General Science Mixed Quiz: Set 41

The correct answer is A, sharing of electrons. A covalent bond is formed when two atoms share one or more pairs of electrons so that each completes its outermost shell. Such bonds form between non-metal atoms, whose electron affinities are similar, so neither can simply pull electrons away from the other. In a hydrogen molecule the two atoms share one pair, giving a single bond; oxygen shares two pairs as a double bond and nitrogen three pairs as a triple bond. Covalent compounds generally have low melting and boiling points and do not conduct electricity, because they contain molecules rather than free ions. B is wrong because protons sit in the nucleus and are never lost in ordinary chemical bonding. C is wrong because transfer of electrons produces an ionic bond, not a covalent one. D is wrong because neutrons take no part in bonding at all. Exam tip: sharing gives a covalent bond, transfer gives an ionic bond.

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 D, Next to diamond, graphite is the second hardest known substance. Graphite is soft and slippery rather than hard, so this is the statement that is not correct. The difference between the allotropes comes from the way the carbon atoms are joined. In diamond every carbon is bonded to four others in a rigid three dimensional network, which makes it the hardest natural substance. In graphite every carbon is bonded to only three others in flat sheets, and these sheets slide over one another, so graphite is soft enough to serve as a lubricant and as pencil lead. A is correct because the fourth electron of each carbon atom in graphite is free to move, which lets graphite conduct electricity. B is correct because diamond is indeed the hardest known natural substance. C is correct because fullerene, such as the sixty atom buckyball, is a third allotrope of carbon. Exam tip: same element but different structure, so diamond is hard while graphite is soft and conducting.

The correct answer is D, 2 gram of carbon. Three grams of hydrogen, 48 grams of oxygen and 42 grams of nitrogen each come to 1.5 moles, while two grams of carbon is only about 0.17 mole, so carbon holds a different number of particles. The number of moles is found by dividing the given mass by the molar mass, and equal moles always mean an equal number of particles. Hydrogen gas is H2 with a molar mass of 2, so 3 divided by 2 is 1.5 moles. Oxygen gas is O2 with a molar mass of 32, so 48 divided by 32 is 1.5 moles. Nitrogen gas is N2 with a molar mass of 28, so 42 divided by 28 is 1.5 moles. Carbon has an atomic mass of 12, so 2 divided by 12 is about 0.17 mole. A is wrong because 3 grams of hydrogen is 1.5 moles. B is wrong because 48 grams of oxygen is also 1.5 moles. C is wrong because 42 grams of nitrogen is 1.5 moles too. Exam tip: convert every mass to moles first.

The correct answer is B, sodium hydrogen carbonate, with the formula NaHCO3. It is a mild, non-corrosive base. In baking it releases carbon dioxide on heating and makes the dough rise, in medicine it neutralises excess acid in the stomach, and in a soda-acid fire extinguisher it reacts with sulphuric acid to produce the carbon dioxide that smothers the flame.

Option A is wrong because sodium carbonate is washing soda, a much stronger base used for cleaning and for softening hard water. Option C is wrong because sodium hydroxide is caustic soda, a corrosive alkali used in soap making and in the chlor-alkali industry. Option D is wrong because sodium chloride is ordinary table salt and is neutral, being the salt of a strong acid and a strong base.

One more point worth carrying: heating baking soda gives sodium carbonate, water and carbon dioxide, which is why it also works as a leavening agent.

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, tendon. A tendon is made of dense white fibrous connective tissue with great tensile strength and almost no elasticity, so when a muscle contracts the whole pull is transmitted to the bone without being lost in stretching. Option A is wrong because a ligament joins one bone to another across a joint; it contains elastic fibre, stretches to a degree and holds the bones in position, and this is the pair examiners most often interchange. Option B is wrong because cartilage is a soft, flexible supporting tissue found at the ends of bones, in the nose, the ear and the windpipe, where it gives shape and cushions movement. Option D is wrong because areolar tissue is a loose packing tissue that fills the space between organs and under the skin and helps in repair. Fix the pair by the words themselves: tendon for muscle to bone, ligament for bone to bone.

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 C, copper sulphate solution. The Tyndall effect is the scattering of a beam of light by particles suspended in a medium, and it needs particles large enough to scatter that light. Copper sulphate solution is a true solution: its particles are smaller than one nanometre, far too small to scatter a beam, so the light passes straight through and no path is seen. A colloid, whose particles measure between one and one thousand nanometres, does scatter the beam and makes its path visible, which is why sunlight becomes visible as it enters a dense forest or a dark room through a small hole. A is wrong because an emulsion is a colloid of two liquids and does show the effect. B is wrong because milk is an emulsion of liquid fat in water, the classic colloid of the chapter. D is wrong because starch solution is also a colloid. Exam tip: colloids and suspensions scatter light, true solutions do not.

