General Science Mixed Quiz: Set 31

The correct answer is B, ionic bond. In an ionic or electrovalent bond a metal atom loses one or more electrons to a non-metal atom; the metal becomes a positive ion and the non-metal a negative ion, and the electrostatic attraction between the two holds the compound together, as in sodium chloride and magnesium oxide. Option A is wrong because a covalent bond is formed by the sharing of electron pairs between two atoms, typically two non-metals, with no transfer at all. Option C is wrong because a coordinate bond is a special covalent bond in which both shared electrons come from one atom, as in the ammonium ion; the electrons are still shared, not transferred. Option D is wrong because a hydrogen bond is a weak attraction between a hydrogen atom already bonded to a very electronegative atom and a lone pair on another such atom, and it is not a true chemical bond of this kind.

The correct answer is A, Capra aegagrus hircus. The domestic goat is treated as a subspecies of the wild goat, so its name carries three parts — the genus Capra, the species aegagrus and the subspecies hircus. It is often shortened to Capra hircus. Carl Linnaeus gave science this binomial system, in which the genus name begins with a capital letter, the species name does not, and the whole name is printed in italics or underlined when handwritten. Goats were among the first animals humans tamed, in western Asia roughly ten thousand years ago, and India keeps one of the largest goat populations in the world. Option B is wrong because Bos indicus is the humped Indian cattle, the zebu. Option C is wrong because Sus scrofa is the wild boar, the ancestor of the domestic pig. Option D is wrong because Ovis aries is the domestic sheep, the animal most often confused with the goat in such questions. Exam tip: goat Capra hircus, sheep Ovis aries, cattle Bos indicus, buffalo Bubalus bubalis.

The correct answer is B, gypsum. Plaster of Paris is calcium sulphate hemihydrate, made by heating gypsum, which is calcium sulphate dihydrate, to about 373 kelvin so that it loses three fourths of its water of crystallisation. When water is added again it sets into a hard mass of gypsum, which is why it is used for plaster casts, moulds, statues and false ceilings, and why it must be stored in a moisture proof container. Option A is wrong because limestone is calcium carbonate and heating it gives quick lime and carbon dioxide. Option C is wrong because quick lime is calcium oxide, the product of that reaction, and adding water to it gives slaked lime. Option D is wrong because washing soda is sodium carbonate decahydrate and heating it simply drives off water of crystallisation. Note also that heating gypsum too strongly destroys the setting property.

The correct answer is A, Crown Gall. Crown gall is caused by the bacterium Agrobacterium tumefaciens, which enters through a wound near the base, or crown, of the stem and makes the plant grow rough tumour-like swellings there. The bacterium works by pushing a piece of its own Ti plasmid into the plant's cells and forcing them to divide, and that natural trick is why laboratories use it as a vector to carry new genes into plants. Grapevine, apple, rose, walnut and stone fruits suffer the most. Option B is wrong because mosaic diseases, such as tobacco mosaic, are caused by viruses that mottle the leaves in light and dark patches. Option C is wrong because smut of wheat, bajra and sugarcane is a fungal disease that fills the ear with black powdery spores. Option D is wrong because white blister, or white rust, of crucifers such as mustard is also fungal and raises white blisters on leaves and stems. Exam tip: bacterial — crown gall, citrus canker, bacterial blight of rice; viral — mosaic, leaf curl; fungal — smut, rust, blight.

The correct answer is B, regeneration. Hydra, starfish and planaria can all grow a lost or cut off part of the body back into a whole animal, and that ability is called regeneration.

Regeneration works because these animals keep a large number of unspecialised cells that can divide and turn into any kind of tissue. A planaria cut into pieces gives a complete worm from each piece, a starfish grows a new arm and can rebuild itself from an arm with part of the central disc, and a piece of Hydra grows the missing mouth and tentacles. It is not the normal way these animals reproduce; it is a repair power that can also produce new individuals.

Option A is wrong because sporulation is the making of spores, seen in fungi like Rhizopus. Option C is wrong because budding is shown by Hydra and yeast but not by starfish or planaria. Option D is wrong because fragmentation is seen in filamentous algae such as Spirogyra.

Exam tip: regeneration in Hydra, planaria and starfish; budding in Hydra and yeast; fragmentation in Spirogyra; spore formation in Rhizopus.

The correct answer is B, Cones. Cones are the photoreceptor cells of the retina that respond to colour in bright light. The human retina carries two kinds of photoreceptors. Cones number about six to seven million, sit densely near the centre of the retina and come in three types, sensitive to red, green and blue light; the brain mixes their signals into every shade we see. They work only when the light is good, which is why colours fade at dusk. Option A is wrong because rods are far more numerous but work in dim light and give only shades of grey, not colour. Option C is wrong because the blind spot is the point where the optic nerve leaves the eyeball and carries no photoreceptors at all. Option D is wrong because the fovea is a small pit in the retina, a place rather than a cell type, although it is packed with cones. Exam tip: rods for night and grey, cones for colour and daylight; colour blindness is a defect of the cones.

