General Science Previous Year Questions (PYQ) with Answers – Page 8

The correct answer is D, Argon. Argon is a noble gas whose outer shell already holds eight electrons, so it has no room to take another one easily; adding an electron would mean starting a new shell, and energy has to be supplied rather than released. Its electron affinity is therefore effectively zero, the lowest of the four. Electron affinity is the energy change when a neutral gaseous atom gains an electron, and it generally rises across a period and falls down a group. The other three are halogens, one electron short of a stable octet, so they release a lot of energy on gaining an electron. Option A is wrong because fluorine has a very high electron affinity, though its small size and crowded electrons push it just below chlorine. Option B is wrong because chlorine has the highest electron affinity of all elements. Option C is wrong because bromine's affinity, while lower than chlorine's, is still far above argon's. Exam tip: order is Cl greater than F greater than Br greater than I, and noble gases sit at the bottom.

The correct answer is C, kinetic energy. At the top the object holds only potential energy; as it falls, that store is steadily converted into kinetic energy, and just as it touches the ground almost all of it is kinetic. Potential energy is mgh and kinetic energy is half of mv squared, and in free fall under gravity the sum of the two stays constant, which is the law of conservation of mechanical energy. At the instant of touching the ground the height is zero, so the potential energy is zero, while the speed is greatest, so the kinetic energy is at its maximum. A is wrong because heat appears only after the impact, from friction and deformation, not at the instant of touching. B is wrong because chemical energy is stored in bonds and plays no part in a falling body. D is wrong because potential energy has already dropped to zero at ground level. Exam tip: in a falling body potential energy decreases, kinetic energy increases, and the total mechanical energy stays the same.

The correct answer is D, Fibrous roots. Fibrous roots are the mark of monocotyledonous plants such as wheat, maize, rice and grasses, not of dicots. A dicot seed carries two cotyledons, and its seedling sends down one strong primary root that thickens into a tap root with smaller lateral branches; gram, pea, mustard, neem and mango are familiar examples. Dicot leaves show reticulate or net-like venation, their flowers usually have petals in fours or fives, and their vascular bundles stand in a ring with cambium, so the stem can thicken year after year. A is wrong because the tap root is precisely what a dicot has. B is wrong because reticulate venation is a dicot feature, while monocots show parallel venation. C is wrong because two cotyledons is the very definition of a dicotyledonous plant. Exam tip: dicot - two cotyledons, tap root, reticulate venation; monocot - one cotyledon, fibrous roots, parallel venation.

The correct answer is D, 80 J. The work done in moving a charge across a potential difference is W = QV, so W = 5 × 16 = 80 joules. Potential difference is itself defined as the work done per unit charge in carrying that charge from one point to another, which is why one volt means one joule of work for every coulomb moved. Here five coulombs are carried across sixteen volts, so the source must supply 80 J of energy. A is wrong because 65 J does not follow from any correct use of 5 C and 16 V. B is wrong because 45 J would need a potential difference of 9 V for the same charge. C is wrong because 40 J would need either 2.5 C at 16 V or 5 C at 8 V. Exam tip: remember V = W/Q, so W = QV; in this chapter joules are simply coulombs multiplied by volts.

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, Nucleus. The nucleus is called the master or control centre of the cell because it directs all the activities of the cell. It holds the chromosomes, which carry DNA, the genetic instructions for making proteins and for passing traits from parents to children. It is wrapped in a double membrane called the nuclear envelope, whose pores let messages move in and out. Robert Brown discovered the nucleus in 1831. Mature red blood cells of mammals have no nucleus. A is wrong because mitochondria release energy as ATP and are called the powerhouse of the cell. B is wrong because the endoplasmic reticulum is a network of membranes that makes and carries proteins and fats. C is wrong because the nucleolus is a small body inside the nucleus that makes ribosomes, so it is only a part of the control centre. Exam tip: Nucleus = control centre, Mitochondria = powerhouse, Ribosome = protein factory, Lysosome = suicide bag, Golgi body = packaging unit.

The correct answer is B, Copper and Zinc. Brass is an alloy of copper and zinc, usually about 70 per cent copper and 30 per cent zinc. Adding zinc makes brass harder than pure copper, gives it a golden-yellow shine and helps it resist corrosion. That is why brass is used for utensils, door handles, taps, musical instruments like the trumpet, and cartridge cases. An alloy is a mixture of a metal with other metals or a non-metal, made to improve properties such as strength, hardness or resistance to rust. A is wrong because copper and tin make bronze, the alloy that gave its name to the Bronze Age. C is wrong because copper with aluminium gives aluminium bronze, not brass. D is wrong because copper and iron do not form a common alloy; iron with a little carbon makes steel. Exam tip: Brass = Cu + Zn, Bronze = Cu + Sn, Solder = Pb + Sn, Stainless steel = Fe + Cr + Ni + C, German silver = Cu + Zn + Ni.

