Materials, Nanotechnology and Industrial Processes

• Statement 3 alone is correct, so the official answer is (b). The test for a chemical change is whether new substances with different chemical identities are formed, which requires the breaking and making of chemical bonds, and whether the change is readily reversible by physical means.
• Statement 1 is a physical change: crystallisation of sodium chloride from solution rearranges ions from the dissolved state into a lattice, but the substance remains sodium chloride throughout and dissolving the crystals restores the solution.
• Statement 2 is a physical change: melting of ice is a phase transition in which hydrogen bonds between water molecules are disrupted but no covalent bond within a water molecule is broken, so the substance is water before and after and freezing reverses it.
• Statement 3 is a chemical change: souring occurs because lactic acid bacteria ferment lactose to lactic acid, a different compound with different properties, and the fall in pH denatures casein and curdles the milk. Nothing can restore the original milk. The item is answered by applying the single criterion of whether a new substance is produced, and it is worth noting that the enthalpy involved in a phase change is comparatively small precisely because intermolecular rather than intramolecular forces are being overcome.

• Statement 1 is correct. Nanoscale carriers such as liposomes, polymeric micelles, dendrimers and gold nanoparticles carry a drug payload to a chosen site, by passive accumulation through the enhanced permeability and retention effect of leaky tumour vasculature and by active targeting through ligands that bind receptors overexpressed on the target cell. Doxorubicin in liposomal form was already in clinical use well before this paper.
• Statement 2 is correct. Non-viral nanoparticle vectors, principally lipid and polymeric nanoparticles, are a principal route for delivering nucleic acids into cells, the delivery problem rather than the editing chemistry being the main obstacle in gene therapy. The official answer (c) follows, and there is no elimination route, both statements having to be judged independently.

All three pairs are correctly matched, so the official answer (d) follows.

• Pair 1 is correct. Lead occurs in lipstick as a contaminant of the mineral pigments and colour additives rather than as an intentional ingredient, and surveys by regulators including the United States Food and Drug Administration have detected it at trace levels across a large majority of tested brands; the concern is cumulative exposure through ingestion, lead being a neurotoxin with no established safe threshold.
• Pair 2 is correct. Brominated vegetable oil was used as an emulsifier in citrus flavoured soft drinks, its function being to keep the flavour oils, which are less dense than water, evenly dispersed rather than separating to the surface; the bromine raises the density of the oil to match the beverage. It is controversial on account of bromine accumulation in tissue, and has been progressively withdrawn and prohibited in several jurisdictions.
• Pair 3 is correct. Monosodium glutamate, the sodium salt of glutamic acid, is used as an umami flavour enhancer and is strongly associated with Chinese restaurant cooking, the reported symptom cluster having historically been named after it, though controlled studies have not reproduced the effect consistently and major food safety authorities regard it as generally safe. Since no pair can be rejected there is no elimination route, and a candidate must know each independently. Note that the stem is carefully worded as 'unwanted or controversial' and 'likely to be found in', which is a lower threshold than harm or intentional addition, and pair 3 in particular satisfies the stem only on that reading, monosodium glutamate being an intentional and lawful ingredient whose controversy is largely popular rather than toxicological.

All three are correct and the official answer (c) follows. The common cause of all three advantages is structural and is worth stating first: an OLED is an emissive device, in which each organic pixel generates its own light when charge carriers recombine in the organic layer, whereas a liquid crystal display is transmissive and merely shutters light from a separate backlight unit. Removing the backlight removes a rigid, bulky and opaque component, and everything else follows.