The correct answer is C, three. A nitrogen atom has five valence electrons and needs three more to complete its octet, so two nitrogen atoms share three pairs of electrons and form a triple bond, written N to N with three lines. That triple bond, made of one sigma and two pi bonds, is very strong, which is why nitrogen gas is so unreactive that it is used to fill food packets and to provide an inert atmosphere. Option A is wrong because one shared pair would make a single bond, as in the hydrogen or chlorine molecule. Option B is wrong because two shared pairs make a double bond, as in the oxygen molecule or in ethylene. Option D is wrong because no ordinary covalent molecule of nitrogen shares four pairs; the octet rule allows at most three here. Remember the set: hydrogen single, oxygen double, nitrogen triple.

The correct answer is C, a mechanism to reduce CO2 emission. A nuclear reactor splits heavy atoms; it burns no fuel, so it releases no carbon dioxide and needs no equipment to cut such emissions. The heat of fission is produced in the reactor core, carried away by a coolant, passed to water in a heat exchanger or steam generator, and the steam then turns a turbine coupled to a generator. Control rods of cadmium or boron absorb spare neutrons and a moderator such as heavy water slows them down so that fission continues steadily. Option A is wrong as a choice because the turbine is the part that actually converts steam energy into electricity. Option B is wrong because the heat exchanger carries heat from the radioactive coolant loop to the clean water loop. Option D is wrong because the reaction chamber, the core holding the fuel rods, is where fission happens. Exam tip: a nuclear plant emits no carbon dioxide; its waste problem is spent radioactive fuel instead.

The correct answer is C, hydrochloric acid. The stomach wall secretes hydrochloric acid, which makes the gastric juice strongly acidic, helps the enzyme pepsin digest proteins and kills many germs swallowed with food. The stomach protects itself with a layer of mucus, and when the acid is produced in excess the burning sensation called acidity follows, treated with an antacid such as milk of magnesia. Option A is wrong because sulphuric acid is an industrial acid used in fertilisers, batteries and detergents and would destroy living tissue. Option B is wrong because nitric acid is likewise a strong laboratory and industrial acid, used in explosives and fertilisers. Option D is wrong because acetic acid is the weak acid of vinegar, which is edible but is not the acid the stomach itself produces. Note that an antacid works by neutralisation, not by stopping the secretion.

The correct answer is D, Black. In the colour code used for fire extinguishers, a black band or label marks a carbon dioxide extinguisher. The body of the cylinder is signal red and a coloured patch near the label tells you the filling at a glance, so the right cylinder can be picked in a hurry. A carbon dioxide extinguisher works by smothering: the gas is heavier than air, so it pushes oxygen away from the fuel, and it also chills the surface as it expands out of the horn. Because it does not conduct electricity and leaves no residue, it is the choice for live electrical equipment, switchboards, laboratories and computer rooms. Option A is wrong because a red band marks a plain water extinguisher, used on wood and paper fires. Option B is wrong because blue marks dry powder, the all-purpose filling for liquid and gas fires. Option C is wrong because cream, not white, is the colour for a foam extinguisher. Exam tip: red — water, cream — foam, blue — dry powder, black — carbon dioxide, yellow — wet chemical.

The correct answer is B, hydrogen. Zinc is above hydrogen in the reactivity series, so it displaces hydrogen from a dilute acid: zinc and dilute hydrochloric acid give zinc chloride and hydrogen gas. This is a displacement reaction, and the gas is tested by bringing a burning splinter to the mouth of the tube, where it burns with a pop sound. Option A is wrong because oxygen is not produced by a metal-acid reaction; it comes from the decomposition of substances such as hydrogen peroxide or potassium chlorate. Option C is wrong because carbon dioxide is released when an acid reacts with a carbonate or a bicarbonate, not with a metal. Option D is wrong because chlorine is not set free here; the chloride ion goes into the salt, zinc chloride. The general rule to remember is that a metal above hydrogen in the reactivity series plus a dilute acid gives a salt and hydrogen.

The correct answer is D, Ligament. A ligament is the connective tissue that joins one bone to another at a joint. It is very elastic and strong, and it holds the bones in place so that a joint can move without the bones slipping apart. Ligaments contain little matrix of their own and are stretched when a joint is twisted, which is what happens in a sprain. A is wrong because a tendon joins a muscle to a bone; tendons are fibrous and strong but have limited stretch, so the pull of a muscle is passed straight to the bone. B is wrong because cartilage is the smooth, flexible tissue that pads the ends of bones and shapes the nose and ear, cushioning the joint rather than tying the bones together. C is wrong because matrix is the material in which the cells of a connective tissue are embedded, not a structure by itself. Exam tip: ligament bone to bone, tendon muscle to bone.

The correct answer is A, acidic. A salt takes its character from the strength of the acid and the base that formed it. Ammonium chloride comes from hydrochloric acid, a strong acid, and ammonium hydroxide, a weak base, so the strong partner dominates and the solution has a pH below seven. Option B is wrong because a basic solution is given by the opposite combination, a weak acid with a strong base, as in sodium carbonate or sodium acetate. Option C is wrong because a neutral solution is given when both partners are strong, the standard example being sodium chloride from hydrochloric acid and sodium hydroxide. Option D is wrong because amphoteric describes a substance such as aluminium oxide or water that can act as both acid and base, which is not what a salt solution is being asked about here. Fix the three combinations and this whole family of questions is settled.

