Polymers and Chemistry in Everyday Life
These two units are the most factual in the Chemistry syllabus, and the efficient way to prepare them is to organise the facts rather than to read them repeatedly. For polymers the organising question is what type of reaction made this, and does it need a functional group at each end? For drugs and detergents it is what does the molecule do to a biological target? Both units carry only a handful of marks, but they are among the cheapest marks in the paper because the questions are almost always direct recall of a classification, a monomer or a use.
Drugs Classified by What They Do
| Class | Action | Examples |
|---|---|---|
| Antacid | Neutralises or suppresses stomach acid | Mg(OH)₂; ranitidine, which blocks histamine |
| Antihistamine | Blocks histamine receptors | Cetirizine, terfenadine |
| Tranquilliser | Acts on the central nervous system | Equanil, luminal, chlordiazepoxide |
| Analgesic | Relieves pain | Aspirin (non-narcotic); morphine (narcotic) |
| Antibiotic | Kills or inhibits micro-organisms | Penicillin (narrow), chloramphenicol (broad) |
| Antiseptic and disinfectant | Applied to living tissue versus to objects | Dettol, 0.2 % phenol; 1 % phenol disinfects |
🎯 The same compound can be an antiseptic or a disinfectant — the concentration decides
Phenol at 0.2 % is an antiseptic, safe enough to put on a wound; at 1 % the same phenol is a disinfectant, used on floors and instruments and too harsh for living tissue. So the distinction is not chemical at all but a matter of dose — an antiseptic is applied to living tissue and a disinfectant to inanimate objects, and many substances cross from one category to the other purely by dilution. This is the standard trap in this section: a question asking whether phenol is an antiseptic or a disinfectant has no single answer unless it gives you the concentration, and when it does give one, that number is the whole question.Soaps, Detergents and Food Additives
Soap is the sodium salt of a long-chain fatty acid, made by saponification — alkaline hydrolysis of a fat or oil, which also yields glycerol. Synthetic detergents are of three kinds: anionic (alkyl benzene sulphonates, used in most washing powders), cationic (quaternary ammonium salts, used in hair conditioners and as germicides) and non-ionic (polyethylene glycol esters, used in liquid dishwashing detergents).
🎯 Why detergents work in hard water and soap does not
Hard water contains Ca²⁺ and Mg²⁺, and the calcium and magnesium salts of fatty acids are insoluble — so soap precipitates as scum before it can form the micelles that trap grease, wasting both the soap and the wash. The calcium salts of alkyl sulphonates, by contrast, are soluble, so a synthetic detergent keeps working regardless of hardness. The trade-off is environmental: branched alkyl chains resist bacterial breakdown and caused persistent foam in rivers, which is why modern detergents use straight chains that are biodegradable. That pairing — works in hard water, but only the straight-chain version is degradable — is what questions on this topic usually turn on.| Substance | Role or property |
|---|---|
| Sodium benzoate | Preservative |
| BHA and BHT | Antioxidants, preventing fats going rancid |
| Saccharin | About 550 times sweeter than sucrose; excreted unchanged |
| Aspartame | About 100 times sweeter, but decomposes on cooking |
| Sucralose | About 600 times sweeter and stable to heat |
| Alitame | About 2000 times sweeter; hard to control sweetness in food |
ℹ️ Aspartame is limited to cold foods, and that is the exam point
Aspartame is a dipeptide, so heating hydrolyses it and destroys the sweetness — which is why it appears in cold drinks and table sweeteners but not in baked goods. Sucralose and saccharin are stable to cooking temperatures and so are used where heat is involved. A question asking which sweetener is unsuitable for cooking is asking for aspartame, and the reason to give is its peptide structure rather than any vague instability.Key takeaways
- Addition polymers need one C=C and lose nothing; condensation polymers need bifunctional monomers.
- Nylon-6 comes from the single monomer caprolactam, unlike nylon-6,6 which needs two.
- Thermoplastics have separate chains and can be remoulded; thermosets are cross-linked and cannot.
- Vulcanisation cross-links rubber just enough to stop it flowing while preserving elasticity.
- Antiseptic versus disinfectant is a matter of concentration and where it is applied, not of chemistry.
- Soap fails in hard water because its calcium salts are insoluble; detergent salts are soluble.
- Only straight-chain detergents are biodegradable — branched ones caused persistent river foam.
- Aspartame is a dipeptide and decomposes on cooking; sucralose and saccharin are heat-stable.
Practice questions (8)
Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.
Which polymer is formed from a single monomer, caprolactam?
Show answer
Answer: B — Nylon-6
Nylon-6 is made by ring-opening polymerisation of caprolactam, which already contains both the amine and the acid function in one molecule. Nylon-6,6 needs two monomers, adipic acid and hexamethylenediamine; Dacron needs terephthalic acid and ethylene glycol; and bakelite needs phenol and formaldehyde. Counting monomers is often enough to answer these directly.Bakelite cannot be remoulded on heating because it is:
Show answer
Answer: B — A cross-linked thermosetting polymer
Bakelite is a three-dimensional covalent network built from phenol and formaldehyde, so there are no separate chains that heat could allow to slide past one another — the material chars instead of softening. Thermoplastics such as polythene do have separate chains held by weak forces, which is why they can be remelted and recycled repeatedly.Soap does not lather well in hard water because:
Show answer
Answer: B — Calcium and magnesium salts of fatty acids are insoluble and precipitate as scum
The Ca²⁺ and Mg²⁺ in hard water form insoluble salts with the fatty acid anion, so the soap is removed from solution as scum before it can assemble the micelles that lift grease. Synthetic detergents avoid the problem because their calcium and magnesium salts are soluble, which is the whole reason they displaced soap for laundry use.Which artificial sweetener is unsuitable for use in cooked foods?
Show answer
Answer: B — Aspartame
Aspartame is a dipeptide, so cooking hydrolyses the peptide bond and the sweetness is lost — hence it appears only in cold drinks and table sweeteners. Saccharin and sucralose survive cooking temperatures and are used in baked products. The reason to quote is the peptide structure, not a general claim of instability.A 1 % solution of phenol is used as a:
Show answer
Answer: B — Disinfectant for floors and instruments
At 1 % phenol is too harsh for living tissue and is used on inanimate objects, making it a disinfectant; the same compound at 0.2 % is mild enough to serve as an antiseptic. So the category depends entirely on concentration and on what the solution is applied to, which is why a question of this type always supplies the percentage.Vulcanisation of rubber involves heating it with:
Show answer
Answer: A — Sulphur
Sulphur forms cross-links between the polyisoprene chains, which stops them sliding past one another and so removes the tackiness and cold flow of natural rubber while leaving enough freedom for the chains to stretch and recoil. Too much sulphur would over-cross-link the material and make it hard and brittle, so the amount is what controls the final elasticity.Which detergent type is used in hair conditioners?
Show answer
Answer: B — Cationic
Cationic detergents are quaternary ammonium salts, and their positive charge lets them bind to the negatively charged surface of hair, which is why they condition and also why they act as germicides. Anionic alkyl benzene sulphonates go into washing powders and non-ionic esters into liquid dishwashing detergents, so each type maps to a distinct use.Why are branched-chain alkyl benzene sulphonates being replaced by straight-chain ones?
Show answer
Answer: B — Branched chains resist bacterial degradation and persist in water bodies
Bacteria degrade straight hydrocarbon chains readily but struggle with branched ones, so the branched detergents accumulated in rivers and produced long-lasting foam. Both types work perfectly well in hard water, since that advantage comes from the sulphonate head rather than the chain shape — so cleaning performance is not the discriminator, environmental persistence is.