What is the botanical name of opium?

The correct answer is A, Propanal. The ending 'al' marks an aldehyde, so propanal is the three-carbon aldehyde, CH3CH2CHO.

An aldehyde carries the group -CHO at the end of the carbon chain, with the carbonyl carbon attached to a hydrogen. In the naming system of organic chemistry the suffix tells the family: 'al' for an aldehyde, 'ol' for an alcohol, 'one' for a ketone, 'oic acid' for a carboxylic acid, 'ane' for an alkane, 'ene' for an alkene and 'yne' for an alkyne. Methanal and ethanal are the first two aldehydes of the series.

B is wrong: propanol ends in 'ol', so it is an alcohol with an -OH group. C is wrong: propyne ends in 'yne' and is an alkyne with a triple bond. D is wrong: propanone ends in 'one' and is a ketone, better known as acetone.

Exam tip: al for aldehyde, ol for alcohol, one for ketone, oic acid for carboxylic acid.

The correct answer is A, The acid present in tomato - Formic acid. This pair is wrong, because tomato contains oxalic acid, along with some citric and malic acid.

Formic acid, or methanoic acid, is the acid of an ant sting and a bee sting, and its name comes from the Latin word for ant. Oxalic acid is found in tomato, spinach and other leafy vegetables. Learning the natural acids as pairs is the quickest way to handle such questions: lemon and orange give citric acid, curd gives lactic acid, vinegar gives acetic acid, apple gives malic acid and tamarind gives tartaric acid.

B is a correct match, so it is not the answer: orange does contain citric acid. C is correct as well, because grapes and tamarind contain tartaric acid. D is also correct, since rancid butter smells the way it does because of butyric acid.

Exam tip: Tomato - oxalic acid; ant sting - formic acid; do not mix the two.

The correct answer is A, carbon dioxide. Urea is made industrially by reacting ammonia with carbon dioxide under high pressure.

The process runs in two steps: ammonia and carbon dioxide first form ammonium carbamate, which then loses a water molecule to give urea. Its formula is CO(NH2)2 and it carries about 46 per cent nitrogen, the highest of the common solid fertilisers, which is why it is the most used nitrogenous fertiliser in India. The ammonia itself comes from the Haber-Bosch process. Urea also has a place in the history of chemistry as the first organic compound synthesised in a laboratory, by Friedrich Wohler in 1828.

B is wrong: ammonia is itself made from nitrogen and hydrogen, and adding hydrogen gives no urea. C is wrong: ammonia with oxygen gives nitric oxide, a step towards nitric acid. D is wrong: ammonia with sulphuric acid gives ammonium sulphate, a different fertiliser.

Exam tip: Urea - ammonia plus carbon dioxide, formula CO(NH2)2, about 46 per cent nitrogen.

The correct answer is C, Two. A normal umbilical cord contains two arteries and one vein, three blood vessels in all.

The umbilical cord joins the developing foetus to the placenta. The two umbilical arteries carry deoxygenated blood, carbon dioxide and wastes from the foetus to the placenta, while the single umbilical vein brings oxygen-rich blood and nutrients from the placenta back to the foetus. This is the reverse of the usual pattern in the body, where arteries carry oxygenated blood. The vessels are cushioned by a jelly-like tissue called Wharton's jelly, and a mature cord is about 50 to 55 cm long.

A is wrong: three is the total number of vessels, not of arteries. B is wrong: one is the number of umbilical veins. D is wrong: four vessels are never present in a normal cord.

Exam tip: Umbilical cord - two arteries plus one vein; the vein is the one carrying oxygenated blood to the foetus.

The correct answer is B, about 17 metres. The sensation of a sound persists in the human brain for roughly one tenth of a second, so a reflected sound must arrive at least that much after the original to be heard as a separate echo. In that tenth of a second sound in air covers about 34 metres, but the sound must travel to the surface and back, so the surface itself must be at least half of that away, roughly 17 metres.

Option A, 1.7 metres, is ten times too small and would give a reflection arriving in about one hundredth of a second, far too soon to be heard separately. Option C, 34 metres, is the total path length travelled by the sound rather than the distance to the wall, and is the most common error in this question. Option D, 170 metres, is ten times too large; an echo is heard well before the surface is that far away.