General Science Mixed Quiz: Set 43

The correct answer is A, —C = C—. An alkene is an unsaturated hydrocarbon in which two carbon atoms are joined by a double bond, so the carbon-to-carbon double bond is its identity. The simplest member is ethene, C2H4, and the whole series follows the general formula CnH2n. The double bond is made of one strong sigma bond and one weaker pi bond; the pi bond breaks easily, so alkenes readily add hydrogen, halogens or water, and they decolourise bromine water, the test used to tell an alkene from an alkane. Ethene is also the gas used to ripen fruit artificially. B is wrong because —C—C— is a single bond, the mark of a saturated alkane. C is wrong because H—O—H is water, which is not a hydrocarbon at all. D is wrong because O = C = O is carbon dioxide, which has two carbon-oxygen double bonds and no carbon-carbon bond. Exam tip: single bond means alkane, double bond means alkene, triple bond means alkyne.

The correct answer is C, Avogadro. The statement in the question is Avogadro's law, given by the Italian scientist Amedeo Avogadro in 1811. It says that equal volumes of all gases at the same temperature and pressure hold the same number of molecules, so the volume of a gas is directly proportional to the number of moles in it. This is why one mole of any gas occupies 22.4 litres at standard temperature and pressure, and why the number of particles in one mole, about 6.022 x 10 to the power 23, is named the Avogadro number after him. A is wrong because Charles law links the volume of a fixed mass of gas to its temperature at constant pressure. B is wrong because Boyle's law says that volume varies inversely with pressure at constant temperature. D is wrong because Gay-Lussac gave the law of combining volumes and the pressure-temperature law. Exam tip: Avogadro ties volume to the number of molecules, Boyle to pressure and Charles to temperature.

The correct answer is A, boiling the water. Temporary hardness is caused by the dissolved bicarbonates of calcium and magnesium. On boiling, these bicarbonates decompose into insoluble carbonates, which settle out as the scale seen in a kettle, so the water that remains is soft. Permanent hardness, caused by the sulphates and chlorides of the same metals, is not removed this way and needs washing soda or an ion exchange resin.

Option B is wrong because common salt does not remove hardness; it is in fact used to regenerate the resin in a water softener, which is a different step. Option C is wrong because filtering removes suspended dirt, not dissolved salts. Option D is wrong because adding vinegar only makes the water acidic and leaves the calcium and magnesium in solution.

The correct answer is C, Stomata. Stomata are the tiny pores on the surface of a leaf through which a plant takes in carbon dioxide and gives out oxygen and water vapour, so the exchange of gases takes place through them. Each stoma is bounded by two bean-shaped guard cells that swell to open the pore and shrink to close it, and this is how the plant controls its water loss. Most stomata lie on the lower surface of a leaf, so less water is lost to direct sunlight. Transpiration also happens largely through these pores. A is wrong because the cell membrane controls what enters and leaves a single cell, not the gas exchange of the whole plant. B is wrong because the nucleus carries the genetic material and directs the activities of the cell. D is wrong because guard cells only open and close the pore, while the exchange itself happens through the stomatal opening. Exam tip: stomata do the gas exchange and transpiration, guard cells only open and close them.

The correct answer is C, lateral meristem. The lateral meristem, better known as the cambium, lies as a ring along the length of the stem and root and divides to add new xylem on its inner side and new phloem on its outer side, so the stem grows thicker year after year; this is secondary growth, and it is what produces the wood of a tree. Option A is wrong because the apical meristem sits at the tip of the root and the shoot and adds to length, which is primary growth. Option B is wrong because the intercalary meristem lies at the base of leaves and at the internodes and lets grasses and bamboo grow again after cutting or grazing, again adding length. Option D is wrong because permanent tissue is made of cells that have lost the power to divide and have taken a fixed shape and function. Keep the three meristems as a set: apical for length, lateral for girth, intercalary for regrowth.

