General Science Mixed Quiz: Set 23

The correct answer is B, a concave lens. In myopia the eyeball is too long or the lens of the eye too strongly converging, so rays from a distant object meet in front of the retina and the person cannot see far objects clearly. A concave lens diverges the incoming rays before they enter the eye, so the image is pushed back and falls exactly on the retina.

Option A, a convex lens, converges light and is used for hypermetropia or long sight, where the image forms behind the retina and near objects look blurred. Option C, a cylindrical lens, corrects astigmatism, which is caused by an uneven curvature of the cornea. Option D, a bifocal lens, has a concave upper part and a convex lower part and is used for presbyopia, the loss of the power of accommodation with age.

The correct answer is C, a and c, that is Spirulina and Chlorella. Both are tiny algae grown in tanks and sold as protein rich food supplements, and both have been studied as food for astronauts. Spirulina is a blue green alga, really a cyanobacterium, whose dried powder is about sixty per cent protein and also carries iron and beta carotene. Chlorella is a single celled green alga that multiplies very fast in a small space and gives off oxygen as it grows, which is why space agencies have tested it in closed life support systems. Such food from microbes is called single cell protein. Option A is wrong because Spirogyra, the slippery filamentous alga of ponds, is not eaten as a supplement. Option B is wrong for the same reason, as it also carries Spirogyra. Option D is wrong because it leaves out both correct names and adds Funaria, which is a moss and not an alga at all. Exam tip: Spirulina and Chlorella give single cell protein.

The correct answer is D, steel. Sound travels fastest in solids because their particles are packed closely and are strongly bound, so a disturbance is passed on to the next particle almost at once. In steel the speed is about five thousand metres per second, in water about fifteen hundred and in air only about three hundred and forty. The order to remember is solid, then liquid, then gas.

Option A, vacuum, carries no sound at all, since there are no particles to vibrate. Option B, air, is the slowest of the three real media listed because its particles are far apart and take longer to pass the disturbance on. Option C, water, is faster than air but far slower than steel. This is also why a rail track carries the sound of an approaching train long before the sound reaches a listener through the air.

The correct answer is B, tuberculosis (TB). The BCG vaccine takes its name from Albert Calmette and Camille Guerin, the two French scientists who developed it from a weakened strain of Mycobacterium bovis. In India it is given as a single dose soon after birth under the Universal Immunisation Programme, and it protects infants mainly from the severe forms of the disease, such as tuberculous meningitis and miliary tuberculosis. Tuberculosis itself is caused by the bacterium Mycobacterium tuberculosis, identified by Robert Koch in 1882, and it spreads through droplets when a patient coughs. Option A is wrong because typhoid, caused by Salmonella typhi, has its own conjugate vaccine. Option C is wrong because mumps is a viral disease covered by the MMR vaccine along with measles and rubella. Option D is wrong because plague, caused by Yersinia pestis and carried by rat fleas, needs a different vaccine. Exam tip: BCG at birth for TB, and World TB Day falls on 24 March.

The correct answer is D, microwave. A microwave oven does not cook by passing a current through a resistance. It generates microwaves in a magnetron, and these waves make the water molecules in the food rotate rapidly, so the food heats from within by dielectric heating. The heating effect of current plays no part in the cooking itself.

Option A, the electric heater, works exactly on Joule heating: current through a nichrome element produces heat proportional to the square of the current, the resistance and the time. Option B, the filament bulb, passes current through a thin tungsten wire that becomes white hot and glows, which is the same effect taken to a very high temperature. Option C, the electric iron, again uses a coiled resistance element inside the sole plate. Since three of the four are ordinary resistance heating devices, the microwave oven is the odd one out.

The correct answer is A, Distillation. Distillation separates a liquid from a solution or from another liquid by boiling it and condensing the vapour, so it is used when the components differ in boiling point, as in getting pure water from salt water or separating acetone from water. It is not the method for purifying a solid mixed with solid impurities. Option B is useful because sublimation purifies a solid that turns straight into vapour on heating, such as camphor, naphthalene, iodine or ammonium chloride, leaving the non-volatile impurity behind. Option C is useful because crystallization dissolves the solid in a hot solvent and lets pure crystals form on cooling, the standard way to purify alum, copper sulphate and sugar. Option D is wrong because two of the three processes do purify solids, so they cannot all be useless. Exam tip: solids are purified by sublimation, crystallization and chromatography; distillation is for liquids.

The correct answer is D, almost all the mass and the positive charge lie in a tiny nucleus. Most alpha particles passed straight through the foil, a few were deflected through small angles and about one in twenty thousand bounced back. Rutherford read this as meaning that the atom is mostly empty space, that its positive charge and nearly all its mass are packed into a very small central nucleus, and that electrons move around it. Option A is wrong because that is Thomson's plum pudding model, which the experiment disproved. Option B is wrong because indivisibility was Dalton's postulate, already broken by the discovery of the electron. Option C is wrong because fixed permitted orbits came later, with Bohr, who introduced them to cure the instability in Rutherford's own model.