• Statement 1 is correct: the organic layers are deposited at low temperature and are thin, so they can be laid on flexible plastic or metal foil substrates, which the glass panel and rigid backlight assembly of a liquid crystal display cannot accommodate.
• Statement 2 is correct and is a direct consequence of statement 1, roll up and conformable displays including textile integrated ones having been demonstrated.
• Statement 3 is correct: with no opaque backlight behind them, transparent OLED panels can be built in which the pixels emit while the substrate remains see through, a configuration used in heads up displays and display cabinets. Since the three statements form a single chain of consequence, a candidate who grasps the emissive against transmissive distinction accepts all three, and the item is therefore decided by one concept rather than by three facts.
• Option (d) is a trap for over caution, inviting a candidate who doubts statement 2 as futuristic to reject the entire set.

The key takes all five and the answer is (d).

• Statement 1 is correct: food printers extrude chocolate, sugar paste and dough layer by layer, and printed confectionery is an established niche in patisserie.
• Statement 2 is correct: the bionic ear demonstrated at Princeton in 2013 was printed from a hydrogel seeded with chondrocytes together with a silver nanoparticle coil, and is the standard cited example of printing a functional organ integrated with electronics.
• Statement 3 is correct: the automotive industry was among the earliest adopters, first for rapid prototyping of body panels and tooling and later for production of jigs, fixtures, brackets and low volume components.
• Statement 4 is correct and is now routine: patient specific titanium cranial and maxillofacial implants and surgical guides are printed from the patient's own computed tomography data, which is precisely the case additive manufacturing serves best, since every implant is a batch of one.
• Statement 5 is the doubtful one, and it survives on a permissive reading: printing is used to fabricate microfluidic devices, photonic and waveguide structures, heat sinks and interconnects, and printed electronics is a working field, so hardware used in data processing can be additively manufactured even though the printing of data processing itself is meaningless. Since no statement is rejected, only option (d) is available.

Hence (d).

• Statement 1 is correct, since functionalised nanotubes are established vehicles for drug and antigen delivery, the high surface area and ability to cross membranes being the attraction.
• Statement 3 is correct, since their electrical conductivity changes on binding of a target molecule, which is the basis of nanotube field effect biosensors.
• Statement 4 is accepted as correct by the key on the strength of work showing enzymatic degradation of carboxylated nanotubes by peroxidases, including myeloperoxidase released by neutrophils.
• Statement 2 is treated as incorrect by the key. The keyed answer is therefore 1, 3 and 4, giving (c), with (a) and (d) rejected for including statement 2 and (b) rejected for dropping statement 1.

The water molecule is bent with an angle of about 104.5 degrees, and because oxygen is far more electronegative than hydrogen the molecule carries a permanent dipole moment. That dipole allows water to surround ions, orienting its negative oxygen end towards cations and its positive hydrogen ends towards anions, which is hydration, and the energy released in doing so overcomes the lattice energy holding the solid together. This is why water is called the universal solvent, so option (a) is correct.

• Option (b) is irrelevant, since thermal conductivity has no bearing on solvation.
• Option (c) is a genuine property of water but explains its thermal buffering of climate and organisms, not its dissolving power.
• Option (d) merely restates the composition without supplying a mechanism, and many oxides are not solvents at all. Hence (a). The question rewards distinguishing a true statement from an explanatory one.

• Statement 1 is correct. Polyethylene terephthalate drawn into fibre is polyester, and polyester and cotton or polyester and wool blends are among the commonest textile constructions, the synthetic component contributing strength, crease resistance and dimensional stability to the natural fibre.
• Statement 2 is incorrect, and the word any is what defeats it. PET is standard for water and carbonated soft drinks, but it is not suitable for the whole range of alcoholic beverages; spirits of high alcohol content act as a solvent on the polymer over time, promoting migration of monomer and additives, and PET's permeability to oxygen and carbon dioxide is a further limitation.
• Statement 3 is correct. PET is among the most widely recycled polymers, mechanically reprocessed into fibre for textiles, into sheet and strapping, and back into bottles, and chemical depolymerisation routes exist as well.
• Statement 4 is incorrect on elementary grounds. PET is a hydrocarbon polymer, so its combustion necessarily yields carbon dioxide, and incineration of plastics is therefore a source of greenhouse gas, quite apart from the other pollutants involved.
• Statements 1 and 3 hold, giving (a).
• Options (b), (c) and (d) each rest on statement 2 or statement 4, and statement 4's chemical impossibility gives a candidate a secure elimination route.

