Free Industrial Chemistry MCQs with Answers

11. A dye differs from a pigment in that a dye

• A. is always black
• B. is insoluble and sits on the surface as fine particles
• C. is soluble in the medium and becomes attached to the fibre
• D. contains no colour at all

Explanation: A dye dissolves and bonds to the fibre by ionic, hydrogen or covalent bonds, so the colour becomes part of the material and resists washing, whereas a pigment is insoluble and must be held in place by a binder as in paint. This is why a dyed cloth keeps its colour through the wash while a painted surface can flake. Fastness to washing and to light is the main measure of dye quality.

12. The part of a dye molecule responsible for its colour is called the

• A. auxochrome
• B. chromophore
• C. monomer
• D. catalyst

Explanation: A chromophore is an unsaturated group such as the azo linkage or a nitro group whose delocalised electrons absorb visible light, and the wavelength absorbed determines the colour seen. An auxochrome such as the hydroxyl or amino group deepens the colour and, more importantly, helps the dye bind to the fibre. A molecule needs both to be a useful dye.

13. Azo dyes contain the group

• A. N=N joining two aromatic rings
• B. C=O
• C. OH only
• D. COOH

Explanation: The azo group links two aromatic systems into one extended conjugated network, and the resulting absorption in the visible region makes these compounds strongly coloured, which is why azo dyes are the largest commercial class. They are made by coupling a diazonium salt with a phenol or an aromatic amine. Varying the rings and their substituents produces the whole range of shades.

14. A mordant is used in dyeing to

• A. bleach the fabric before dyeing
• B. dissolve the dye in water
• C. remove excess dye after the process
• D. fix a dye to a fibre it would not otherwise bind to, usually by forming a metal complex

Explanation: A mordant, commonly a salt of aluminium, chromium or iron, is absorbed by the fibre first and then forms a coordination complex with the dye, anchoring it to the cloth. The metal used affects the final shade, so one dye with different mordants gives different colours. Direct dyes need no mordant because they bind to the fibre unaided.

15. Terylene, a polyester, is formed by the condensation of

• A. two amino acids
• B. a diol with a dicarboxylic acid
• C. two molecules of ethene
• D. glucose units

Explanation: Ethane-1,2-diol reacts with benzene-1,4-dicarboxylic acid so that ester linkages form and water is eliminated, giving a polymer used for clothing fibre and drink bottles. A diamine with a diacid would give a polyamide such as nylon instead. Each monomer must be difunctional for a chain rather than a single small molecule to result.

16. Teflon, used for non stick coatings, is a polymer of

• A. tetrafluoroethene
• B. chloroethene
• C. propene
• D. styrene

Explanation: Every hydrogen of ethene is replaced by fluorine, and the resulting carbon to fluorine bonds are so strong and the surface so non polar that the polymer is chemically inert, heat resistant and extremely slippery. This is why teflon is used in cookware and in laboratory taps and seals. Chloroethene gives PVC and styrene gives polystyrene.

17. Natural rubber is made harder and more elastic over a wide temperature range by vulcanisation, in which it is heated with

• A. carbon black
• B. sulphur, which forms cross links between the polymer chains
• C. water under pressure
• D. sodium hydroxide

Explanation: Short chains of sulphur atoms bridge the polyisoprene chains, so they can no longer slide past one another permanently and the rubber recovers its shape instead of deforming, remaining useful when hot or cold. The more sulphur is added, the harder the product becomes, with ebonite at the extreme. Carbon black is also added to tyres but acts as a reinforcing filler rather than a cross linking agent.

18. Recycling thermoplastics is possible but recycling thermosetting plastics is not, because thermosetting plastics

• A. are always coloured
• B. are made from natural materials
• C. cannot be melted and reshaped once their cross links have formed
• D. dissolve in water

Explanation: The permanent covalent network of a thermoset means heating it decomposes the material rather than softening it, so it can only be ground into filler or burnt for energy recovery. A thermoplastic has independent chains held by weak forces, so it can be melted and re-formed many times. This is why sorting plastic waste by type is the first step in any recycling scheme.

19. Which polymer is used to make drinking bottles and is widely recycled?

• A. Bakelite
• B. Polyethylene terephthalate
• C. Urea formaldehyde resin
• D. Melamine resin

Explanation: Polyethylene terephthalate is a thermoplastic polyester that is clear, tough and impermeable to carbon dioxide, and because it melts without decomposing it can be recycled into fibre or new bottles. The other three are thermosetting resins used for electrical fittings, adhesives and tableware and cannot be remelted. Recycling codes on packaging identify the polymer for exactly this reason.

20. The general requirement for a monomer to undergo addition polymerisation is that it must contain

• A. at least two different functional groups
• B. a benzene ring
• C. a hydroxyl group
• D. a carbon to carbon double bond

Explanation: The pi bond opens so that each unit can bond to the next with nothing eliminated, which is why ethene, propene, chloroethene and styrene all polymerise this way. Two different functional groups are what condensation polymerisation needs, as in the diamine and diacid of nylon. Checking for the double bond is the quickest way to classify an unfamiliar polymerisation.