A mixture of common salt and ammonium chloride can be separated by

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.