• Statement 1 is incorrect, and decisively so. Nanoscale particles are abundant in nature and always have been: volcanic ash and sea spray aerosols, soot from combustion and forest fire, clay mineral colloids in soil and water, and biological structures such as ferritin and viruses all fall in the nanometre range. Engineered nanoparticles are a recent addition to a natural population, not a new class of matter.
• Statement 2 is correct. Nanoparticles of titanium dioxide and zinc oxide are used in sunscreens and other cosmetics, where reducing the particle size preserves the ultraviolet blocking action while removing the opaque white film that larger particles leave on the skin.
• Statement 3 is correct as the key treats it. Nanoparticles released from commercial products into the environment raise recognised safety concerns, since their small size and large surface area allow them to cross biological barriers and interact with tissues in ways the same material in bulk does not, and silver and titanium dioxide nanoparticles have attracted particular attention.
• Statements 2 and 3 holding, (d) is correct; options (a) and (c) accept the false claim about nature, and (b) drops the well established cosmetic use.

• Statement 1 is correct. Carbon fibre reinforced polymers combine very high specific strength and stiffness with low density and fatigue resistance, which is why they are used in aircraft primary structures, in automotive body panels and chassis components, and in wind turbine blades and pressure vessels.
• Statement 2 is incorrect. Carbon fibre composites are recycled at commercial scale, principally by pyrolysis in which the polymer matrix is thermally decomposed and the fibres recovered, and also by solvolysis and mechanical grinding. The recovered fibres are shorter and lose some strength, so the process is downcycling rather than closed loop recovery, but recycling certainly occurs. Only statement 1 stands, giving (a).
• Options (b) and (c) require the recyclability claim to be denied, and (d) denies an obvious use.

• Statement I is correct: rare earth elements are essential to display manufacture, with europium providing red emission, terbium green, and yttrium serving as the host lattice in many phosphor compositions, alongside cerium and gadolinium in related applications.
• Statement II is correct: several lanthanides are strongly luminescent, and Statement II supplies the reason for Statement I, so the explanatory link holds and the answer is (a). The physical basis is worth stating precisely. Emission in the lanthanides arises from transitions between four f orbitals, which lie inside the filled five s and five p shells and are therefore shielded from the surrounding crystal field. Two consequences follow: the emission occurs at sharply defined wavelengths largely independent of the host material, which gives displays their colour purity, and the transitions are formally forbidden by the Laporte selection rule, which makes them slow and the excited states long-lived.
• Options (c) and (d) each reject one accurate statement.
• Option (b) concedes both but denies the causal connection, which cannot stand, since the luminescent property is precisely why these elements are chosen for screens.

• Statements 1 and 3 are correct, giving (a).
• Statement 1 is sound, since neodymium, praseodymium, dysprosium and terbium go into the high strength permanent magnets that drive robotic actuators, hard disk assemblies, satellite reaction wheels and electric traction motors, while other lanthanides serve in phosphors, lasers and catalysts.
• Statement 3 is sound, since the National Critical Mineral Mission was approved in January 2025 covering exploration, domestic and overseas acquisition, recycling and stockpiling.
• Statement 2 is wrong on the second name, because although China dominates mining with a large majority of world output, the countries following it are the United States, Myanmar and Australia, while India holds substantial monazite reserves but a very small share of actual mined production, and confusing reserves with output is the trap.
• Statement 4 is wrong on the count, because rare earth elements number seventeen, the fifteen lanthanides together with scandium and yttrium, which are grouped with them because they share ionic radius and chemical behaviour and occur in the same ores. (b) discards the applications statement, and (c) and (d) accept the count of thirteen.