General Science Mixed Quiz: Set 4

The correct answer is A, luminous intensity, that is the brightness of a source of light in a given direction. The candela takes its name from the candle, which was once used as a rough standard of light, and it is one of the seven base units of the SI system.

Option B, electric current, is measured in ampere, named after the French scientist Andre-Marie Ampere. Option C, amount of substance, is measured in mole, which is the chemist's counting unit for atoms and molecules. Option D, pressure, is measured in pascal, a derived unit equal to one newton per square metre. Two related units are worth knowing alongside the candela: the lumen for luminous flux and the lux for illuminance, that is the light falling on a surface.

The correct answer is D, Four. In diamond every carbon atom is joined to four other carbon atoms by strong covalent bonds. These bonds point towards the corners of a tetrahedron and build a rigid three-dimensional network that runs through the whole crystal. That network makes diamond the hardest natural substance, which is why it is used in cutting and drilling tools. Since all four valence electrons are used in bonding, no free electrons are left, so diamond does not conduct electricity. In graphite, the other common form of carbon, each atom bonds to only three others in flat hexagonal layers, and the spare electron lets graphite conduct. A is wrong because two bonds per carbon cannot build a solid network. B is wrong because carbon has a valency of four and cannot form five bonds. C is wrong because three bonds per atom describes graphite, not diamond. Exam tip: diamond = 4 bonds, hard, insulator; graphite = 3 bonds, soft, conductor.

The correct answer is D, It has no unit. Magnification is a ratio of two lengths, so the units cancel and it is a pure number. For a mirror or a lens, magnification m equals the height of the image divided by the height of the object (h′/h). For a lens it also equals v/u, the image distance divided by the object distance, and for a mirror it equals −v/u. A value of 2 means the image is twice the size of the object; a value less than 1 means it is smaller. With the usual sign convention a negative m means a real, inverted image, and a positive m means a virtual, erect image. A is wrong because the dioptre is the unit of the power of a lens, P = 1/f with f in metres. B is wrong because the newton is the SI unit of force. C is wrong because the centimetre measures lengths such as focal length and image distance, not their ratio. Exam tip: magnification = no unit; power of a lens = dioptre; focal length = metre or centimetre.

The correct answer is C, more than the atmospheric pressure. Air always moves from a region of higher pressure to one of lower pressure, so it flows out of the lungs only when the pressure inside them rises above that of the outside air. During expiration the diaphragm relaxes and moves up into its dome shape, and the intercostal muscles between the ribs relax, so the ribs move down and inward. This reduces the volume of the chest cavity, which raises the pressure in the lungs and pushes the air out. Inspiration is the reverse: the diaphragm contracts and flattens, the chest cavity grows, lung pressure falls below atmospheric pressure and air rushes in. Option A is wrong because when the pressures are equal no air moves; this happens for a moment between breaths. Option B is wrong because lower pressure inside the lungs draws air in, which is inspiration. Option D is wrong because contraction of the diaphragm enlarges the chest and causes inspiration, not expiration. Exam tip: inspiration means the diaphragm contracts and lung pressure falls; expiration means it relaxes and lung pressure rises.

The correct answer is D, Heating effect. An iron, toaster, oven, kettle and heater all turn electrical energy into heat when current flows through a coil of high resistance. This is explained by Joule's law of heating: heat produced H = I²Rt, so the heat grows with the square of the current, with the resistance and with the time. The heating coils are made of nichrome, an alloy of nickel and chromium, because it has high resistance and does not melt or oxidise easily when red hot. The electric fuse and the filament bulb also work on this effect. A is wrong because the chemical effect of current is used in electroplating and in charging batteries. B is wrong because the magnetic effect runs electric bells, fans, motors and electromagnets. C is wrong because the physiological effect is the shock or muscle twitch that current causes in a living body. Exam tip: H = I²Rt; heater coil = nichrome, bulb filament = tungsten, fuse wire = tin-lead alloy with a low melting point.