The correct answer is D, Colourless. Hydrogen is a colourless, odourless and tasteless gas, so none of the colours listed fits it. It is the lightest element, with one proton and one electron, and it is the most abundant element in the universe, although on the earth it is found mostly combined in water and in organic compounds. Hydrogen exists as the diatomic molecule H2, burns in air with a pale blue flame and gives only water as the product, which is why it is studied as a clean fuel. It is used to make ammonia by the Haber process and to harden vegetable oils into vanaspati. A is wrong because no common form of hydrogen is yellow. B is wrong because orange is the colour seen in sodium vapour lamps and not in hydrogen. C is wrong because none of the gases here is black; carbon in its soot form is black, but that is a solid. Exam tip: hydrogen, oxygen and nitrogen are all colourless and odourless gases.

The correct answer is B, alkyne only. Put n equal to 3 in the alkyne general formula CnH2n-2 and the result is C3H4, which is propyne, the second member of the alkyne series. An alkyne carries one carbon-to-carbon triple bond, and that bond costs the molecule four hydrogen atoms compared with the alkane of the same carbon count, so propane C3H8 becomes propyne C3H4. The triple bond makes alkynes highly unsaturated, so they add hydrogen and halogens readily; the first member, ethyne or acetylene, gives the very hot oxy-acetylene welding flame. A is wrong because an alkene of three carbon atoms follows CnH2n and would be C3H6, propene. C is wrong because an alkane of three carbon atoms follows CnH2n+2 and would be C3H8, propane. D is wrong because an alcohol carries a hydroxyl group and a three-carbon alcohol is written C3H7OH, which contains oxygen. Exam tip: for the same carbon count each extra bond removes two hydrogen atoms - C3H8, C3H6, C3H4.

The correct answer is B, In the same direction. In a class 3 lever the fulcrum is at one end, the effort is applied in the middle and the load sits at the far end. Since both the effort and the load lie on the same side of the fulcrum, the beam turns one way and they move in the same direction. The effort arm is shorter than the load arm, so the mechanical advantage is less than one and more effort is needed than the load; what such a lever gains is speed and a wider sweep of movement. The human forearm, a pair of sugar tongs, a fishing rod and a stapler work this way. A is wrong because effort and load move oppositely in a class 1 lever, where the fulcrum lies between them, as in a see-saw or scissors. C is wrong because a lever turns about a fixed point and its two ends never move at right angles to each other. D is wrong because the size of the load changes the force needed, not the direction of motion. Exam tip: class 1 has the fulcrum in the middle, class 2 the load, class 3 the effort.

The correct answer is B, ethyl mercaptan. Liquefied petroleum gas is a mixture of butane and propane and is by itself almost odourless, so a leak could fill a room unnoticed. A trace of ethyl mercaptan, a sulphur compound with a strong unpleasant smell, is therefore added as an odourant, and it is that smell people recognise as the smell of cooking gas.

Option A is wrong because carbon monoxide is odourless and highly poisonous, so it would be useless and dangerous as a warning agent. Option C is wrong because nitrous oxide is laughing gas, used as an anaesthetic, and has only a faintly sweet smell. Option D is wrong because chlorine is a corrosive greenish gas used to disinfect water and would attack the cylinder and the burner.

A related fact is that compressed natural gas, used in vehicles, is mainly methane and is also odourised for the same reason.

The correct answer is C, 2 s. A simple pendulum about one metre long completes one full swing in roughly two seconds, and a pendulum of this length is called the seconds pendulum. The time period of a simple pendulum is T = 2 pi times the square root of L divided by g, where L is the length of the thread and g is the acceleration due to gravity, about 9.8 metres per second squared. Putting L = 1 metre gives T = 2 x 3.14 x the square root of 1 divided by 9.8, which comes to about 2.01 seconds. The period depends only on the length and on g, not on the mass of the bob or on how far it is pulled aside, so long as the swing is small. A is wrong because a 6 second period needs a thread about 9 metres long. B is wrong because a 4 second period needs a thread about 4 metres long. D is wrong because a 1 second period belongs to a thread of only about 25 centimetres. Exam tip: the period doubles only when the length becomes four times.

