All Free Chemistry MCQs with Answers

341. 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.

342. 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.

343. 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.

344. 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.

345. The linkage that joins amino acid units in a protein is

• A. a glycosidic linkage
• B. a peptide linkage, formed by condensation between a carboxyl and an amino group
• C. an ester linkage
• D. a hydrogen bond

Explanation: The carboxyl group of one amino acid condenses with the amino group of the next, releasing water and leaving a CO-NH amide link known as the peptide bond. Glycosidic links join sugars and ester links join glycerol to fatty acids, so each class of macromolecule has its own characteristic linkage. Hydrogen bonds are important in proteins but hold the chain in shape rather than joining the units.

346. An amino acid is described as amphoteric because it

• A. dissolves only in organic solvents
• B. contains a benzene ring
• C. contains both an acidic carboxyl group and a basic amino group, so it reacts with both acids and bases
• D. has no charge under any conditions

Explanation: The carboxyl group can donate a proton and the amino group can accept one, so an amino acid neutralises both acids and alkalis. Within a single molecule the proton transfers from one group to the other, giving the doubly charged zwitterion that exists at intermediate pH. This ionic character is why amino acids are crystalline solids with high melting points and dissolve readily in water.

347. The pH at which an amino acid exists mainly as a zwitterion and does not migrate in an electric field is called its

• A. neutral point
• B. isoelectric point
• C. equivalence point
• D. melting point

Explanation: At the isoelectric point the positive and negative charges within the molecule exactly balance, so the net charge is zero and the amino acid is at its least soluble and does not move towards either electrode. The value differs for each amino acid according to its side chain. Electrophoresis separates amino acids and proteins by exploiting these differences.

348. Proteins are classified as fibrous or globular. A fibrous protein such as keratin is

• A. insoluble in water and structural in function
• B. soluble in water and catalytic in function
• C. always an enzyme
• D. made of a single amino acid repeated

Explanation: Fibrous proteins consist of long parallel chains held in bundles, which makes them tough, insoluble and suited to structural roles in hair, nails, tendons and skin. Globular proteins fold into compact soluble balls with hydrophilic groups outside, which is what enzymes, antibodies and haemoglobin require. Both types are built from the same twenty amino acids in different sequences.

349. The secondary structure of a protein, such as the alpha helix, is maintained by

• A. peptide bonds between amino acids
• B. ionic bonds between side chains only
• C. hydrogen bonds between the carbonyl and amide groups of the backbone
• D. disulphide bridges only

Explanation: Regular coiling or pleating arises because each carbonyl oxygen hydrogen bonds to an amide hydrogen further along the same chain, giving the alpha helix and the beta pleated sheet. Peptide bonds create the primary sequence, and side chain interactions such as disulphide bridges belong to the tertiary level. Heat breaks the weak hydrogen bonds and the structure unwinds, which is denaturation.

350. Denaturation of a protein involves

• A. hydrolysis of all its peptide bonds
• B. loss of its three dimensional shape while the amino acid sequence remains intact
• C. a change in its amino acid sequence
• D. conversion of the protein into a carbohydrate

Explanation: Heat, extreme pH, heavy metal ions and alcohol break the weak interactions holding the fold, so the chain unravels and biological activity is lost, but the covalent peptide backbone survives. This is why a boiled egg sets irreversibly without breaking down into amino acids. Hydrolysis of the peptide bonds is digestion, a different process requiring enzymes or prolonged heating with acid.