Chemistry Questions and Answers

Answer: B

The fibre known as artificial silk is Rayon.

Rayon is made by regenerating cellulose from wood pulp (it is semi-synthetic). Its smooth, lustrous fibres look and feel like silk, and it was first marketed as 'artificial silk'.

Nylon was the first fully synthetic fibre and was developed as a substitute for silk in stockings, which makes it a common trap, but the name 'artificial silk' belongs to rayon. Terylene is a polyester, and cotton is a natural fibre.

Answer: C

The metallic constituents of hard water are calcium, magnesium and iron.

Water becomes hard by dissolving salts of calcium and magnesium, as bicarbonates, chlorides and sulphates, from rocks such as limestone and dolomite. These ions form scum with soap and scale in boilers. Dissolved iron, common in groundwater, also contributes to hardness and staining.

Tin is not a constituent of natural hard water, so every option that includes tin is wrong.

Answer: A

The formula keyed is KAl(SO₄)₂ (printed KAI, standing for K and Al).

Potash alum is potassium aluminium sulphate, a double salt of potassium sulphate and aluminium sulphate. Its full formula is KAl(SO₄)₂·12H₂O, often written K₂SO₄·Al₂(SO₄)₃·24H₂O; the option gives the salt without its water of crystallisation. Each formula unit holds one K⁺, one Al³⁺ and two SO₄²⁻ ions, so the charges balance.

KAl(H₂O)₂ has no sulphate at all, and KAlSO₄ has only one sulphate for four positive charges, so it does not balance. 'None of these' is wrong because the first option is the alum.

Answer: A

The pH will be less than 6.

Milk turns sour when lactic acid bacteria ferment its sugar, lactose, into lactic acid. More acid means a higher concentration of hydrogen ions, and a lower pH. Fresh milk is close to pH 6.5 to 6.8; sour milk and curd fall to about 4.5.

'More than 6' would mean the milk had become less acidic, the opposite of souring. 'Neutral (pH=7)' is wrong because souring adds acid. 'No change in pH' ignores the lactic acid altogether.

Answer: B

CNG is mainly methane.

Compressed Natural Gas is natural gas compressed to about 200 to 250 times atmospheric pressure for use as a vehicle fuel. Natural gas is mostly methane (CH₄), typically 80 to 90 per cent or more, with small amounts of ethane, propane and other gases. Because methane burns more cleanly than petrol or diesel, CNG produces less particulate pollution.

Butane and propane are the main components of LPG (liquefied petroleum gas), the cooking gas, and ethane is only a minor component of natural gas.

Answer: B

The hardest substance among these is diamond.

Diamond is a crystalline form of carbon in which each atom is bonded strongly to four others in a rigid three-dimensional network. That makes it the hardest naturally occurring substance, rated 10 on the Mohs scale, which is why it is used to cut glass and drill rock.

Platinum, silver and gold are metals whose atoms can slide past one another, so they are soft and malleable by comparison (gold and silver about 2.5 to 3 on the Mohs scale, platinum about 4 to 4.5).

Answer: B

The gas used in an electric bulb is an inert gas (argon).

The tungsten filament of an incandescent bulb glows at about 2,500 °C. If the bulb held air, the oxygen would burn the filament instantly. It is therefore filled with an inert gas, usually argon with some nitrogen, which does not react with the hot tungsten and also reduces its evaporation, so the filament lasts longer.

Oxygen would make the filament burn out, hydrogen is flammable, and nitrogen oxide is a reactive pollutant gas, not a bulb filler.

Answer: D

The one that is not a synthetic polymer is resins.

Resins in the ordinary sense are natural products: viscous secretions of plants, especially conifers, such as rosin and lac. They occur in nature rather than being made in a factory.

Polythene (from ethene), nylon-6,6 (from hexamethylenediamine and adipic acid) and Buna-S (styrene-butadiene rubber) are all man-made polymers. Synthetic resins also exist, but the plain term 'resins' is the natural material, which is why it is the odd one out.

Answer: A

The correct matching is (a)-(iii), (b)-(iv), (c)-(i), (d)-(ii).

1. Neoprene is a polymer of chloroprene (2-chloro-1,3-butadiene).
2. Buna-N is a copolymer of 1,3-butadiene and acrylonitrile.
3. Glyptal is a polyester of ethylene glycol and phthalic acid.
4. Bakelite is a condensation polymer of phenol and formaldehyde.

Option B swaps glyptal and bakelite, and options C and D give neoprene the butadiene-acrylonitrile pair that belongs to Buna-N.

Answer: B

The synthetic polymer is Nylon-6, 6.

Nylon-6,6 is a polyamide made in the laboratory and in factories by condensation polymerisation of hexamethylenediamine and adipic acid, each of which has six carbon atoms, hence the name. It is used for fibres, ropes, tyre cords and moulded parts.

Starch and cellulose are natural polymers of glucose made by plants, and proteins are natural polymers of amino acids made by living cells, so all three are natural rather than synthetic polymers.