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 C, C-12. Relative atomic mass is measured against the carbon-12 atom, one-twelfth of whose mass is taken as the standard unit. That unit is the atomic mass unit, written u, and it is defined as exactly one-twelfth of the mass of one atom of carbon-12. So when we say the relative atomic mass of oxygen is 16 u, we mean one oxygen atom is sixteen times as heavy as one-twelfth of a carbon-12 atom. The International Union of Pure and Applied Chemistry fixed carbon-12 as the standard in 1961 because it gave the most consistent values. A is wrong because nitrogen-14 was never used as the standard. B is wrong because oxygen-16 was the older standard, replaced by carbon-12. D is wrong because C-16 is not even a known isotope of carbon used here. Exam tip: 1 u equals one-twelfth the mass of a carbon-12 atom.

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 C, 1.6 sec. Time period is the time taken for one full oscillation, so it is the total time divided by the number of oscillations: 64 divided by 40 gives 1.6 seconds. Frequency is the reverse quantity, the number of oscillations in one second, which here is 40 divided by 64, or 0.625 hertz; period and frequency are always reciprocals of each other. For a simple pendulum the period depends only on the length of the thread and on the acceleration due to gravity, through T equal to 2 pi times the square root of L by g. It does not depend on the mass of the bob, nor, for small swings, on how wide the swing is. A pendulum about one metre long has a period of close to two seconds. A is wrong because 0.6 second would give more than 100 oscillations in 64 seconds. B and D are wrong because 6 and 16 seconds are far too long for 40 swings in just over a minute. Exam tip: time period equals total time divided by the number of oscillations.

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, strong electrostatic forces between ions. An ionic compound is a lattice of positive and negative ions held by strong electrostatic attraction, and that force explains all its other properties. Because the attraction is strong, a great deal of heat is needed to break the lattice, which is why sodium chloride melts only near 801 degrees Celsius. The same rigid lattice makes these solids hard and brittle. B is wrong because ionic compounds have high, not low, melting and boiling points. C is wrong because they dissolve readily in water, a polar solvent that surrounds and separates the ions, though they do not dissolve in kerosene or petrol. D is wrong because in the molten state the ions are free to move and the compound conducts electricity well; it is only the solid that does not conduct, since its ions are fixed in place. Exam tip: one strong force explains high melting point, water solubility and conduction when molten.

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, metallic character. Carbon is a non-metal, so it does not show metallic character; it neither loses electrons to form positive ions nor shows the lustre and malleability of metals. Carbon has four electrons in its outermost shell and completes its octet by sharing rather than by transfer, which makes covalent bonding its natural behaviour. A is therefore a property it does show, as carbon is tetravalent and forms four covalent bonds, as in methane. C is also shown: catenation is the ability of an element to link its own atoms into long chains and rings, and carbon does this better than any other element, which is why millions of organic compounds exist. D is shown as well, because carbon exists in several allotropes - diamond, graphite and fullerenes - each with a different structure and properties. Exam tip: carbon is tetravalent and a champion of catenation, but it is never metallic.

The correct answer is B, gypsum. Gypsum is calcium sulphate dihydrate, and heating it carefully to about 373 kelvin drives off three quarters of its water to leave calcium sulphate hemihydrate, which is Plaster of Paris. When water is added again it sets back into a hard mass of gypsum, which is why it is used for plaster casts in hospitals and for decorative moulding.

Option A is wrong because heating limestone, which is calcium carbonate, gives quick lime and carbon dioxide. Option C is wrong because bauxite is the ore of aluminium and is processed to alumina, not to plaster. Option D is wrong because quartz is silicon dioxide and is used in glass and in watches. Note also that the plaster must not be overheated, or it turns into the dead plaster that will not set.

The correct answer is B, xylem. Xylem is a complex permanent tissue of tracheids, vessels, xylem parenchyma and xylem fibres; the vessels and tracheids are dead, hollow and joined end to end, which makes them ideal pipes for carrying water and minerals from the root to the leaves in one direction. Option A is wrong because phloem conducts the food prepared in the leaves and moves it in both directions, upward and downward, to wherever it is stored or used. Option C is wrong because collenchyma is a simple permanent tissue that gives flexibility and mechanical support to young and climbing stems; it conducts nothing. Option D is wrong because cambium is a lateral meristem whose business is division, adding new xylem inward and new phloem outward and so increasing the girth of the stem. A useful memory aid is that only the parenchyma is alive in xylem, while in phloem only the fibres are dead.

The correct answer is C, volume by mass percentage of a solution. Concentration tells how much solute is present in a given amount of solution, and the three standard percentage forms are mass by mass, mass by volume and volume by volume; volume by mass is not one of them. Mass by mass percentage is the mass of solute dissolved in 100 grams of solution, mass by volume percentage is the mass of solute in 100 millilitres of solution, and volume by volume percentage is the volume of solute in 100 millilitres of solution, the form used when both solute and solvent are liquids, as with alcohol mixed in water. A is wrong because volume by volume percentage is a standard form. B is wrong because mass by volume percentage is standard as well and is common in medicine. D is wrong because mass by mass percentage is the most used of the three. Exam tip: remember the trio - mass by mass, mass by volume and volume by volume.