General Science Mixed Quiz: Set 2

The correct answer is C, seven. The SI system, adopted in 1960 by the General Conference on Weights and Measures, rests on seven base units: the metre, the kilogram, the second, the ampere, the kelvin, the mole and the candela. Every other unit in science is derived from these by multiplication and division.

Options A and B fall short of the list and usually come from forgetting the mole or the candela, which are the two least familiar. Option D, nine, comes from adding the radian and the steradian, the units of plane angle and solid angle; these were long treated as a separate class called supplementary units and are not counted among the base units. A safe way to remember the seven is by their quantities: length, mass, time, current, temperature, amount of substance and luminous intensity.

The correct answer is B, newton. One newton is the force that gives a mass of one kilogram an acceleration of one metre per second squared, which follows directly from the equation force equals mass times acceleration. Its dimensional formula is therefore M L T⁻².

Option A, the joule, is the unit of work and energy: one joule is the work done when a force of one newton moves its point of application through one metre. Option C, the watt, is the unit of power, that is the rate of doing work, and equals one joule per second. Option D, the pascal, is the unit of pressure and equals one newton per square metre. All four are derived units named after scientists, and papers often list them together to see whether you can match each with its quantity.

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, 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 C, erect, virtual and larger than the object. For a lens, magnification m is the image height divided by the object height, which also equals v divided by u. A plus sign means the image is erect and virtual, and a value of 2 means it is twice as tall as the object. Only a convex lens can form such an image, when the object is placed between the optical centre and the principal focus; this is exactly how a magnifying glass or a simple microscope works. A concave lens also forms erect, virtual images, but they are always smaller, so its magnification stays between 0 and plus 1. A is wrong, because an erect, virtual, smaller image needs m between 0 and 1, as with a concave lens. B is wrong, because a real, inverted image always has a negative magnification. D is wrong, because a real, inverted, enlarged image would have m below minus 1, such as minus 2. Exam tip: the sign tells the type, plus for virtual and erect, minus for real and inverted; a value above 1 means enlarged.

The correct answer is C, 277. Water is densest at about 4 degrees Celsius, and 4 plus 273 gives roughly 277 kelvin. Most liquids keep shrinking as they cool, but water behaves oddly: below 4 degrees Celsius it starts to expand again because its molecules begin forming the open, hexagonal pattern of ice. This anomalous expansion is why ice is lighter than water and floats, and why a lake freezes from the top while the 4-degree water sinks to the bottom, letting fish survive the winter. Its density at that point is very close to 1 gram per cubic centimetre. D is wrong because 273 kelvin is 0 degrees Celsius, the freezing point, where water is already less dense than at 4 degrees. A is wrong because 0 kelvin is absolute zero, where no liquid water can exist. B is wrong because 227 kelvin is about minus 46 degrees Celsius, far below freezing. Exam tip: maximum density of water at 4 degrees Celsius equals 277 K; freezing at 273 K, boiling at 373 K.

The correct answer is B, 1831. In 1831 the British scientist Michael Faraday showed that a changing magnetic field produces an electric current in a nearby coil. He moved a magnet in and out of a coil of wire and saw a galvanometer needle move, and he also found that switching a current on and off in one coil induced a current in a second coil wound on the same iron ring. This is electromagnetic induction, the working principle of the electric generator (dynamo) and the transformer. The SI unit of capacitance, the farad, is named after him. A is wrong because 1853 is more than twenty years after the discovery. C is wrong because 1820 is the year Hans Christian Oersted found that an electric current produces a magnetic field, the reverse effect. D is wrong because 1875 has no link with this discovery. Exam tip: Oersted 1820, current makes magnetism; Faraday 1831, changing magnetism makes current.

The correct answer is C, refraction of starlight. Starlight has to cross many layers of air before it reaches the eye, and these layers keep changing in temperature and density, so their refractive index keeps changing too. The light is bent a little differently every moment, the apparent position of the star shifts slightly and the amount of light entering the eye rises and falls. This flickering is what we call twinkling. A star is so far away that it acts as a point source, so the small shifts are easily seen. Planets are nearer and appear as tiny discs; light from the many points of the disc averages out, so planets do not twinkle. A, Diffraction, is the bending of light around edges and is not the cause. B, Reflection, sends light back from a surface and cannot make a star flicker. D, Scattering, of sunlight by air makes the sky blue but does not cause twinkling. Exam tip: twinkling and the early sunrise both come from atmospheric refraction; the blue sky and the red sunset come from scattering.

