Which of the following is a process in which hot, less dense materials rise upward and are replaced by colder, more dense materials?

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.

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 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 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 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.