Chemistry Questions and Answers

Answer: A

Polyvinyl chloride is a good electrical insulator.

PVC has no free electrons or ions to carry current and resists moisture, so it is widely used to insulate electric wires and cables and to make switch boards and conduits.

A conductor is the opposite: metals such as copper and aluminium conduct because they have free electrons. A bulb and a transmitter are devices, not properties of a material, so they cannot complete the sentence.

Answer: A

Gypsum is added to Portland cement to increase the setting time.

Without gypsum, the tricalcium aluminate in cement clinker reacts with water almost instantly (flash setting), leaving no time to mix and place the concrete. Gypsum (calcium sulfate) reacts with the aluminate to form ettringite, which coats the grains and slows the reaction, so the cement sets in a workable time.

Gypsum therefore retards setting rather than speeding it up, and it is not added for softness or colour.

Answer: D

None of these alloys contains silver, so the answer is None of these.

German silver (nickel silver) is an alloy of copper, zinc and nickel; it gets its name only from its silvery appearance. Brass is copper and zinc, and bronze is copper and tin.

'German silver' is the trap: despite the name, it contains no silver at all. It was developed in Germany in the nineteenth century as a cheap imitation of silver for cutlery and ornaments, which is how it got its name.

Answer: B

Common salt is Sodium chloride, NaCl.

Table salt is mainly sodium chloride, obtained by evaporating sea water or mining rock salt. It is often iodised by adding a little potassium iodide or iodate.

Sodium bicarbonate (NaHCO₃) is baking soda. Sodium hydroxide (NaOH) is caustic soda, a strong alkali. Sodium salicylate is a salt of salicylic acid used as a painkiller. None of them is eaten as salt.

Answer: C

The gas essential for fire is oxygen.

Burning is a rapid reaction of a fuel with oxygen, and a fire needs three things together: fuel, heat and oxygen (the fire triangle). Take any one away and the fire goes out, which is why smothering a fire works.

Nitrogen does not support burning; it makes up most of the air but only dilutes the oxygen. Carbon dioxide actually puts fires out, which is why it is used in extinguishers. Since oxygen is the answer, 'None of these' is wrong.

Answer: B

The gas present in the largest proportion in the atmosphere is nitrogen.

Dry air is about 78% nitrogen and 21% oxygen by volume, with argon close to 1% and carbon dioxide only about 0.04%.

Oxygen is second, well behind nitrogen. Acetylene is not a natural component of air at all; it is a fuel gas made from calcium carbide for welding and cutting. Carbon dioxide, though important for climate, is a trace gas.

Answer: A

Iodised salt is a mixture of potassium iodide and common salt.

Common salt (sodium chloride) is fortified by adding a small amount of an iodine compound, traditionally potassium iodide and now often potassium iodate, to prevent iodine deficiency disorders such as goitre. The additive is simply mixed in, and each substance keeps its own properties, so the product is a mixture, not a compound.

Molecular iodine is not used, because it is volatile and would escape, and it does not combine with sodium chloride to form a compound. 'Only potassium iodide' is wrong because most of iodised salt is still common salt.

Answer: A

Vinegar is a solution of 5 - 20% acetic acid in water.

Vinegar is made by fermenting an alcohol solution with acetic acid bacteria, which oxidise the ethanol to acetic acid (ethanoic acid). Table vinegar usually has around 5 to 8 percent acetic acid, and stronger grades used for pickling and cleaning go higher, which is why textbooks give it as a dilute solution within this range.

Solutions of 25 to 30 percent, 40 to 45 percent and 50 percent are concentrated acetic acid, which is corrosive and not used as food vinegar. Only the first option describes a dilute solution of the strength vinegar actually has.

Answer: B

Only statement 2 is correct, so the answer is 2 only.

A soap molecule is the sodium or potassium salt of a long-chain fatty acid. The ionic carboxylate end is the hydrophilic head and the long hydrocarbon chain is the hydrophobic tail. Statement 1 has them the wrong way round.

Above the critical micelle concentration, soap molecules cluster into micelles of colloidal size, with tails inside and heads outside. A colloidal solution scatters a beam of light, the Tyndall effect, so statement 2 is correct.

Answer: A

Both A and R are true and R is the correct explanation of A.

Stainless steel contains at least about 10.5% chromium. Chromium at the surface reacts with oxygen to form a very thin, tightly bound layer of chromium oxide. This passive film keeps oxygen and moisture away from the iron beneath and re-forms quickly if scratched, which is why stainless steel resists rusting.

So the Reason is not merely true alongside the Assertion; it is the mechanism behind it.