The correct answer is C, 82. Lead has 82 protons in its nucleus, so its atomic number is 82. Its symbol is Pb, from the Latin plumbum, the root of the word "plumber", because Roman water pipes were made of lead. It sits in group 14 and period 6 of the periodic table, below carbon, silicon, germanium and tin, and it is the heaviest element that has stable isotopes; the decay chains of uranium and thorium end at lead. Lead is soft, dense and poisonous, which is why it has been removed from petrol and paint, and it is used in car batteries and radiation shields. The four options are neighbours in the table, so the exam is testing the exact number. A, 81, is thallium (Tl). B, 78, is platinum (Pt). D, 79, is gold (Au). Exam tip: the run of heavy metals goes 78 platinum, 79 gold, 80 mercury, 81 thallium, 82 lead.

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 B, 14 : 3. Ammonia is NH3, so one nitrogen atom of mass 14 is joined to three hydrogen atoms of mass 1 each, giving 14 : 3 by mass. This is the law of constant proportions given by Proust in 1799: a pure compound always contains the same elements in the same ratio by mass, whatever its source. The NCERT Class 9 chapter uses exactly these examples, water H2O with hydrogen to oxygen 1 : 8 and ammonia NH3 with nitrogen to hydrogen 14 : 3. Remember that the ratio by mass is not the ratio by atoms; by atoms ammonia is 1 : 3, by mass it is 14 : 3 because nitrogen is fourteen times heavier than hydrogen. A is wrong because 11 is not the atomic mass of nitrogen. C is wrong for the same reason, even though the hydrogen part is right. D is wrong because it counts only one hydrogen atom instead of three. Exam tip: water 1 : 8, ammonia 14 : 3, carbon dioxide 3 : 8, all by mass.

The correct answer is D, Gustav Robert Kirchhoff. This is Kirchhoff's first law, the current law or junction rule, which he stated in 1845. It says the total current entering a junction equals the total current leaving it, because charge cannot pile up or vanish at a point. So it is really the law of conservation of charge applied to a circuit node. Kirchhoff's second law, the voltage law or loop rule, says the algebraic sum of potential differences around any closed loop is zero, which follows from conservation of energy. Together the two laws let us solve circuits that Ohm's law alone cannot handle. A is wrong because Faraday's laws deal with electromagnetic induction and electrolysis, not junctions. B is wrong because Ohm's law relates voltage, current and resistance in a single conductor (V = IR). C is wrong because Victor Hess discovered cosmic rays and won the 1936 Nobel Prize in Physics. Exam tip: Kirchhoff current law = charge conserved at a junction; Kirchhoff voltage law = energy conserved around a loop.

The correct answer is B, milk of magnesia. Milk of magnesia is a milky white suspension of magnesium hydroxide, Mg(OH)2, in water. It is a mild base, so it neutralises the extra hydrochloric acid in the stomach that causes acidity and heartburn; the reaction gives magnesium chloride and water. In larger doses it draws water into the intestine and works as a laxative. This antacid is a textbook example of neutralisation, where an acid and a base react to form a salt and water. A is wrong because fused magnesia is magnesium oxide melted in an electric arc furnace and used as a heat-resistant material in furnace linings. C is wrong because magnesia alba is a basic magnesium carbonate, a white powder. D is wrong because caustic-calcined magnesia is magnesium oxide made by heating at lower temperatures and used in industry and farming. Exam tip: milk of magnesia is Mg(OH)2, an antacid base that neutralises stomach HCl.

The correct answer is C, Zn. Zinc has atomic number 30, meaning every zinc atom has 30 protons in its nucleus. It is the last element of the first transition series in period 4 of the modern periodic table, with the electronic configuration [Ar] 3d10 4s2, and it is placed in group 12. Zinc is a bluish-white metal used to galvanise iron against rust, to make brass with copper, and in dry cells. All four options are neighbours in the same period, which is why paper setters like this question: iron is 26, cobalt 27, nickel 28, copper 29 and zinc 30. A is wrong because Ni, nickel, has atomic number 28. B is wrong because Fe, iron, has atomic number 26. D is wrong because Co, cobalt, has atomic number 27. Exam tip: remember the run Fe 26, Co 27, Ni 28, Cu 29, Zn 30.

