Which of the following are equal for a chemical system at equilibrium?

The correct answer is B, 90 degrees. When the ray makes 90 degrees with the surface it is travelling along the normal, so the angle of incidence measured from the normal is zero and the ray goes straight on without bending. Refraction is the bending of light as its speed changes on entering a new medium, and by Snell's law the sine of the angle of incidence divided by the sine of the angle of refraction equals the refractive index; with an angle of incidence of zero both sines are zero and no bending is possible, though the speed and the wavelength still change. A is wrong because 120 degrees to the surface means the ray strikes at a slant and bends. C is wrong because at 45 degrees to the surface the angle of incidence is also 45 degrees, so the ray refracts. D is wrong because 0 degrees means the ray grazes along the surface and does not enter medium Y. Exam tip: along the normal means no bending; bending grows as the ray slants more.

The correct answer is A, Ranikhet. Newcastle disease is called Ranikhet disease in India because the first serious outbreak in the country was reported from Ranikhet in the Kumaon hills, while the name Newcastle comes from Newcastle upon Tyne in England, where it was identified in 1926. It is caused by a paramyxovirus, spreads fast through a flock by droppings and droplets, and shows itself as gasping, greenish diarrhoea, twisted neck and a sharp fall in egg laying, so losses can be heavy. There is no cure, and poultry keepers rely on vaccination with the F and R2B strains along with strict hygiene. B is wrong because IBD, infectious bursal disease or Gumboro, attacks the bursa and the young bird's immunity. C is wrong because coccidiosis is caused by Eimeria, a protozoan, and brings bloody droppings. D is wrong because fowl pox is a different viral disease, marked by warty scabs on the comb and wattles. Exam tip: Ranikhet equals Newcastle disease, a viral disease met by vaccination.

The correct answer is A, Goat. Capra hircus is the scientific name of the domestic goat, in which Capra is the genus and hircus the species, written in the two-word form that Carl Linnaeus gave to biology. The goat is a ruminant of the family Bovidae and is called the poor man's cow because it gives milk and meat on little feed and thrives in dry country; Rajasthan leads the states in goat population and Sirohi, Marwari, Jakhrana and Barbari are among its known breeds. B is wrong because cattle are Bos indicus, the humped Indian cattle, or Bos taurus. C is wrong because the sheep is Ovis aries. D is wrong because the Indian water buffalo is Bubalus bubalis, the animal that gives most of India's milk. Exam tip: fix the four names together, goat Capra hircus, sheep Ovis aries, cattle Bos indicus and buffalo Bubalus bubalis.

The correct answer is C, Bakelite. Bakelite is a thermosetting polymer, made by condensing phenol with formaldehyde, and once it has set under heat its chains are cross-linked, so it cannot be melted and moulded again. That hardness and poor conduction of heat and electricity are why bakelite is used for switches, plugs, handles of pans and radio cabinets; it was prepared by Leo Baekeland and was the first fully synthetic plastic. Melamine and urea-formaldehyde resins behave in the same way. A is wrong because polythene is a thermoplastic that softens on heating and can be recycled into new articles. B is wrong because PVC is also a thermoplastic, used for pipes, cable covers and raincoats. D is wrong because neoprene is a synthetic rubber, that is an elastomer, valued for resisting oil and petrol. Exam tip: thermosetting means bakelite, melamine and urea-formaldehyde; thermoplastic means polythene, PVC and polystyrene.

The correct answer is C, poisons. A catalytic poison is a substance that lowers or destroys the activity of a catalyst, usually because it sticks firmly to the active surface and leaves no room for the reactants. Carbon monoxide and sulphur compounds poison platinum and nickel catalysts, which is why petrol for vehicles with a catalytic converter must be low in sulphur, and arsenic compounds poison the iron catalyst used in making ammonia. Such poisoning is often irreversible, so industrial gases are cleaned before they reach the catalyst bed. A is wrong because controller is not a term used in catalysis. B is wrong because a promoter does the opposite and raises the activity of a catalyst, as molybdenum does for iron in the Haber process. D is wrong because an initiator starts a chain reaction, for example in polymerisation, and is used up in it. Exam tip: promoter raises, poison lowers; molybdenum promotes iron in the Haber process.