Why do plastics not degrade easily?

Correct — B, They have strong covalent bonds. A plastic is a polymer: thousands of small molecules joined end to end into a very long chain whose spine is carbon joined to carbon, with the occasional oxygen or nitrogen, by covalent bonds. Those bonds are strong, and nothing in an ordinary soil or seawater environment carries enough energy to break them at random. Biodegradation is not a matter of energy alone, though — it is enzymatic. Microorganisms break down leaf litter, wood and food waste because they possess enzymes shaped to attack the particular linkages in cellulose, starch and protein. The carbon-carbon chain of polythene or polypropylene is a linkage they have never had to evolve against, so the chain simply sits there, fragmenting mechanically into ever smaller pieces without ever being consumed. That is why plastic waste persists for decades and why microplastics accumulate. Strength of bond plus absence of an enzyme that fits is the full explanation, and of the four options only (b) names the bond correctly.

• (a)They have strong ionic bonds. — Plastics are not ionic. Ionic solids such as common salt are held together by attraction between oppositely charged ions, have high melting points, and dissolve in water to give a conducting solution — none of which describes a polythene bag.
• (c)They have strong metallic bonds. — Metallic bonding is the attraction between a lattice of positive metal ions and a sea of free electrons, and it is what makes metals conduct heat and electricity. Plastics are famously insulators, which is why wires are sheathed in them.
• (d)They have very high melting points (> 500 °C). — False as a statement of fact, quite apart from being the wrong explanation. Common plastics melt far below 500 °C — polythene softens around 130 °C and a plastic bottle will deform in boiling water. Plastics resist microbes, not heat; many are shaped by melting them.

Polymers are long chains built from repeating small units called monomers. In the plastics of everyday use — polythene, polypropylene, polystyrene, PVC, PET — the chain's backbone is a run of carbon atoms held to one another by covalent bonds, formed by sharing electron pairs. NCERT's Class 10 chemistry makes the same point about carbon compounds generally: the bonds within the molecule are strong, while the forces between molecules are weak. Biodegradation depends on microbes having an enzyme that recognises and cuts a particular bond, and no common soil organism has one shaped for a plain carbon-carbon chain.

The question is really asking you to say what kind of bond holds a plastic together, and three of the four options can be ruled out by thinking about what those bonds would imply. Ionic would mean a salt-like, water-soluble solid; metallic would mean an electrical conductor; and the melting-point option is simply not true of plastics. That leaves covalent, which is also the right answer on the merits. A useful refinement to carry away is that strong bonds are only half the story — diamond has very strong covalent bonds too and is not a pollution problem, because it is inert and inedible rather than persistent in the environment. What makes plastic a problem is that its bonds are both strong and unfamiliar to living things, so the material lasts for decades while breaking into smaller and smaller fragments.

• A plastic is a polymer — a long chain of repeating monomer units joined by covalent bonds along a carbon backbone.
• Covalently bonded molecules have strong bonds inside the molecule but weak forces between molecules, which is why plastics melt at low temperatures yet resist chemical attack.
• Biodegradation is enzymatic: microbes decompose cellulose, starch and protein because they carry enzymes shaped for those linkages, and none is shaped for a plain carbon-carbon chain.
• Polythene softens at around 130 °C, so the option claiming melting points above 500 °C is factually wrong about common plastics.
• Because the chain is never consumed, plastic fragments mechanically into microplastics that persist and enter water and food.

• Reaching for a high melting point as the reason plastics survive; persistence in the environment is about enzymes, not heat.
• Assuming strong bonds alone make a material non-biodegradable — cellulose is also covalently bonded and decays readily, because organisms have cellulase.
• Calling plastics unbreakable. They fragment easily; what they do not do is decompose.

Either as a why-question like this one, or as a statement item on the environmental problems plastics cause, or as a bonding-type recall test dressed in an everyday example.

• practice — not a real PYQ

  Plastics resist decay in soil chiefly because

  • (a)they are radioactive
  • (b)soil microbes lack enzymes able to break their carbon-carbon chains
  • (c)they dissolve in water and are washed away
  • (d)they are held together by hydrogen bonds

  Answer(b) soil microbes lack enzymes able to break their carbon-carbon chains — decomposition is enzymatic, and no common soil organism carries an enzyme shaped for a plain polymer backbone.

• practice — not a real PYQ

  Which one of the following is a property that plastics do NOT generally show?

  • (a)Poor conduction of electricity
  • (b)Low density
  • (c)Melting points above 500 °C
  • (d)Resistance to corrosion

  Answer(c) Melting points above 500 °C — common plastics soften well below 300 °C, which is exactly why they can be moulded by heating.