The correct answer is C, Guava. Guava is the classic jam fruit because it is rich in both pectin and acid, the two things a jam needs to set into a firm gel. Jam is made by boiling fruit pulp with sugar until the mixture reaches about 68 per cent soluble solids; without enough pectin it stays syrupy and never sets. Ripe guava carries close to one per cent pectin, so guava jam sets without any added gelling agent, and its strong aroma survives the boiling. Option A, banana, has very little pectin, and its pulp darkens and turns pasty on cooking, so it goes into chips, powder and puree. Option B, pineapple, is low in pectin and high in water, which is why it is preferred for squash, juice and slices in syrup. Option D, sweet orange, is low in acid and its segments break down, so oranges are used for squash and marmalade. Exam tip: jam needs pectin plus acid — guava, apple and aonla are the standard pectin-rich fruits.

The correct answer is A, Diffusion. Gases move across the cell membrane by diffusion, the spread of particles from a region where they are more concentrated to a region where they are less concentrated. No energy is spent by the cell, because the particles simply move down the concentration gradient. In a living cell the carbon dioxide made during respiration builds up inside, so it diffuses out, while oxygen is used up inside and so diffuses in from outside. The same process works in the lungs and in the leaves of plants through stomata. B is wrong because osmosis is the movement of water, not gases, through a selectively permeable membrane. C is wrong because endocytosis is the engulfing of large particles by folding of the membrane, as an amoeba takes in food. D is wrong because absorption is a general uptake of substances, usually of digested food in the intestine. Exam tip: diffusion moves gases, osmosis moves water.

The correct answer is D, 0.7 V. A silicon diode in forward bias drops about 0.7 volt across itself, whatever the supply voltage is. This value is the barrier or knee voltage of a silicon p-n junction: current begins to flow only after the applied voltage crosses about 0.7 V, and after that the drop across the diode stays near 0.7 V while the rest of the supply appears across the resistor. Here 12 V minus 0.7 V, that is 11.3 V, falls across the 240 ohm resistor and fixes the current in the circuit. A germanium diode has a lower knee voltage of about 0.3 V. Option A is wrong because 1.5 V is the voltage of a dry cell and has nothing to do with the junction. Option B is wrong because half the supply would appear across the diode only if it behaved like a resistor of the same value. Option C is wrong because the full 12 V appears across the diode only when it is reverse biased or the circuit is broken. Exam tip: silicon 0.7 V, germanium 0.3 V.

The correct answer is D, Buccal cavity. In humans the digestion of starch begins in the buccal cavity, that is the mouth. The salivary glands pour out about 1.5 litres of saliva a day, and the enzyme in it, salivary amylase or ptyalin, breaks starch into the simpler sugar maltose. This is why a piece of chapati or rice starts tasting slightly sweet if it is chewed for a while. The saliva also wets the food so the tongue can roll it into a bolus for swallowing. A is wrong because the rectum only stores the waste left after digestion and absorption. B is wrong because in the small intestine pancreatic amylase carries this work forward and maltase finishes it into glucose, but it does not begin there. C is wrong because the oesophagus is only a food pipe that pushes the bolus down by peristalsis and has no digestive enzyme. Exam tip: starch begins in the mouth with salivary amylase, protein in the stomach with pepsin, fat in the small intestine with lipase and bile.

The correct answer is D, gold. Calaverite is gold telluride, with the formula AuTe2.

It is one of the very few minerals in which gold occurs chemically combined rather than as the free metal, because gold is a noble metal that usually resists reaction. Calaverite was named after Calaveras County in California, where it was found in 1861, and it is an important gold ore in Kalgoorlie in Australia and in Cripple Creek in Colorado. It looks brassy yellow to silvery white and is brittle.

Option A is wrong because the common silver telluride is hessite, and silver's chief ore is argentite. Option B is wrong as zinc is mined mainly as zinc blende, a sulphide. Option C is wrong because copper comes largely from copper pyrites, malachite and cuprite.

Exam tip: link ore to metal - calaverite gold, argentite silver, zinc blende zinc, bauxite aluminium, haematite iron, cinnabar mercury.

The correct answer is A, Size 18-25 micrometres, because it is the statement that does not hold. White blood cells are usually about 8 to 20 micrometres across, larger than red cells but not in the range given here. The other three statements describe them correctly. White blood cells keep a nucleus all their life, unlike mature red cells which lose theirs, and they have no fixed shape, so they can squeeze through capillary walls into tissue. Many of them, such as neutrophils and monocytes, swallow and destroy bacteria, which is why they are called the soldiers of the body. They are made in the bone marrow and are far fewer than red cells. B, C and D are therefore wrong choices for this question because each one is a true statement about white blood cells. Exam tip: WBCs are nucleated, irregular in shape and about 8 to 20 micrometres wide.

The correct answer is D, Thallophyta. Algae belong to Thallophyta, the simplest division of the plant kingdom. Their body is a thallus, which means it is not divided into root, stem and leaf, and they have no vascular tissue. Most algae live in water, make their own food by photosynthesis and reproduce by spores; Spirogyra, Ulothrix, Chara and Volvox are common examples. Option A is wrong because angiosperms are flowering plants whose seeds lie enclosed in a fruit, the most advanced group of all. Option B is wrong because pteridophytes, such as ferns, are the first plants with proper vascular tissue, though they bear no seeds. Option C is wrong because bryophytes, such as mosses and Marchantia, are called the amphibians of the plant kingdom; they stand a step above thallophytes and need water for reproduction. Exam tip: the order of the plant kingdom is Thallophyta, Bryophyta, Pteridophyta, Gymnosperms and Angiosperms.

