All Free Chemistry MCQs with Answers

331. The major environmental problem with most addition polymers is that they

• A. dissolve in rainwater and pollute rivers
• B. react violently with air
• C. are non biodegradable, because their inert carbon backbone resists attack by microorganisms
• D. release oxygen as they age

Explanation: The saturated carbon chain has no polar groups for enzymes to attack, so plastic waste persists in landfill and in the oceans for centuries. Burning it disposes of the material but can release toxic gases, PVC giving hydrogen chloride and dioxins. Research into biodegradable and photodegradable polymers, and improved recycling of thermoplastics, addresses this directly.

332. An adhesive works by

• A. melting the two surfaces together
• B. forming intermolecular forces or chemical bonds with both surfaces while cohering strongly within itself
• C. removing all air from between the surfaces only
• D. converting one surface into the other

Explanation: A good adhesive must both adhere, sticking to each surface, and cohere, holding together internally, and it usually starts as a liquid so that it can flow into the microscopic irregularities before setting. Failure at the interface is adhesive failure while failure within the glue itself is cohesive failure. Welding, by contrast, does melt the surfaces and is not adhesion.

333. Epoxy resin adhesives are supplied as two tubes that must be mixed before use because

• A. one tube contains the solvent that evaporates
• B. the two components are the same substance at different concentrations
• C. mixing lowers the temperature of the joint
• D. the resin and the hardener react together to form cross links, setting the adhesive

Explanation: Setting is a chemical reaction between the resin and a hardener that builds a rigid cross linked network, so the two must be kept apart until the moment of use. Because no solvent has to evaporate, epoxies set even in a sealed joint and bond metals, glass and ceramics strongly. Adhesives that set by solvent evaporation, such as rubber cement, work on a quite different principle.

334. Thermoplastic adhesives such as hot melt glue set by

• A. cooling and solidifying, with no chemical change
• B. reacting chemically with the surface
• C. absorbing water from the air
• D. releasing carbon dioxide

Explanation: The adhesive is applied molten, flows into the surface irregularities and then simply freezes as it cools, so the change is physical and reheating the joint softens it again. This makes such adhesives quick and convenient but unsuitable where the joint may become warm. Thermosetting adhesives such as epoxies form permanent cross links instead.

335. 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.

336. 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.

337. 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.

338. 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.

339. 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.

340. 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.