The correct answer is A, Cell will lose water. A medium that holds less water than the cell is a hypertonic medium, so water moves out of the cell by osmosis and the cell shrinks. Osmosis is the movement of water through a semi-permeable membrane from the side where water is in higher concentration to the side where it is lower. In an animal cell this loss of water makes the cell shrivel, and the matching change in a plant cell is called plasmolysis, where the protoplasm pulls away from the cell wall. In a hypotonic medium the opposite happens and the cell swells, which is why red blood cells burst in pure water, while in an isotonic medium there is no net movement at all. B is wrong because water always moves when the two concentrations differ. C and D are wrong because gaining water and swelling happen in a hypotonic medium and not in a hypertonic one. Exam tip: hypertonic means water out and shrinking, hypotonic means water in and swelling.

The correct answer is A, low coefficient of friction. Teflon is the trade name of polytetrafluoroethylene, a polymer whose surface is one of the most slippery among solids, with a coefficient of friction of about 0.04. A bearing has to let a shaft turn with as little rubbing as possible, so this slipperiness is the reason Teflon is chosen; it also lets many such bearings work without any oil or grease. The polymer was found by accident by Roy Plunkett in 1938, resists almost every chemical and holds up to about 260 degrees Celsius, which is why it also coats non-stick cookware and is wound as sealing tape on pipe threads. B is wrong because Teflon is a poor conductor of heat, so it is not chosen to carry heat away. C is wrong because saving space depends on the design of the bearing, not on the material. D is wrong because only the first reason holds. Exam tip: Teflon is PTFE, the non-stick polymer with the lowest friction.

The correct answer is A, C3H4. C3H4 is propyne, and it fits the alkyne general formula CnH2n-2 when n equals 3. Alkynes are unsaturated hydrocarbons that carry at least one carbon-to-carbon triple bond, and the first member of the family is ethyne, C2H2, better known as acetylene and burnt with oxygen in welding torches. Because a triple bond is rich in electrons, alkynes add hydrogen, halogens and hydrogen halides easily and burn with a very hot but sooty flame, since the proportion of carbon in them is high. B is wrong because C2H4 is ethene, an alkene with a double bond, fitting CnH2n. C is wrong because C3H8 is propane, a saturated alkane fitting CnH2n+2. D is wrong because C6H6 is benzene, an aromatic ring compound with alternating double bonds, not an open-chain alkyne. Exam tip: for three carbon atoms, propane is C3H8, propene is C3H6 and propyne is C3H4.

The correct answer is B, 111 u. Calcium chloride is an ionic compound, so it has a formula unit mass rather than a molecular mass, and the value is found by adding the atomic masses of all the atoms in the formula. Calcium has an atomic mass of 40 u and chlorine of 35.5 u, and the formula carries two chlorine atoms, so the sum is 40 plus 2 multiplied by 35.5, that is 40 plus 71, which gives 111 u. The half unit in chlorine comes from its two isotopes, chlorine-35 and chlorine-37, mixed in roughly a three to one ratio, and forgetting that 0.5 is the usual slip in this sum. A is wrong because 105 u would follow from taking chlorine as 32.5 u. C is wrong because 121 u would need chlorine at 40.5 u. D is wrong because 100 u comes from rounding chlorine down to 30 u. Exam tip: Ca is 40 u and Cl is 35.5 u, so CaCl2 is 111 u while NaCl is 58.5 u.

The correct answer is B, mercuric sulphide, with the formula HgS. The natural mineral form of mercuric sulphide is cinnabar, the chief ore of mercury, and its bright red powdered form has been used as the pigment vermilion for centuries. Because mercury compounds are toxic, modern sindoor is usually made from safer synthetic dyes.

Option A is wrong because lead oxide includes red lead, used in primer paint, and litharge, but neither is called vermilion. Option C is wrong because zinc oxide is the white pigment known as Chinese white or philosopher's wool and is used in ointments and sunscreen. Option D is wrong because ferric oxide is the reddish brown pigment of haematite and of rust, a different red from vermilion.

Learning the pigment names in pairs helps here: vermilion with mercury, litharge with lead, Chinese white with zinc and haematite with iron, because papers rotate these four.

