxygen demand of 70,000 to 98,000, together with the dark melanoidin colour that blocks light. Those are what deoxygenate a river and kill its life, and Indian regulation has responded by requiring distilleries to move to zero discharge into inland surface watercourses rather than to a permitted pH. In short, the distillery is the standard example of organic pollution and oxygen depletion, while the battery plant is the standard example of a strongly acidic discharge.
• (b)Plastic cup manufacturing industries — Manufacturing plastic cups is a forming operation rather than a chemical one. Sheets or granules of a thermoplastic such as polystyrene or polypropylene are heated and pressed or moulded into shape, and the water a plant of this kind uses goes mostly to cooling the moulds and the formed product; it comes back warm rather than acidic. The characteristic environmental burden of the industry is therefore solid: offcuts, rejects and, at the end of the cup's very short life, an item of single-use plastic that persists in the environment and fragments into microplastic. Those are serious problems, and they are the reason the sector is regulated, but they are not problems that register on a pH meter downstream. A question that asks specifically about the pH of river water is asking which industry puts a strong acid or a strong alkali into its wastewater, and a moulding shop does neither.
• (c)Soap and detergent industries — This industry changes the pH of receiving water in the opposite direction. Soap is made by saponification, in which a fat or oil is boiled with a caustic alkali, normally sodium hydroxide, and the product is the sodium salt of a long-chain fatty acid. A salt of a strong base and a weak acid hydrolyses in water to give an alkaline solution, which is why soap solution turns red litmus blue and feels slippery, and why the washings and spent lyes of a soap works are alkaline rather than acidic. Detergent manufacture adds builders such as sodium carbonate, sodium silicate and phosphates, all of which push the same way. The sector's real water problems are foaming, the nutrient load from phosphate builders that drives eutrophication, and the poor biodegradability of some surfactants — but a rise in pH, not a fall. An option that moves the measurement the wrong way can be eliminated as soon as the chemistry of the product is recalled, without knowing anything about the industry's discharge volumes.

pH measures the hydrogen-ion activity of a solution on a logarithmic scale, so a change of one unit is a tenfold change in concentration; 7 is neutral at ordinary temperatures, below 7 is acidic and above 7 alkaline. Most healthy fresh waters sit a little either side of neutral, between about 6·5 and 8·5, and they hold that range because dissolved bicarbonate acts as a buffer, absorbing added hydrogen ions until the buffering capacity is used up. An industrial discharge changes river pH according to what the industry's process chemistry puts into its wastewater, and the useful way to hold the topic is to sort industries by that chemistry rather than to memorise a list. Industries built on a strong mineral acid discharge acid and pull pH down: lead-acid battery manufacture with sulphuric acid, metal pickling and electroplating with sulphuric and hydrochloric acid, and acid mine drainage, where sulphide minerals oxidise to sulphuric acid. Industries built on a caustic alkali push pH up: soap and detergent manufacture, pulp and paper, textile mercerising. Industries whose burden is organic — distilleries, breweries, sugar mills, tanneries, food processing — are mildly acidic but do their damage through biochemical oxygen demand, stripping dissolved oxygen out of the water as microbes consume the load. Why pH matters at all is that aquatic life has narrow tolerances, with eggs and larvae failing first, and that acidity mobilises heavy metals from sediment into solution, so a pH change converts a settled pollutant into a circulating one. Indian regulation acts on all of this through the Water (Prevention and Control of Pollution) Act, 1974 and the Environment (Protection) Act, 1986, under which effluent standards including a permitted pH range are prescribed and enforced by the pollution control boards.

The general science block of this paper reaches into environmental chemistry two or three times, and its items are built to be answered from process chemistry rather than from any recalled table of industrial effluent quality. That is the skill worth naming, because it converts an unfamiliar question into a familiar one. Ask what chemical the industry's process actually uses in bulk. A lead-acid battery is a device whose electrolyte is sulphuric acid, so a plant making them handles sulphuric acid at every stage and its washings carry it away. Soap is made with caustic alkali, so a soap works discharges alkali. A moulding shop melts and shapes a polymer and uses water mainly for cooling. A distillery ferments and distils, so what leaves it is an organic load. Once the four options are read that way, the question of which lowers pH answers itself, and the reading takes less time than trying to remember effluent data. The item also rewards reading the measurement being asked about. Two of these industries pollute severely without lowering pH — one raises it and one contributes no significant acidity at all — so an answer chosen on the general ground that an industry is dirty will not survive. This paper elsewhere sets the same discipline on a global scale, asking what causes ocean acidification, where the answer is again a matter of following the chemistry rather than the reputation of the pollutant.