The correct answer is C, Only (b). Of the three, only citrus canker is caused by bacteria; rust of wheat is caused by a fungus and yellow vein mosaic of bhindi by a virus. Citrus canker produces raised, corky brown spots on the leaves, twigs and fruits of lemon, orange and other citrus plants, and it spreads through the air. Rust of wheat forms reddish-brown powdery patches on the leaves and stems, which is how it got its name. Yellow vein mosaic turns the veins of bhindi, or okra, leaves yellow, and insects carry the virus from plant to plant. NCERT's Class 8 science book gives exactly these three as its examples of bacterial, fungal and viral diseases of plants. A is wrong because yellow vein mosaic is a viral disease, not a bacterial one. B is wrong because it adds rust of wheat, which is a fungal disease. D is wrong because it adds the viral yellow vein mosaic. Exam tip: citrus canker, bacteria; rust of wheat, fungus; yellow vein mosaic of bhindi, virus.

The correct answer is B, Potassium – Po. The symbol of potassium is K, from its Latin name kalium; Po is the symbol of polonium, the radioactive element discovered by Marie Curie in 1898 and named after Poland. Potassium has atomic number 19, sits in group 1 as an alkali metal, and is soft enough to cut with a knife. Several common elements carry symbols from Latin names rather than English ones, and examiners love them: sodium Na (natrium), iron Fe (ferrum), copper Cu (cuprum), silver Ag (argentum), gold Au (aurum), tin Sn (stannum), lead Pb (plumbum), mercury Hg (hydrargyrum) and tungsten W (wolfram). A is a correct pair because phosphorus is P, atomic number 15. C is a correct pair because iron is Fe from ferrum, atomic number 26. D is a correct pair because iodine is I, atomic number 53, a halogen. Exam tip: K is potassium and Po is polonium; when a symbol looks like the first two letters of the English name, check whether the element actually uses a Latin name.

The correct answer is C, Abscisic acid. It is the main growth inhibitor among plant hormones and works against the growth promoters. Abscisic acid, or ABA, is called the stress hormone because it helps plants survive drought, cold and salt: it makes the stomata close to save water, keeps seeds and buds dormant so they do not sprout in bad weather, and helps leaves and fruits to age and fall. So when a question asks which hormone slows or stops growth, ABA is the answer; ethylene is the other hormone linked with ageing and ripening. A is wrong, because auxins promote growth by making cells elongate, and they make shoots bend towards light. B is wrong, because cytokinins promote cell division and delay the ageing of leaves. D is wrong, because gibberellins promote stem elongation, break seed dormancy and are sprayed on grapes to increase their size. Exam tip: the growth promoters are auxin, gibberellin and cytokinin; the inhibitor is abscisic acid, the stress hormone.

The correct answer is C, Nitrogen. Nitrogen is an unreactive gas, so it drives the oxygen out of the packet and stops the oil in the chips from getting oxidised. Rancidity is the oxidation of fats and oils in food, which spoils their smell and taste. Chips are fried in oil, so they turn rancid quickly once they meet air. Filling the pack with nitrogen keeps oxygen out and also acts as a cushion that stops the chips from breaking in transport. Other ways to slow rancidity are adding antioxidants, storing food in airtight containers and keeping it in a refrigerator. Option A is wrong because carbon dioxide is used for fizzy drinks and some other foods, but the standard gas for chips is nitrogen, which is cheap and very unreactive. Option B is wrong because hydrogen is highly inflammable and unsafe inside food packs. Option D is wrong because oxygen is the very gas that causes rancidity in the first place. Exam tip: rancidity is oxidation of fats; nitrogen flushing and antioxidants prevent it.

The correct answer is D, Sickle cell anaemia, Haemophilia, Down syndrome. All three are genetic disorders, caused by changes in genes or chromosomes and passed on through birth, not by germs. Sickle cell anaemia is an autosomal recessive disease in which a single change in the haemoglobin gene makes red blood cells sickle-shaped. Haemophilia is a sex-linked recessive disorder carried on the X chromosome, in which blood does not clot properly; it mostly affects males, and it is called the royal disease because it spread among Queen Victoria's descendants. Down syndrome is a chromosomal disorder caused by an extra copy of chromosome 21, also called trisomy 21. Option A is wrong because AIDS is caused by the HIV virus and is not inherited through genes. Option B is wrong because AIDS and Hepatitis B are viral infections, and most cancers come from mutations acquired during life. Option C is wrong because Hepatitis B is a viral infection, even though haemophilia and colour blindness are genetic. Exam tip: genetic disorders include sickle cell anaemia, haemophilia, colour blindness, thalassaemia and Down syndrome.

