Elements, Compounds and Mixtures: Separation Methods and PYQs

The correct answer is C, common salt. Common salt is sodium chloride, in which sodium and chlorine are chemically combined in a fixed ratio by mass, and the result has properties quite unlike either element, since sodium reacts violently with water and chlorine is a poisonous gas while their compound is eaten daily. It has a sharp melting point and can be broken up only by chemical means such as electrolysis. Option A, air, is a homogeneous mixture of nitrogen, oxygen, argon, carbon dioxide and water vapour, whose proportions vary from place to place and whose gases can be separated physically by the fractional distillation of liquid air. Option B, brass, is an alloy of copper and zinc, and an alloy is a homogeneous mixture rather than a compound, which is why its composition can be adjusted. Option D, sea water, is a mixture of water with many dissolved salts, and the salt can be recovered simply by letting the water evaporate.

The correct answer is B, the Tyndall effect. The particles of a colloid are between one and one thousand nanometres across, which is large enough to scatter a beam of light sideways and make the path of the beam visible. That is why a torch beam shows up in fog, why car headlights cut a visible cone through mist and why sunlight entering a thick forest canopy appears as separate shafts. A true solution has far smaller particles and shows no such effect, so this is the standard test for telling a colloid from a solution. Option A, total internal reflection, is the complete reflection of light at a boundary beyond the critical angle and is the principle of the optical fibre. Option C, the Doppler effect, is the change in the observed frequency of a wave when the source and observer move relative to each other. Option D, diffraction, is the bending and spreading of waves at an obstacle or a narrow slit, and although it also involves light it is a different phenomenon altogether.

The correct answer is B, milk. Milk is an emulsion in which tiny globules of fat are dispersed through water, and those globules fall within the colloidal size range, so milk looks uniform to the eye, does not separate on standing, passes through filter paper and shows the Tyndall effect. Fog, smoke, ink, butter and cheese are the other standard colloids. Option A, sugar solution, is a true solution, because the sugar breaks up into individual molecules far smaller than colloidal particles and the mixture shows no Tyndall effect. Option C, muddy water, is a suspension: the soil particles are large, the mixture is visibly cloudy, the particles settle on standing and they are held back by filter paper. Option D, brass, is a solid solution of zinc in copper, that is an alloy, and so counts as a homogeneous mixture rather than a colloid. The distinguishing test in each case is particle size.

The correct answer is B, separating funnel. Kerosene and water do not mix, so on standing they form two distinct layers with the lighter kerosene above the denser water. A separating funnel has a stopcock at the bottom, and opening it lets the lower layer run out while the upper layer is held back, which separates the two cleanly and without heating. Option A, distillation, is used for miscible liquids whose boiling points differ widely, such as acetone and water, and would be a wasteful and unnecessary way to deal with liquids that have already separated by themselves. Option C, sublimation, separates a solid that passes directly into vapour, such as ammonium chloride or camphor, from one that does not, and has nothing to do with two liquids. Option D, crystallisation, recovers a pure solid from its solution by slow cooling. The word that decides the answer is immiscible.

The correct answer is B, less than 25 kelvin. When two miscible liquids boil at temperatures far apart, ordinary distillation is enough, because the more volatile liquid boils off almost completely before the other begins. When the boiling points are close, the vapour carries both liquids and a single distillation cannot separate them, so a fractionating column is fitted between the flask and the condenser. The vapour condenses and evaporates repeatedly as it climbs the column, giving in effect many distillations in one, and the conventional limit at which this becomes necessary is a boiling point difference of less than twenty-five kelvin. Petroleum refining and the separation of oxygen, nitrogen and argon from liquid air both use it. Option A, more than 100 kelvin, describes exactly the case in which simple distillation suffices. Option C is an arbitrary figure with no standing. Option D, zero, would mean the liquids boil together and could not be separated this way at all.

The correct answer is B, copper and zinc. Brass is made by melting copper with zinc, and the resulting homogeneous mixture is harder than copper, easier to cast and machine and gives the familiar yellow colour used for utensils, fittings and musical instruments. Option A, copper and tin, is bronze, which is the alloy of the Bronze Age and is harder still; brass and bronze are the pair most often exchanged in the options, so the way to hold them is zinc for brass and tin for bronze. Option C, copper and nickel, gives cupronickel, used for coins and for condenser tubes, and copper with zinc and nickel together gives German silver. Option D, iron and carbon, is steel, in which a small proportion of carbon makes iron far stronger, and adding chromium and nickel to it gives stainless steel. Every one of these is a homogeneous mixture and not a compound, which is why the proportions can be varied.

