Which of the following is the chemical symbol of Curium?

The correct answer is A, Milk. Milk is an emulsion because it is tiny drops of liquid fat spread through water, and an emulsion is a colloid in which one liquid is dispersed in another liquid. The fat droplets in milk do not join together because casein, a milk protein, acts as the emulsifying agent and coats each drop. Butter, cream, mayonnaise and cold creams are other everyday emulsions; in butter and cream the arrangement is reversed, with water dispersed in fat. B is wrong because mist is tiny water drops spread in air, which makes it an aerosol and not an emulsion. C is wrong because a coloured gemstone is a solid dispersed in a solid, so it is a solid sol. D is wrong because clouds are also water drops in a gas and belong with aerosols. Exam tip: liquid in liquid is an emulsion as in milk, liquid in gas is an aerosol as in cloud and mist, and solid in liquid is a sol.

The correct answer is B, Nitrogen. An ordinary incandescent bulb is filled with an unreactive gas, nitrogen usually mixed with argon, so that the white-hot tungsten filament does not burn away. The filament works at about 2500 degrees Celsius, and in air the tungsten would oxidise within seconds. Nitrogen and argon do not react with tungsten, and the gas pressure inside also slows the evaporation of the metal, so the filament lasts longer and the glass does not blacken quickly. A is wrong because halogens such as iodine and bromine are added only in halogen lamps, a special high-temperature type, not in the common bulb. C is wrong because carbon dioxide would release oxygen at that temperature and attack the filament. D is wrong because hydrogen is highly inflammable and would make the bulb dangerous. Exam tip: filament is tungsten, melting point about 3400 degrees Celsius, and the bulb is filled with nitrogen and argon.

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.