The correct answer is B, Na₂CO₃·10H₂O. Washing soda is sodium carbonate decahydrate: each unit of sodium carbonate holds ten molecules of water of crystallisation. It is made from common salt in steps: baking soda is heated to get anhydrous sodium carbonate, called soda ash, which is then recrystallised from water to give washing soda crystals. It is a basic salt used for cleaning, in the glass, soap and paper industries, and to remove the permanent hardness of water. Paper setters like to put these related compounds side by side as options, so learn them together. A is wrong because NaHCO₃ is baking soda (sodium hydrogencarbonate), used in cooking and in antacids. C is wrong because Na₂CO₃ without water is soda ash, not the crystals called washing soda. D is wrong because NaCl is common salt, the starting material for all of these. Exam tip: NaCl → NaHCO₃ (baking soda) → Na₂CO₃ (soda ash) → Na₂CO₃·10H₂O (washing soda).

The correct answer is A, The pressure in blood vessels exceeds outside pressure. At great heights the air is thin, so the atmospheric pressure acting on the body is much lower than at sea level. The blood inside our vessels keeps pushing outwards at nearly its usual pressure. In the nose the blood vessels are very fine and lie close to the surface, so when the pressure inside them becomes greater than the pressure outside, some of them burst and the nose bleeds. The cold, dry mountain air also makes the lining of the nose crack more easily. The same fall in air pressure is why water boils below 100 °C on mountains and food takes longer to cook. B is wrong because the pressure inside the vessels becomes more than, not less than, the outside pressure. C is wrong because equal pressures would give no push to burst the vessels. D is wrong because blood does not become thick enough at a height to cause nose bleeding. Exam tip: higher altitude, lower air pressure, so the pressure in blood vessels is relatively higher and the nose can bleed.

The correct answer is A, Tin and copper. Bronze is made by melting copper and tin together, with copper as the main metal. It is harder than pure copper and does not corrode easily, so it has been used for statues, bells, medals and utensils for thousands of years. The Harappans cast the famous bronze figure of the 'dancing girl', and the Chola rulers of south India are known for their bronze idols such as Nataraja. An alloy is a mixture of a metal with other metals or with a non-metal, made to improve qualities like hardness, strength and resistance to rust. B is wrong because copper and zinc make brass, not bronze. C is wrong because aluminium and zinc do not make bronze; alloys of aluminium such as duralumin are used in aircraft. D is wrong because zinc and tin without copper do not form bronze. Exam tip: bronze, copper plus tin; brass, copper plus zinc; steel, iron plus carbon; stainless steel adds chromium and nickel.

The correct answer is C, 1 and 3 only. The cooking temperature is set by the pressure inside the cooker, and that pressure is fixed by the weight sitting on the vent and the area of the hole under it. The boiling point of water rises with pressure. In an open pan water boils at 100°C, but when steam is trapped, pressure builds up until it can lift the weight, the whistle, off the vent. Pressure is force divided by area, so a heavier weight or a smaller hole means a higher pressure before steam escapes, and water then boils at about 120°C, which is why food cooks faster. Statement 2 is wrong because a bigger flame only makes the water boil and the whistle blow sooner; once steam escapes, the temperature stays at the boiling point for that pressure, and extra heat just makes more steam. Statements 1 and 3 are right for the reason above. So options A, B and D, which include statement 2, are wrong. Exam tip: higher pressure means a higher boiling point; the flame changes the speed, not the temperature.