The correct answer is B, sublimation. Ammonium chloride passes directly from the solid to the vapour state on heating, without melting, while common salt does not. If the mixture is heated in a china dish covered with an inverted funnel, the ammonium chloride sublimes, travels up as white fumes and deposits again as a solid on the cooler inner surface of the funnel, leaving the salt behind in the dish. Camphor, naphthalene and iodine behave in the same way and are separated by the same method. Option A, filtration, separates an insoluble solid from a liquid and cannot act on two dry solids. Option C, centrifugation, spins a liquid so that denser particles move outward, as when cream is taken from milk, so it too needs a liquid. Option D, a separating funnel, is for two immiscible liquids. The clue in any such question is the presence of one subliming substance.

The correct answer is A, chromatography. Chromatography separates substances that are dissolved in the same solvent and present in very small quantities, by letting the solution creep along a strip of filter paper. Each dye is held by the paper and carried by the solvent to a different degree, so the dyes come to rest at different distances up the strip and appear as separate bands of colour. The method is also used to separate the pigments of a leaf and to detect drugs in blood or urine. Option B, evaporation, would simply drive off the solvent and leave all the dyes mixed together in the residue. Option C, sieving, separates solid particles of different sizes and cannot act on substances dissolved in a liquid. Option D, magnetic separation, works only when one component is attracted by a magnet, as iron filings are when mixed with sand, and dyes are not magnetic.

The correct answer is C, a salt solution. When salt dissolves in water it breaks up into separate ions, which are far smaller than the wavelength of visible light, so they cannot scatter a beam sideways and the path of light through the liquid stays invisible. This absence of scattering is the mark of a true solution. Option A, fog, is a colloid of water droplets dispersed in air, and it scatters light so strongly that a torch beam or a headlight becomes a visible cone. Option B, smoke, is a colloid of solid carbon and ash particles in air and scatters light in the same way, which is why a shaft of sunlight shows up in a smoky room. Option D, milk, is a colloidal emulsion of fat globules in water and is the standard classroom example of the effect. The underlying rule is that only particles of colloidal size, between one and one thousand nanometres, scatter light appreciably.

The correct answer is B, impurities remain dissolved in the mother liquor. In crystallisation a hot saturated solution is allowed to cool slowly, and as it cools only the required substance comes out of solution in the regular form of crystals while the impurities, present in smaller amounts, stay dissolved in the liquid that is left behind, which is called the mother liquor. Filtering off the crystals therefore leaves the impurities behind and gives a far purer solid than evaporation, which drives away the solvent and leaves everything else in the dish together. Option A is wrong because crystallisation is slower, not quicker, since slow cooling is essential for good crystals. Option C is wrong because the solution has to be heated first to make it saturated; what crystallisation avoids is the strong prolonged heating that would decompose a substance such as sugar. Option D is wrong because the method applies only to soluble solids.

The correct answer is A, one to eight by mass. A molecule of water contains two atoms of hydrogen and one of oxygen. Taking the atomic masses as one for hydrogen and sixteen for oxygen, the two hydrogen atoms contribute two units of mass and the oxygen atom sixteen, so the ratio by mass is two to sixteen, which reduces to one to eight. Option B, eight to one by mass, is the same figure with the order reversed, and reversing it is the commonest mistake, so note that oxygen is the heavier part. Options C and D confuse mass with volume: by volume, or equivalently by the number of atoms, hydrogen and oxygen are in the ratio of two to one, which is what the formula for water states directly. A fixed ratio of this kind is exactly what makes water a compound rather than a mixture, and it holds for every sample of pure water from any source.

The correct answer is D, only B, that is a liquid. A colloid is named according to the states of its two parts, the dispersed phase and the dispersing medium, and an emulsion is defined as a colloid in which a liquid is dispersed in another liquid, so the dispersing medium must itself be a liquid. Milk, in which fat globules are dispersed in water, and butter, in which water is dispersed in fat, are the standard examples. Option B, only A, names a gas, but a liquid dispersed in a gas is an aerosol, which is what fog, mist and a cloud are, and not an emulsion. Option A, A and C only, names a gas and a solid, so it rules out the one correct medium. Option C, B and C only, correctly includes liquid but wrongly adds solid, and a liquid dispersed in a solid is called a gel, as in cheese and jelly. Fix the set as sol for a solid in a liquid, emulsion for a liquid in a liquid, aerosol for either in a gas and gel for a liquid in a solid.