The correct answer is B, Chlorine. Bleaching powder gives off chlorine when it reacts with water, and this chlorine does the bleaching and kills germs. Bleaching powder is calcium oxychloride, CaOCl2, made by passing chlorine gas over dry slaked lime, Ca(OH)2. With water it gives back calcium hydroxide and chlorine: CaOCl2 + H2O → Ca(OH)2 + Cl2. The chlorine it can release is called "available chlorine", which is why the powder smells of chlorine and is used to disinfect drinking water and swimming pools and to bleach cotton and paper. A is wrong because bleaching powder does not give off hydrogen gas. C is wrong because carbon monoxide comes from the incomplete burning of fuels. D is wrong because carbon dioxide is released when an acid reacts with a carbonate such as washing soda or baking soda. Exam tip: Bleaching powder = CaOCl2 (releases chlorine), Baking soda = NaHCO3, Washing soda = Na2CO3.10H2O, Plaster of Paris = CaSO4.½H2O.

The correct answer is C, Calcium Carbonate. Carbon dioxide reacts with lime water to form calcium carbonate, which does not dissolve, and its fine white particles make the water look milky. Lime water is a clear solution of calcium hydroxide, Ca(OH)2, and the reaction is Ca(OH)2 + CO2 → CaCO3 + H2O. This is the standard test for carbon dioxide, for example to show that the air we breathe out contains CO2. If CO2 is passed for a long time, the milkiness disappears because soluble calcium hydrogen carbonate forms. A is wrong because lime water contains no barium, so barium carbonate cannot form. B is wrong because calcium oxide (quicklime) is made by heating limestone, not by bubbling CO2. D is wrong because calcium hydrogen carbonate forms only with excess CO2, and since it dissolves in water it clears the milkiness instead of causing it. Exam tip: CO2 in lime water = milky (CaCO3); excess CO2 = clear again (Ca(HCO3)2); limestone, chalk and marble are all CaCO3.

The correct answer is D, Vitamin K. Vitamin K is needed to make the clotting proteins, so without it even a small cut keeps bleeding. The liver uses vitamin K to make prothrombin and other clotting factors that turn blood into a clot at a wound. The letter K comes from the German word "Koagulation", meaning clotting. Green leafy vegetables such as spinach and cabbage are rich sources, and bacteria in the large intestine also make some of it. Newborn babies are usually given a vitamin K injection because they are born with very little. A is wrong because lack of the vitamin B group causes diseases like beriberi (B1) and pellagra (B3). B is wrong because lack of vitamin A causes night blindness. C is wrong because lack of vitamin C causes scurvy with bleeding gums, but the vitamin that makes blood clot is K. Exam tip: A = night blindness, B1 = beriberi, C = scurvy, D = rickets, K = bleeding; vitamins A, D, E and K dissolve in fat.

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 A, 3 to 12. The d-block occupies the middle of the modern periodic table, groups 3 to 12, and its members are also called transition elements. In these elements the last electron enters a d orbital of the second-last shell, which is why they show variable valency, form coloured compounds and work as good catalysts. Iron, copper, zinc, silver, gold and mercury all belong here. The block has four series, filling the 3d, 4d, 5d and 6d orbitals, one in each of periods 4 to 7. B is wrong because groups 1 and 2 are the s-block, the alkali and alkaline earth metals. C is wrong because groups 13 to 18 form the p-block, which holds most non-metals and the noble gases. D is wrong because the d-block does not stop at group 10; zinc, cadmium and mercury in group 12 are part of it. Exam tip: s-block = groups 1 and 2, d-block = groups 3 to 12, p-block = groups 13 to 18, f-block = lanthanides and actinides placed below the table.

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 C, 27 K. A rise of 27°C is a rise of 27 K, because one degree Celsius and one kelvin are exactly the same size. The two scales differ only in where they begin: the melting point of ice is 0°C but 273.15 K. So the formula T(K) = t(°C) + 273.15 converts a reading, not a difference. When two temperatures are subtracted the 273.15 cancels out, which is why a rise, a fall or an interval carries the same number on both scales. Option D, 300 K, is the trap in this question: 27 + 273 is the Kelvin reading of a body that is at 27°C, not the size of a 27-degree rise. Options A and B follow from no step at all; they would need a scale whose divisions are smaller than the Celsius degree, and the kelvin is not such a unit. Exam tip: add 273.15 to a reading, but copy an interval straight across.