Everyday Chemistry and Common Names of Compounds

The correct answer is A, it has free electrons. In graphite each carbon atom is joined to only three other carbon atoms in flat hexagonal layers, so the fourth valence electron of every atom is not used in bonding and can move freely along the layer. These mobile electrons carry current, which is why graphite is used to make electrodes and the brushes of electric motors.

Option B is wrong because being an allotrope of carbon explains nothing by itself: diamond is also an allotrope of carbon and does not conduct electricity at all, since all four of its electrons are locked in bonds. Option C is wrong because brittleness is a mechanical property and has no bearing on conduction. Option D is wrong because graphite does not form a basic oxide; carbon dioxide is acidic in nature, and oxide character is unrelated to electrical conductivity.

The correct answer is D, carbohydrates. A dilute solution of iodine gives a deep blue-black colour with starch, which is a polysaccharide and therefore a carbohydrate. The colour appears because iodine molecules slip into the coiled amylose chains of starch, and the test is used on rice, potato and bread in school laboratories and in food testing.

Option A is wrong because cholesterol is detected by the Liebermann-Burchard or Salkowski reaction, not by iodine. Option B is wrong because fat is shown by the translucent spot test on paper or by Sudan III staining. Option C is wrong because protein is detected by the biuret test, which gives a violet colour, or by the ninhydrin test. Iodine belongs only with starch, and that is the pairing to remember.

Remember also that iodine itself is an element, and the blue-black colour disappears on heating and returns on cooling.

The correct answer is B, rayon. Rayon is made by dissolving the cellulose of wood pulp in a chemical and forcing the solution through fine holes to form filaments. Because it is made from a natural polymer rather than built from small molecules, it is called a regenerated or semi-synthetic fibre, and its silk-like sheen has earned it the name artificial silk.

Option A is wrong because nylon is fully synthetic, the first fibre made entirely from coal, water and air, with no plant material in it. Option C is wrong because silk is a natural animal fibre obtained from the cocoon of the silkworm and is not man-made at all. Option D is wrong because polyester, of which terylene is the best known form, is a fully synthetic fibre made from petroleum products. Only rayon starts from wood pulp.

The correct answer is D, boric acid. Boric acid dissolves in water to give a true solution, in which the dispersed particles are individual molecules small enough to pass through a filter paper and even a parchment membrane, and the mixture shows no Tyndall effect. A true solution is therefore not a colloid.

Option A is wrong because milk is a colloid, an emulsion of liquid fat droplets dispersed in water. Option B is wrong because mud is a colloidal dispersion of fine soil particles in water, which is why it stays turbid for a long time before settling. Option C is wrong because butter is also a colloid, with water dispersed in fat, that is a gel-like emulsion. The rule to apply is that only a true solution is excluded from the colloidal state.

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 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, 3 : 1. Aqua regia is a freshly prepared mixture of three parts concentrated hydrochloric acid to one part concentrated nitric acid. Neither acid alone attacks gold, but together they do, because the nitric acid oxidises the metal while the chloride ions hold the gold in solution as a complex. It is used to refine and test gold and platinum.

Option A is wrong because it reverses the proportion and would leave too little hydrochloric acid to form the complex. Option C is wrong because an equal mixture is not the recognised composition. Option D is wrong because no standard preparation uses two to three. The name itself is a clue: aqua regia means royal water, since it dissolves the royal metals, while aqua fortis, or strong water, is nitric acid alone.

The correct answer is B, lactic acid. When milk is set into curd, the bacteria Lactobacillus convert the milk sugar lactose into lactic acid, and it is this acid that gives curd its sour taste and makes the milk protein clot. The same acid accumulates in muscles during heavy exercise when oxygen supply is insufficient.

Option A is wrong because citric acid is found in lemon, orange and other citrus fruits. Option C is wrong because tartaric acid is found in tamarind and in grapes, and is also a component of baking powder along with baking soda. Option D is wrong because oxalic acid is found in tomato, spinach and sorrel, and is the acid whose calcium salt can form kidney stones. The pairing of each food with one acid is what examiners test.

The correct answer is B, sodium hydrogen carbonate. In a soda-acid extinguisher a solution of sodium hydrogen carbonate is kept in the body of the cylinder and sulphuric acid in a sealed glass phial. When the knob is struck the phial breaks, the acid meets the bicarbonate and a large volume of carbon dioxide is produced. The gas drives the water out of the nozzle and also blankets the fire, cutting off oxygen.

Option A is wrong because sodium chloride does not react with sulphuric acid to give a gas that puts out fire. Option C is wrong because calcium oxide reacts with water giving off heat, which would make matters worse. Option D is wrong because potassium nitrate is an oxidising agent used in gunpowder and would feed a fire rather than stop it.

The correct answer is A, boiling the water. Temporary hardness is caused by the dissolved bicarbonates of calcium and magnesium. On boiling, these bicarbonates decompose into insoluble carbonates, which settle out as the scale seen in a kettle, so the water that remains is soft. Permanent hardness, caused by the sulphates and chlorides of the same metals, is not removed this way and needs washing soda or an ion exchange resin.

Option B is wrong because common salt does not remove hardness; it is in fact used to regenerate the resin in a water softener, which is a different step. Option C is wrong because filtering removes suspended dirt, not dissolved salts. Option D is wrong because adding vinegar only makes the water acidic and leaves the calcium and magnesium in solution.

The correct answer is B, ethyl mercaptan. Liquefied petroleum gas is a mixture of butane and propane and is by itself almost odourless, so a leak could fill a room unnoticed. A trace of ethyl mercaptan, a sulphur compound with a strong unpleasant smell, is therefore added as an odourant, and it is that smell people recognise as the smell of cooking gas.

Option A is wrong because carbon monoxide is odourless and highly poisonous, so it would be useless and dangerous as a warning agent. Option C is wrong because nitrous oxide is laughing gas, used as an anaesthetic, and has only a faintly sweet smell. Option D is wrong because chlorine is a corrosive greenish gas used to disinfect water and would attack the cylinder and the burner.

A related fact is that compressed natural gas, used in vehicles, is mainly methane and is also odourised for the same reason.

The correct answer is B, mercuric sulphide, with the formula HgS. The natural mineral form of mercuric sulphide is cinnabar, the chief ore of mercury, and its bright red powdered form has been used as the pigment vermilion for centuries. Because mercury compounds are toxic, modern sindoor is usually made from safer synthetic dyes.

Option A is wrong because lead oxide includes red lead, used in primer paint, and litharge, but neither is called vermilion. Option C is wrong because zinc oxide is the white pigment known as Chinese white or philosopher's wool and is used in ointments and sunscreen. Option D is wrong because ferric oxide is the reddish brown pigment of haematite and of rust, a different red from vermilion.

Learning the pigment names in pairs helps here: vermilion with mercury, litharge with lead, Chinese white with zinc and haematite with iron, because papers rotate these four.