What is the main reason copper turns green over time?

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, 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.

The correct answer is C, Thallophyta. Litmus is a purple dye taken from lichens, and in the classification used in school science lichens are placed in Thallophyta, the group of simple plants whose body is an undifferentiated thallus with no true root, stem or leaf. Algae and fungi belong to the same group, and a lichen is in fact a partnership of an alga and a fungus living together. Litmus is a natural indicator: it stays purple in a neutral solution, turns red in acid and blue in base, and is used both as a solution and as litmus paper. Option A is wrong because gymnosperms are well developed plants with naked seeds, such as pine and cycas. Option B is wrong because pteridophytes, the ferns, are the first plants with vascular tissue. Option D is wrong because bryophytes such as moss and Marchantia are called the amphibians of the plant kingdom. Exam tip: litmus is red in acid, blue in base and purple when neutral.