Which gas is added to liquefied petroleum gas so that a leakage can be detected?

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.