• Lead-acid battery manufacture uses sulphuric acid as the battery electrolyte, so plate formation, acid filling and the washing of equipment and floors all send free sulphuric acid into the plant's wastewater. Untreated effluent from such plants has been reported at around pH 2, together with lead of the order of ten milligrams per litre.
• Sulphuric acid is a strong mineral acid and dissociates essentially completely in water, which is why it drives pH far lower than the weak organic acids present in fermentation wastes. The pH scale is logarithmic, so water at pH 2 carries a hundred thousand times the hydrogen-ion concentration of neutral water at pH 7.
• The Central Pollution Control Board's general standards for discharge into inland surface water require pH between 5·5 and 9·0, with biochemical oxygen demand not above 30 milligrams per litre, chemical oxygen demand not above 250, suspended solids not above 100 and oil and grease not above 10.
• Soap and detergent effluent is alkaline, not acidic. Saponification uses caustic alkali and a soap is the salt of a strong base and a weak fatty acid, so it hydrolyses to an alkaline solution; detergent builders such as carbonates, silicates and phosphates push the same way, and the sector's characteristic problems are foaming and phosphate-driven eutrophication.
• Distillery spent wash is acidic at about pH 3·8 to 4·4, but its defining hazard is organic: a biochemical oxygen demand of roughly 45,000 to 60,000 milligrams per litre, a chemical oxygen demand of 70,000 to 98,000, and a dark melanoidin colour. Indian regulation has pushed the sector towards zero discharge into inland surface watercourses.
• Low pH harms aquatic ecosystems in two ways: the direct physiological effect, which eggs and larvae feel first, and the mobilisation of heavy metals from sediment into solution, which is the greater danger where the same discharge is also adding a metal such as lead.

• Choosing an industry because it is notoriously polluting rather than because it lowers pH. Two of these four options are serious polluters that do not acidify a river, and one of them alkalises it.
• Assuming that soap and detergent effluent is acidic because a fatty acid is named in the chemistry. Soap is the salt of a strong base and a weak acid, so its solution is alkaline and it raises pH.
• Overlooking the difference between a strong mineral acid and a weak organic acid. Fermentation wastes are acidic, but sulphuric acid dissociates completely and takes the pH several units lower.
• Forgetting that pH is logarithmic. A shift from pH 7 to pH 2 is not a fivefold change but a hundred-thousandfold change in hydrogen-ion concentration, which is why an untreated acid discharge overwhelms a river's natural buffering.
• Confusing acidification of a river by an acid effluent with ocean acidification, which is driven by dissolved carbon dioxide forming carbonic acid and is a different mechanism on a different scale.

Environmental chemistry in this paper is asked in three shapes. The first is this one: four industries and a named water quality parameter, where the answer follows from what chemical the industry's process uses in bulk. The second names a pollutant and asks for its source or its effect — lead, mercury, fluoride, arsenic, phosphate — and rewards a short table of pollutant, source, effect and the disease or ecological damage associated with it. The third asks about the regulatory framework: which Act, which board, which standard. Preparation that covers all three is a single page listing the major polluting industries with their characteristic effluent, the direction in which each moves pH, the general discharge standards for inland surface water, and the two Acts under which those standards are made. Expect at least one option in every such set to be an industry that is genuinely damaging in a way the question is not asking about, since that is how these items separate a candidate who reads the parameter from one who recognises a polluter.

No directly related past PYQ was found.

• practice — not a real PYQ

  The untreated washings and spent lyes of a soap manufacturing unit are discharged into a small stream. What is the most likely immediate effect on the pH of the stream water?

  • (a)It falls sharply to below 5
  • (b)It rises above 8
  • (c)It is unaffected, because soap is neutral
  • (d)It settles at exactly 7 in all conditions

  Answer(b) it rises above 8 — soap is made by boiling a fat or oil with caustic alkali, and the product is the salt of a strong base and a weak fatty acid, which hydrolyses in water to give an alkaline solution. The residual free alkali in the washings pushes the same way, so this effluent raises pH rather than lowering it. That is the opposite direction from the effluent of an industry built on a strong mineral acid.

• practice — not a real PYQ

  Under the general standards prescribed for the discharge of industrial effluents into inland surface water in India, the permissible range of pH is

  • (a)4·0 to 8·0
  • (b)5·5 to 9·0
  • (c)6·5 to 8·5
  • (d)7·0 to 9·5

  Answer(b) 5·5 to 9·0 — this is the general discharge standard for inland surface water, and it is prescribed alongside limits on biochemical oxygen demand, chemical oxygen demand, suspended solids and oil and grease. The narrower band of about 6·5 to 8·5 in the third option is the range in which healthy natural fresh water is usually found, which is a different quantity from the range an effluent may be discharged at.