The correct answer is D, Fore-brain. The brain is divided into fore-brain, mid-brain and hind-brain, and the fore-brain is the main thinking part. Its largest portion is the cerebrum, whose folded outer layer holds separate areas for hearing, sight, smell and touch, for reasoning and memory, and for ordering voluntary movement. The fore-brain also carries centres for hunger and thirst. A is wrong because the hind-brain looks after balance and the automatic actions that keep us alive, not thinking. B is wrong because the cerebellum is a part of the hind-brain and controls posture, balance and the precision of voluntary movement, as in writing or picking up a pencil. C is wrong because the mid-brain mainly relays signals and manages reflexes of the eye and ear, such as the change in pupil size. The medulla, also in the hind-brain, controls breathing, blood pressure and vomiting. Exam tip: fore-brain thinks, cerebellum balances, medulla breathes.

The correct answer is A, Ant. Among the four, the ant has the largest brain in proportion to its body. An ant’s brain is tiny in absolute terms, with about two hundred and fifty thousand neurons, yet it makes up roughly a sixth of the insect’s body weight, a share no vertebrate comes close to. This is a general rule of body size: as animals get smaller, the brain shrinks more slowly than the body, so small creatures carry proportionately bigger brains. Option B is wrong because an elephant has the largest brain in absolute weight on land, about five kilograms, but that is a very small fraction of a body of several tonnes. Option C is wrong because a dolphin, though highly intelligent, has a brain near one per cent of its body weight. Option D is wrong because a human brain is about 1.4 kilograms, roughly two per cent of body weight, high for a vertebrate but far below an ant’s share. Exam tip: elephant biggest brain by weight, ant biggest in proportion.

The correct answer is B, Spirogyra. Spirogyra is a green filamentous alga found in ponds, and when its filament grows long it simply breaks into two or more pieces. Each piece then grows into a full new filament, and this way of multiplying is called fragmentation. It works because the organism has a simple body in which every part can carry out all the life processes, so no piece is lost by the break. A is wrong because Hydra reproduces by budding, in which a small outgrowth appears on the body wall, grows its own mouth and tentacles and then detaches. C is wrong because yeast also reproduces by budding, a bud forming on the parent cell and often staying attached in a short chain. D is wrong because Planaria reproduces by regeneration; if it is cut, each part grows the missing organs back, but that follows an injury and is not its normal way of multiplying. Exam tip: Spirogyra fragments, Hydra and yeast bud, Planaria regenerates.

The correct answer is C, increases. Water behaves unlike most liquids between 0 and 4 degrees Celsius, a behaviour called the anomalous expansion of water. Almost every substance shrinks as it is cooled, but water shrinks only until it reaches 4 degrees Celsius. At that temperature it has its smallest volume and therefore its greatest density, about one gram per cubic centimetre. Cool it further, from 4 degrees down to 0 degrees, and it expands again, so one litre takes up more space at 0 degrees than at 4 degrees. This is why ice floats and why, in winter, a lake freezes only at the top while the water below stays at about 4 degrees, allowing fish to live through the cold. A and D are wrong because water only expands over this range, it does not shrink. B is wrong because the volume clearly changes. Exam tip: water is densest at 4 degrees Celsius.

The correct answer is A, a suspension is a heterogeneous mixture. In a suspension the particles of the solute are large enough to be seen and do not dissolve; they stay spread through the liquid only while it is stirred and settle to the bottom when it is left standing. Chalk powder in water, muddy river water and a shaken bottle of paint are everyday examples. Because the particles are big, a suspension scatters a beam of light, showing the Tyndall effect, and the particles can be separated by filtration. B is wrong because solids spread in a gas, such as smoke, are aerosols, a kind of colloid, not a suspension. C is wrong because the particles of a suspension are large enough to be seen with the naked eye; it is the particles of a solution that cannot be seen. D is wrong because a homogeneous mixture is a solution, which is uniform throughout. Exam tip: solution homogeneous, suspension heterogeneous and settles, colloid in between.

The correct answer is D, Tunnel diode. A tunnel diode, also called an Esaki diode after Leo Esaki who made it in 1957, is heavily doped on both sides, so its depletion layer is extremely thin and electrons cross it by quantum tunnelling. Over part of its forward characteristic the current falls as the voltage rises, giving a negative resistance region, and it is this property that lets the diode both amplify a signal and sustain oscillations in a circuit. It switches extremely fast and is used in microwave oscillators and high frequency circuits. A is wrong because a point contact diode is used to detect and rectify high frequency signals. B is wrong because a Zener diode works in reverse breakdown and is used to regulate voltage. C is wrong because an ordinary junction diode only rectifies, allowing current one way. Exam tip: tunnel diode means negative resistance, so it can amplify and oscillate; Zener diode means voltage regulation.