The correct answer is A, Papaya. A unisexual flower carries only one kind of reproductive organ, either stamens or a pistil, while a bisexual flower carries both. Papaya is dioecious: male and female flowers are borne on separate plants, so pollen must travel from a male tree to a female tree before fruit can set, which is why growers keep a few male trees in a papaya orchard. Other familiar unisexual examples are maize, cucumber, watermelon and coconut, in which the two kinds of flower sit on the same plant and the plant is called monoecious. Option B is wrong because the disc florets of a sunflower head are bisexual. Option C is wrong because the hibiscus flower is a standard bisexual flower carrying both stamens and a pistil. Option D is wrong because mustard, a member of the family Brassicaceae, also bears bisexual flowers. Exam tip: unisexual flowers include papaya, maize, cucumber and watermelon, while hibiscus, mustard, rose and pea are bisexual.

The correct answer is B, Nucleoid. In a prokaryotic cell the genetic material lies free in the cytoplasm with no nuclear membrane around it, and this poorly defined nuclear region is called the nucleoid. Bacteria and blue green algae are prokaryotes, and besides the lack of a true nucleus they have no membrane bound organelles such as mitochondria, plastids or the endoplasmic reticulum; their ribosomes are of the 70S type and their cell wall carries peptidoglycan. Eukaryotes such as fungi, plants and animals keep their DNA inside a double membrane called the nuclear envelope. A is wrong because nucleic acid is the chemical name for DNA and RNA and not a region of the cell. C is wrong because the nucleolus is a dense body inside a true nucleus where ribosomes are put together. D is wrong because a nucleosome is the bead of DNA wound round histone proteins, found in eukaryotes. Exam tip: prokaryote means a nucleoid and no membrane bound organelles.

The correct answer is D, C3H6. The alkene series begins at two carbon atoms, so its first member is ethene, C2H4, and its second member is propene, C3H6. Alkenes follow the general formula CnH2n and must carry at least one carbon-to-carbon double bond, which is why the series cannot start at one carbon atom: a lone carbon has no second carbon to form a double bond with. Each member differs from the one before it by a CH2 unit, the mark of a homologous series, so the members share chemical properties while their boiling points rise steadily with chain length. A is wrong because C6H12 is hexene, the fifth member of the series. B is wrong because C2H4 is ethene, which is the first member and not the second. C is wrong because C4H8 is butene, the third member. Exam tip: the alkenes run C2H4, C3H6, C4H8 and C5H10, so the nth member has n+1 carbon atoms.

The correct answer is D, Quick lime. Quick lime is calcium oxide, CaO, and not calcium carbonate, so it is the odd one out. Limestone, chalk and marble are all natural forms of calcium carbonate, CaCO3, differing only in how they were formed and in how hard they are. Quick lime is made by heating limestone strongly, a step called calcination, in which calcium carbonate breaks up into calcium oxide and carbon dioxide. Add water to quick lime and a great deal of heat is given out as slaked lime, calcium hydroxide, is formed; this is the whitewash put on walls. A is wrong because limestone is the common sedimentary rock form of calcium carbonate and the raw material of cement. B is wrong because chalk is a soft, powdery form of the same compound. C is wrong because marble is its hard, crystalline form, used in buildings such as the Taj Mahal. Exam tip: limestone CaCO3 heated gives quick lime CaO, and quick lime with water gives slaked lime Ca(OH)2.

The correct answer is A, C6H12. Cyclohexane is a closed ring of six carbon atoms joined by single bonds, and each carbon carries two hydrogen atoms, which gives C6H12. Closing an open chain into a ring costs the molecule two hydrogen atoms, so the cycloalkanes follow the general formula CnH2n, the same formula as the alkenes even though a cycloalkane is fully saturated. The ring is not flat; it takes a puckered chair shape that keeps the bond angles near 109.5 degrees and makes cyclohexane unusually stable. It is a colourless liquid used as a solvent and in making nylon. B is wrong because C6H14 is hexane, the open-chain alkane of six carbon atoms. C is wrong because C6H6 is benzene, an aromatic ring with alternating double bonds. D is wrong because C6H10 is cyclohexene, a six-carbon ring that carries one double bond. Exam tip: cycloalkanes follow CnH2n, so cyclopropane is C3H6, cyclobutane C4H8 and cyclohexane C6H12.