At STP, the number of molecules present in 5.6 L of O2 gas is:

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.

The correct answer is D, None of the above. None of the three substances listed slows a reaction down; the substance that does that is called a negative catalyst or an inhibitor. A catalyst changes the rate of a reaction without being used up in it, and it works by lowering the activation energy the reactants must cross. A positive catalyst speeds the reaction up, while a negative catalyst holds it back, the way glycerol slows the decomposition of hydrogen peroxide. A is wrong because a catalytic promoter is added to make the catalyst more active, so it raises the rate instead of lowering it. B is wrong because a homogeneous catalyst is one present in the same phase as the reactants, and it speeds the reaction up. C is wrong because a heterogeneous catalyst sits in a different phase, like iron in the Haber process, and it also speeds the reaction up. Exam tip: remember the pair - a promoter increases a catalyst's activity, a poison or inhibitor decreases it.

The correct answer is A, Carbon Dioxide. Dry ice is solid carbon dioxide. At ordinary pressure it does not melt into a liquid at all: it changes straight from solid to gas, a process called sublimation, at about minus 78.5 degrees Celsius. Because no liquid is left behind, the solid stays dry, which is how it got its name. It is used to keep ice cream, frozen food, vaccines and blood samples cold while they travel, and the thick white fog it makes on stage is only cold carbon dioxide gas condensing the moisture of the air. Option B is wrong because the oxides of nitrogen are gases, with nitrous oxide being the laughing gas used by dentists. Option C is wrong because oxidane is a systematic name for water, whose solid form is ordinary ice. Option D is wrong because sulphur dioxide is the pungent gas used as a preservative and bleach. Exam tip: dry ice is solid carbon dioxide and it sublimes, never melts.