The correct answer is B, Roasting. Roasting means heating a sulphide ore strongly in plenty of air, so that the sulphide turns into an oxide and the sulphur escapes as sulphur dioxide gas. The standard example is zinc blende: 2ZnS + 3O2 gives 2ZnO + 2SO2. An oxide is easier to reduce to the metal with carbon, which is why ores of zinc, lead and copper first pass through this step. Remember the key contrast: roasting uses excess air and suits sulphide ores, while calcination uses limited or no air and suits carbonate ores. A is wrong, because calcination heats carbonate ores such as calamine in limited air, as when ZnCO3 gives ZnO and CO2. C is wrong, because smelting is the later step in which the oxide is reduced to metal in a furnace with a reducing agent and a flux. D is wrong, because incineration is burning waste at a high temperature, not a step in extracting metals. Exam tip: sulphide ore, roasting, excess air; carbonate ore, calcination, little or no air.

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 D, Magnesium hydroxide. Milk of magnesia is a white suspension of magnesium hydroxide, Mg(OH)2, in water, and its milky look gives it the name. It is a mild base, so it is taken as an antacid to neutralise excess hydrochloric acid in the stomach and relieve acidity; in larger doses it works as a laxative. Being only slightly soluble, it does not make the stomach strongly alkaline, which is why it is safe as a medicine. A, Magnesium bicarbonate, exists only in solution and is not used as a medicine. B, Magnesium carbonate, is used in chalk and as a drying powder for the hands in sports, and is not called milk of magnesia. C, Magnesium sulphate, is Epsom salt, used in bath salts and as a purgative. Exam tip: pair the common names: milk of magnesia is Mg(OH)2, Epsom salt is MgSO4.7H2O, milk of lime is a Ca(OH)2 suspension.

The correct answer is B, Both I and II. Both pairs are wrong. Amyloplasts store starch, a carbohydrate, and aleuroplasts store proteins; oils and fats are stored by elaioplasts. Plastids are organelles found only in plant cells. Chloroplasts hold chlorophyll and do photosynthesis, chromoplasts hold the yellow, orange and red pigments of flowers and fruits, and leucoplasts are colourless storage plastids. The leucoplasts are named after what they keep: amyloplasts keep starch (amylum), as in potato tubers; elaioplasts keep oils (elaion means oil), as in castor seeds; aleuroplasts keep proteins (aleurone), as in the outer layer of wheat grains. A is wrong because it accepts pair II, which links aleuroplasts to carbohydrates instead of proteins. C is wrong because it treats both pairs as correct. D is wrong because it accepts pair I, which links amyloplasts to fats instead of starch. Exam tip: amylo = starch, elaio = oil, aleuro = protein; all three are leucoplasts.

The correct answer is D, Bowman's capsule. Each human kidney has about one million nephrons, and every nephron begins with a cup-shaped Bowman's capsule that encloses a knot of capillaries called the glomerulus. Blood enters the glomerulus under pressure, and water, salts, glucose, urea and other small molecules are pushed out through its walls into the capsule; this is ultrafiltration, the first step of urine formation. Proteins and blood cells are too large to pass and stay in the blood. The filtrate then flows through the tubule, where useful substances such as glucose and most of the water are reabsorbed. A, the Collecting duct, only gathers urine from many nephrons and carries it to the pelvis of the kidney. B, the Renal vein, carries filtered blood away from the kidney and is not part of a nephron. C, the Ureter, is the tube that takes urine from the kidney to the bladder. Exam tip: Bowman's capsule plus glomerulus is the Malpighian body, the filter of the nephron.

The correct answer is C, Insulin. The pancreas is both an exocrine gland, pouring digestive enzymes into the intestine, and an endocrine gland. Its endocrine part is made of small clusters of cells called the islets of Langerhans. The beta cells in these islets release insulin, which lowers blood sugar by helping body cells take in glucose and by making the liver store extra glucose as glycogen. When the body makes too little insulin, or cannot use it properly, blood sugar stays high, which is diabetes mellitus. Insulin was discovered in 1921 by Frederick Banting and Charles Best. Option A is wrong because adrenaline, the ‘fight or flight’ hormone, comes from the medulla of the adrenal gland. Option B is wrong because glucagon is made by the alpha cells of the same islets, and it raises blood sugar, the opposite of insulin. Option D is wrong because aldosterone comes from the adrenal cortex and helps the kidneys keep back sodium and water. Exam tip: alpha cells make glucagon and raise sugar; beta cells make insulin and lower it.