What is the chemical formula for the compound commonly known as 'laughing gas'?

The correct answer is A, Night blindness. Vitamin A, chemically retinol, is needed to make rhodopsin, the light-sensitive pigment in the rod cells of the retina that lets us see in dim light. When the vitamin is short, the eye cannot adjust to darkness, and the person cannot see properly at night or in poor light; this is night blindness (nyctalopia). If the deficiency continues, the eye becomes dry and the cornea is damaged, a condition called xerophthalmia, which can end in total blindness. Vitamin A is fat-soluble and comes from carrots, papaya, mango, green leafy vegetables, milk, eggs and fish liver oil. B is wrong because scurvy, with bleeding gums, is caused by lack of vitamin C. C is wrong because marasmus is caused by a severe shortage of both calories and protein in young children. D is wrong because kwashiorkor is a protein deficiency disease marked by a swollen belly. Exam tip: vitamin A night blindness, C scurvy, D rickets, K bleeding; marasmus and kwashiorkor are protein-energy problems, not vitamin diseases.

The correct answer is B, SF6. The name itself gives the formula: 'hexa' means six, so sulphur hexafluoride has one sulphur atom joined to six fluorine atoms, SF6. The molecule is octahedral in shape, with the six fluorine atoms arranged symmetrically around the sulphur. SF6 is a colourless, odourless, non-flammable and very unreactive gas, about five times heavier than air. Because it does not conduct electricity, it is used as an insulating gas inside high-voltage circuit breakers and switchgear. It is also one of the strongest greenhouse gases known, with a global warming potential thousands of times that of carbon dioxide, and it is listed under the Kyoto Protocol. A is wrong because S2F6 would need two sulphur atoms, which the name does not say. C is wrong because SF2 is sulphur difluoride, with only two fluorine atoms. D is wrong because S6F reverses the numbers and is not a real compound. Exam tip: mono 1, di 2, tri 3, tetra 4, penta 5, hexa 6; SF6 = one sulphur, six fluorine, used in electrical switchgear.

The correct answer is A, B3. Pellagra is caused by a lack of vitamin B3, also called niacin or nicotinic acid. The body can also make niacin from the amino acid tryptophan, so the disease is common where people live mainly on maize, which is poor in both. Pellagra is remembered by the 'four Ds': dermatitis (rough, dark skin on parts exposed to the sun), diarrhoea, dementia and, if untreated, death. Niacin is found in meat, fish, groundnuts, whole grains and milk. B is wrong because vitamin B6 (pyridoxine) deficiency causes anaemia, skin problems and convulsions in infants. C is wrong because vitamin B2 (riboflavin) deficiency causes cracked lips and sores at the corners of the mouth, called ariboflavinosis. D is wrong because vitamin B12 (cobalamin) deficiency causes pernicious anaemia and nerve damage. Exam tip: B1 beriberi, B2 ariboflavinosis, B3 pellagra, B12 pernicious anaemia; pellagra = the four Ds.

The correct answer is B, Pellagra. Pellagra is caused by a lack of vitamin B3, niacin, also called nicotinic acid, and is remembered by the three Ds: dermatitis, a rough dark rash on skin exposed to sunlight, diarrhoea from the gut, and dementia from the nervous system; untreated, a fourth D, death, follows. It was common where maize was the staple food, because the niacin in maize is bound and hard to absorb and maize is poor in tryptophan, the amino acid from which the body makes niacin. Niacin is found in meat, fish, groundnuts, whole grains and pulses. Option A is wrong because anaemia comes from a shortage of iron, vitamin B12 or folic acid, not B3. Option C is wrong because beriberi is the deficiency disease of vitamin B1, thiamine, affecting nerves and the heart. Option D is wrong because scurvy, with bleeding gums, is caused by a lack of vitamin C, ascorbic acid. Exam tip: B1 = beriberi, B2 = cheilosis, B3 = pellagra, B12 = pernicious anaemia, C = scurvy, D = rickets.

The correct answer is D, [M L T⁻²]. Newton's second law gives force = mass × acceleration. Mass has the dimension [M], and acceleration is velocity divided by time, that is [L T⁻¹] ÷ [T] = [L T⁻²]. Multiplying, force = [M L T⁻²]. Its SI unit is the newton, 1 N = 1 kg m s⁻², and its CGS unit is the dyne, with 1 N = 10⁵ dyne. Weight, tension, thrust and friction are all forces, so they share the same dimensional formula, which is a favourite exam trick. Option A is wrong because [M⁰ L³ T⁰] is simply [L³], the dimension of volume. Option B is wrong because [M L⁻³ T⁰] is mass per unit volume, that is density. Option C is wrong because [M⁰ L T⁻¹] is length per unit time, the dimension of speed or velocity. Exam tip: build from the definition: velocity [L T⁻¹], acceleration [L T⁻²], force [M L T⁻²], work and energy [M L² T⁻²], power [M L² T⁻³], pressure [M L⁻¹ T⁻²].