Which of the following statements is INCORRECT? a) Methanoic acid is secreted by Nettle plant leaves. b) Acetic acid is found in vinegar. c) Tooth decay starts when mouth pH is lower than 5.5. d) Bacteria present in the mouth produce base by degradation of the remaining food particles present in the mouth.

The correct answer is B, AgBr. Silver bromide is the standard example of an ionic crystal that carries both kinds of point defect at the same time. A Schottky defect is a missing pair, one cation vacancy with one anion vacancy, so the crystal stays electrically neutral but loses mass and its density falls; it appears where the two ions are close in size and the coordination number is high. A Frenkel defect is the smaller ion, usually the cation, slipping out of its lattice site into an interstitial space, so the mass and the density are unchanged; it appears where the two ions differ widely in size. In silver bromide the small silver ion can move into an interstitial hole while vacancy pairs also form, so both defects are seen. A is wrong because AgF, whose ions are comparable in size, shows only the Schottky defect. C is wrong because AgCl shows the Frenkel defect. D is wrong because NaCl is the classic Schottky example. Exam tip: a Schottky defect lowers density, a Frenkel defect does not, and AgBr shows both.

The correct answer is B, are long chain molecules held together by secondary bonds. In a thermoplastic the polymer exists as long linear or lightly branched chains, and one chain is held to the next only by weak secondary forces such as van der Waals attraction. Heat gives enough energy to loosen those weak links, so the material softens, can be moulded into a new shape and hardens again on cooling, and the cycle can be repeated, which is why thermoplastic waste is recyclable. Polyethylene, polypropylene, PVC, polystyrene and nylon all belong to this group. A is wrong because a three-dimensional network of strong primary bonds describes a thermosetting plastic such as bakelite, which sets once and cannot be remoulded. C is wrong because the ability to deform plastically rises with temperature instead of falling. D is wrong because the secondary bonds do break as thermal energy rises, and that is exactly why the plastic softens. Exam tip: thermoplastic means weak secondary bonds and remoulding, thermosetting means cross-linked primary bonds and a shape set for good.

The correct answer is C, Platyhelminthes. Flatworms, the phylum Platyhelminthes, have an incomplete digestive system with just one opening, so the same mouth takes the food in and throws the waste out. Their gut is a blind sac with no anus. Planaria, the liver fluke and the tapeworm belong here, and the tapeworm has no gut at all, absorbing food through its body wall. Flatworms are triploblastic, acoelomate and bilaterally symmetrical, and many of them are parasites. A complete digestive system, with a separate mouth and anus, appears from the roundworms onwards. A is wrong because arachnids such as spiders and scorpions are arthropods and have a complete gut. B is wrong because echinoderms like the starfish and sea urchin have both a mouth and an anus. D is wrong because arthropods, the largest phylum of the animal kingdom, have a complete alimentary canal. Exam tip: Coelenterata and Platyhelminthes have a single opening; from Aschelminthes onwards the gut has two.

The correct answer is D, f. The focal length of a mirror does not depend on the medium around it, so a convex mirror of focal length f in air keeps the focal length f inside the liquid. A mirror works by reflection, and for a spherical mirror f = R/2, where R is the radius of curvature of its surface. R is fixed by the shape of the mirror alone, so replacing air with water, oil or any other liquid changes nothing. A lens is the opposite case, because it works by refraction: its focal length depends on the refractive index of the glass relative to the medium around it, so a lens does change its focal length in a liquid. A is wrong because multiplying f by 4/3 treats the mirror as if it were a lens. B is wrong for the same reason, with the factor merely inverted. C is wrong because 7/3 has no basis here at all. Exam tip: mirror focal length is R/2 and stays fixed in every medium, a lens focal length does not.

The correct answer is D, Formation of an oxide layer. Copper turns green because its surface slowly reacts with the air and gets covered by a layer of corrosion products, and that layer is green. In moist air copper first forms a dark copper oxide; carbon dioxide and moisture then change it into basic copper carbonate, the green patina seen on old copper roofs, temple domes and the Statue of Liberty. The coating clings to the metal and shields what lies beneath, which is why copper does not waste away the way iron rusts. A is wrong because hydrogen embrittlement makes a metal brittle and crack-prone and does not colour its surface. B is wrong because copper does not take up nitrogen from the air, nitrogen being almost unreactive at ordinary temperatures. C is wrong because ultraviolet light does not react with copper to build a green coating. Exam tip: the green layer on copper is basic copper carbonate, and tamarind or lemon cleans copper vessels because its acid dissolves that layer.