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 B, oxidation. Rusting is a slow oxidation in which iron combines with oxygen in the presence of moisture to form hydrated iron oxide, the reddish-brown flaky substance we call rust. Both air and water must be present, which is why iron does not rust in dry air or in boiled water free of dissolved oxygen, and why it is protected by painting, oiling, galvanising with zinc or alloying into stainless steel. Option A is wrong because reduction is the removal of oxygen or the gain of electrons, the opposite of what happens to iron here. Option C is wrong because neutralisation is the reaction between an acid and a base to give salt and water, which has nothing to do with rusting. Option D is wrong because decomposition means one compound splitting into simpler substances, whereas rusting builds a new compound. Rusting is one form of corrosion.

The correct answer is C, Anabaena. Anabaena is a cyanobacterium, a prokaryote with no true nucleus, and all prokaryotes are placed in Monera. In R.H. Whittaker's five-kingdom classification (1969), Monera holds the bacteria and the cyanobacteria, or blue-green algae, whose cells have no nuclear membrane and no membrane-bound organelles. Anabaena grows as chains of cells and has special thick-walled cells called heterocysts that fix nitrogen from the air. It lives inside the small water fern Azolla, which is why Azolla is used as a biofertiliser in paddy fields. Option A is wrong because diatoms are single-celled eukaryotes with glass-like silica walls, placed in Protista; their deposits form diatomaceous earth. Option B is wrong because Euglena is a protist that makes its own food in light but feeds like an animal in the dark. Option D is wrong because Ulothrix is a filamentous green alga with a true nucleus, grouped with plants. Exam tip: no true nucleus means Monera; Anabaena and Nostoc are the nitrogen-fixing cyanobacteria to remember.

The correct answer is C, 5.5. Tooth enamel is made of calcium phosphate, the hardest substance in the body, and it begins to corrode when the pH in the mouth drops below about 5.5. Bacteria in the mouth break down leftover sugar into acid, which is why sweets left on the teeth cause decay and why cleaning the teeth with a mildly basic toothpaste neutralises the excess acid. Option A is wrong because 7.0 is the neutral point, at which enamel is perfectly safe. Option B is wrong because at 6.5 the mouth is only slightly acidic and the enamel still holds. Option D is wrong because 3.5 is far more acidic than the threshold; damage would already have begun well before that value is reached. Note the neighbouring figure often confused with this one: rain is called acid rain when its pH falls below about 5.6.

The correct answer is C, tetrahedral. In methane the carbon atom is joined to four hydrogen atoms by four single bonds and has no lone pair, so the four bonding pairs push one another as far apart as possible and point to the corners of a regular tetrahedron, with bond angles of about 109.5 degrees. Option A is wrong because a linear shape, with a bond angle of 180 degrees, belongs to molecules such as carbon dioxide and acetylene. Option B is wrong because a trigonal planar shape, with angles of 120 degrees, belongs to molecules such as boron trifluoride, which has only three bonding pairs. Option D is wrong because a pyramidal shape belongs to ammonia, where one lone pair on nitrogen pushes the three bonds down to an angle of about 107 degrees. Water, with two lone pairs, is bent at about 104.5 degrees; learn these four shapes together.

The correct answer is A, dry slaked lime. Bleaching powder, calcium oxychloride, is made in industry by passing chlorine gas over dry slaked lime, that is calcium hydroxide. It is used for bleaching cotton, linen and wood pulp, for disinfecting drinking water, and as an oxidising agent in chemical industries. Option B is wrong because limestone is calcium carbonate, and heating it gives quick lime and carbon dioxide rather than a bleaching agent. Option C is wrong because gypsum is calcium sulphate dihydrate, the raw material of plaster of Paris. Option D is wrong because sodium carbonate is washing soda, used for cleaning and for softening hard water. The chain worth memorising runs from limestone to quick lime on heating, from quick lime to slaked lime on adding water, and from dry slaked lime to bleaching powder on passing chlorine.

The correct answer is B, hydrogen. Soil acidity is measured on the pH scale, and pH depends on the concentration of hydrogen ions in the soil water. The more hydrogen ions there are, the lower the pH and the more acidic the soil. A pH below 7 is acidic, 7 is neutral and above 7 is alkaline, and most crops grow best between about 6 and 7.5. Acidic soils are common in areas of heavy rainfall, where water washes away bases such as calcium and magnesium, and they are corrected by adding lime, that is calcium carbonate. Option A is wrong because phosphorus is a plant nutrient whose shortage stunts growth but does not define acidity. Option C is wrong because oxygen in soil pores affects aeration and root respiration, not pH. Option D is wrong because nitrogen is a nutrient taken up as nitrate or ammonium and acidity is still read from hydrogen ions. Exam tip: pH measures hydrogen ions, below 7 is acidic, and lime cures an acidic soil.