The correct answer is A, Glass powder and red phosphorus. The rough strip of a safety matchbox is coated with powdered glass and a little red phosphorus, while the head of the matchstick carries antimony trisulphide and potassium chlorate. When the match is struck, friction turns some red phosphorus into white phosphorus. This reacts at once with the potassium chlorate in the head and produces enough heat to set the antimony trisulphide burning. Keeping the phosphorus on the box and the other chemicals on the stick is what makes the match safe. NCERT's Class 8 science book adds that early matches used white phosphorus, which proved dangerous for the workers who made them and for users. B is wrong because white phosphorus is not used on the rubbing surface of a safety match. C is wrong because magnesium is not part of the striking strip. D is wrong because neither potassium nor white phosphorus is found on the strip. Exam tip: box strip, red phosphorus plus glass powder; match head, antimony trisulphide plus potassium chlorate.

The correct answer is A, 20 Hz – 20000 Hz. A healthy human ear can hear sound waves with frequencies from about 20 hertz to 20,000 hertz (20 kHz); this band is called the audible range. Sounds below 20 Hz are called infrasonic, and animals such as elephants and whales use them to communicate over long distances. Sounds above 20 kHz are called ultrasonic; bats, dogs and dolphins can hear them, and ultrasound is used in medical scans, in cleaning delicate parts and in SONAR. Children under five and some animals can hear up to about 25 kHz, and the upper limit falls as people grow older. Option B is wrong because 80 to 100 Hz is only a tiny slice of the audible range. Option C is wrong because 2 lakh to 4 lakh Hz, that is 200 to 400 kHz, lies far inside the ultrasonic range. Option D is wrong because 0 to 20 Hz is the infrasonic range, which humans cannot hear. Exam tip: infrasonic below 20 Hz, audible from 20 Hz to 20 kHz, ultrasonic above 20 kHz.

The correct answer is D, Golgi apparatus. The Golgi apparatus is the main site where glycoproteins and glycolipids are formed. It is a stack of flat, disc-shaped sacs called cisternae, first described by Camillo Golgi in 1898. Proteins made on the endoplasmic reticulum reach the Golgi, where sugars are added to them, and they are then packed into vesicles and sent to places inside the cell or out of it. That is why the Golgi is called the packaging and dispatch centre of the cell, and it also helps to form lysosomes. Option A is wrong because plastids, such as chloroplasts, carry out photosynthesis or store food and pigments in plant cells. Option B is wrong because lysosomes are small sacs of digestive enzymes and are called the suicide bags of the cell. Option C is wrong because mitochondria make energy in the form of ATP and are called the powerhouse of the cell. Exam tip: Golgi means packaging and adding sugars; mitochondria mean ATP; a lysosome is the suicide bag.

The correct answer is D, Shape - Amoeboid or circular. A red blood cell is a biconcave disc, thin in the middle and thick at the edge, not an amoeboid cell. This shape gives it more surface area to pick up oxygen and lets it bend through narrow capillaries. Mature human RBCs have no nucleus, are about 7.5 micrometres wide, live for about 120 days and are made in the red bone marrow. Their red colour comes from haemoglobin, which carries oxygen from the lungs to the tissues and part of the carbon dioxide back. A is wrong as a choice because 7.5 μm really is the size of an RBC. B is wrong because carrying oxygen and carbon dioxide is exactly what RBCs do. C is wrong because a haemocytometer is indeed the counting chamber used to count blood cells under a microscope. Exam tip: RBC = biconcave, no nucleus, 120 days; WBC = amoeboid, has a nucleus; the amoeboid shape belongs to white cells, not red.

The correct answer is D, Medulla. The medulla oblongata is the breathing centre of the brain, which is why we keep breathing even while asleep. It is the lowest part of the hindbrain and joins the brain to the spinal cord. It controls involuntary actions such as breathing, heartbeat, blood pressure, swallowing, coughing, sneezing and vomiting. It senses the level of carbon dioxide in the blood and speeds up or slows down breathing to match, so an injury at the base of the skull can be fatal. A is wrong because the forebrain (cerebrum) handles thinking, memory, speech and voluntary actions. B is wrong because the cerebellum keeps the body balanced and makes voluntary movements smooth and precise. C is wrong because the midbrain mainly carries reflexes linked to seeing and hearing. Exam tip: Medulla = breathing and heartbeat, Cerebellum = balance, Cerebrum = thinking and memory, Hypothalamus = body temperature, hunger and thirst.

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.