To which of the following family of elements does potassium belong?

The correct answer is B, London dispersion forces. These are the weak attractions that appear when the electron cloud of an atom becomes momentarily lopsided, creating a temporary dipole which then induces a matching dipole in a neighbouring atom. Named after Fritz London, they are the weakest of the van der Waals forces and fall away very sharply with distance, so they count only over a few hundred picometres. They exist between all atoms and molecules and are the only forces holding non-polar substances such as helium, argon and methane together, which is why such gases liquefy only at very low temperatures. Their strength grows as the electron cloud gets larger. A is wrong because dipole-dipole forces act between molecules that already carry permanent dipoles. C is wrong because dipole-induced dipole forces need one permanent dipole to begin with. D is wrong because a hydrogen bond is a far stronger attraction, formed where hydrogen is bonded to nitrogen, oxygen or fluorine. Exam tip: strength order - hydrogen bond, then dipole-dipole, then London dispersion.

The correct answer is D, Sodium benzoate. Sodium benzoate is the sodium salt of benzoic acid and is the preservative most often added to sauces, salad dressings, jams, pickles and soft drinks. It works best in acidic foods, where part of it turns into benzoic acid, and that stops the growth of moulds, yeasts and many bacteria. It dissolves easily in water, costs little and does not change the taste of the food, which is why it is so widely used; on labels it appears as INS 211 or E211. A is wrong because sodium bicarbonate is baking soda, used as a raising agent in cooking and as an antacid. B is wrong because sodium fluoride is added to toothpaste and drinking water to protect teeth, not to preserve food. C is wrong because sodium hypochlorite is the bleaching and disinfecting agent used in household bleach. Exam tip: common food preservatives - sodium benzoate, sodium metabisulphite and potassium metabisulphite; salt and sugar are the oldest of all.

The correct answer is C, Intercellular spaces are present. Permanent tissues are made of cells that have stopped dividing and have taken on a fixed shape, size and function, and in simple permanent tissues such as parenchyma the loosely packed cells leave clear intercellular spaces through which gases can move. Permanent tissue is formed when cells produced by meristematic tissue undergo differentiation; the simple kinds are parenchyma, collenchyma and sclerenchyma, while xylem and phloem are the complex kinds. Such cells are usually large, with a big central vacuole and a thin lining of cytoplasm. A is wrong because repeated division is the mark of meristematic tissue, not of permanent tissue. B is wrong because permanent cells often hold inorganic inclusions such as crystals of calcium oxalate. D is wrong because large vacuoles are typical of permanent cells, while it is meristematic cells that lack them. Exam tip: meristematic - small cells, dense cytoplasm, no vacuole, no intercellular spaces; permanent tissue is the opposite on every count.

The correct answer is C, Scandium, gallium and germanium. Mendeleev arranged the elements by increasing atomic mass in 1869 and left blank spaces where his pattern demanded an element that was not yet known. He described in advance the mass, density and compounds of three of them, calling them eka-boron, eka-aluminium and eka-silicon, and when scandium, gallium and germanium were discovered they matched his descriptions closely, which won the periodic law wide acceptance. Option A is wrong because sodium, potassium and calcium were all well known long before 1869. Option B is wrong because cobalt, nickel and copper were known too, and cobalt and nickel were in fact the pair he had to place out of strict mass order. Option D is wrong because radium, radon and rhenium were found much later and were not among his three predictions.

The correct answer is B, J. J. Thomson. After discovering the electron in 1897, J. J. Thomson pictured the atom as a sphere of positive charge with negatively charged electrons stuck in it, a picture often compared to plums in a pudding or seeds in a watermelon. It explained why an atom is electrically neutral, but it had no nucleus and could not survive Rutherford's scattering experiment. Option A is wrong because John Dalton's 1808 theory treated the atom as indivisible and offered no internal structure at all. Option C is wrong because Rutherford replaced this model with the nuclear one after the gold foil experiment. Option D is wrong because Niels Bohr then added fixed permitted shells to Rutherford's nucleus in order to explain why the atom is stable.