Soaps, Detergents and Polymers
~8 min read
- Soaps: Sodium/potassium salts of long-chain fatty acids. Made by saponification — fat/oil + NaOH/KOH. Don't lather in hard water.
- Detergents: Synthetic; sodium salts of sulphonic acids. Lather even in hard water. Source of water pollution if non-biodegradable.
- Polymers: Large molecules from repeating monomer units. Natural (rubber, cellulose, proteins). Synthetic (polythene, PVC, nylon, polyester).
Soaps and detergents make modern life cleaner; polymers shape it. NDA tests their composition and properties.
Soap
- Chemically: sodium (or potassium) salt of long-chain fatty acid.
- Saponification: Fat (oil) + NaOH → Soap + Glycerol (glycerine).
- Hard soap: Sodium-based. Bath soap.
- Soft soap: Potassium-based. Liquid soaps, shaving cream.
- How it works: Soap molecule has hydrophilic head (water-loving, -COO⁻Na⁺ end) and hydrophobic tail (water-repelling, long hydrocarbon). Tails embed in oil/grease; heads stay in water → emulsify the dirt → washed away.
- Hard water problem: Soap reacts with Ca²⁺/Mg²⁺ in hard water → forms insoluble scum → doesn't lather.
Detergents
- Synthetic cleansing agents. Sodium salts of long-chain sulphonic acids (or alkyl sulphonates).
- Sulphonates don't form scum with Ca²⁺/Mg²⁺ — work in hard water.
- Branched-chain detergents are not biodegradable — caused river pollution. Replaced by linear-chain (biodegradable) detergents in the 1960s-70s.
- Synthetic detergents include:
- Laundry detergents (Surf, Tide).
- Dish-wash liquids.
- Shampoos.
- Industrial cleaners.
Polymers — Natural and Synthetic
- Polymer: Large molecule of many repeating units (monomers).
- Natural polymers: Cellulose, starch, proteins, DNA, rubber, silk, wool, cotton.
- Synthetic polymers:
| Polymer | Monomer | Use |
|---|---|---|
| Polythene (Polyethylene) | Ethene (CH₂=CH₂) | Plastic bags, bottles, sheets |
| PVC (Polyvinyl chloride) | Vinyl chloride | Pipes, electrical insulation, raincoats |
| Polypropylene | Propene | Bottles, films, ropes |
| Polystyrene | Styrene | Disposable cups, packaging foam (Thermocol) |
| Teflon (PTFE) | Tetrafluoroethylene | Non-stick cookware |
| Nylon | Hexamethylenediamine + Adipic acid | Ropes, fabric, fishing lines, parachutes |
| Polyester (Terylene) | Ethylene glycol + Terephthalic acid | Fabrics, bottles (PET), films |
| Bakelite | Phenol + Formaldehyde | Switches, plug bases, telephone handsets |
| Melamine | Melamine + Formaldehyde | Dinnerware (unbreakable plates), laminates |
Natural Rubber and Vulcanisation
- Natural rubber: From latex of Hevea brasiliensis tree. Polymer of isoprene.
- Soft, sticky in heat. Not durable for industrial use.
- Vulcanisation (Goodyear, 1839): heating rubber with sulphur. Creates cross-links between polymer chains → stronger, harder, more elastic, more durable.
- Used in: tires, gloves, balls, conveyor belts, elastic items.
Plastic Pollution
- Most plastics are non-biodegradable — persist in environment for centuries.
- Microplastics: Small fragments now found in soil, water, air, food.
- Mitigation: Reduce, reuse, recycle. Replace with biodegradable alternatives. Ban single-use plastics (India banned in 2022).
- Biodegradable plastics: PLA (polylactic acid from corn), starch-based, PHA. Break down by microbes.
NDA PYQ Examples
Q: Soap is sodium salt of:
(a) Carbonic acid (b) Long-chain fatty acid (c) Sulphuric acid (d) Citric acid
Answer: (b) Long-chain fatty acid.
Q: Saponification produces:
(a) Soap and water (b) Soap and glycerol (c) Soap and CO₂ (d) Acid and base
Answer: (b) Soap and glycerol.
Q: PVC stands for:
(a) Polyvinyl carbonate (b) Polyvinyl chloride (c) Polyvinyl chromate (d) Polyvinyl carbide
Answer: (b) Polyvinyl chloride.
Q: Vulcanisation involves:
(a) Heating rubber with sulphur (b) Cooling rubber (c) Mixing with metal (d) Adding water
Answer: (a) Heating rubber with sulphur.
Q: Nylon, polyester, polythene are all:
(a) Natural polymers (b) Synthetic polymers (c) Soaps (d) Salts
Answer: (b) Synthetic polymers.
Drill Soaps, Detergents and Polymers for NDA
NDA-pattern items on Soaps, Detergents and Polymers with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why doesn't soap lather in hard water?
Ca²⁺ and Mg²⁺ ions in hard water react with soap (sodium stearate) to form insoluble calcium/magnesium stearate — a scummy white precipitate. This wastes soap and leaves residue. Detergents (sulphonate-based) don't form scum and work in hard water.
How does soap clean?
Soap molecules have a hydrophilic (water-loving) head and hydrophobic (oil-loving) tail. Tails embed in oil/grease droplets; heads face outward into water. The oil is surrounded by soap molecules (micelle), suspended in water, and washed away.
Why was vulcanisation a major invention?
Natural rubber was soft, sticky, deformed easily. Vulcanisation (Goodyear, 1839) treated rubber with sulphur and heat, creating cross-links between chains. Result: stronger, harder, more elastic, durable rubber — enabling tires, industrial belts, the entire rubber industry.
What is the difference between plastic and polymer?
All plastics are polymers, but not all polymers are plastic. Polymer = any large molecule of repeating units (proteins, DNA, cellulose are polymers). Plastic = synthetic polymer that can be moulded (polythene, PVC, nylon). Common usage of 'plastic' usually means synthetic polymer.
Why is Bakelite considered the first true plastic?
Bakelite (phenol + formaldehyde, Leo Baekeland 1907) was the first fully synthetic plastic — entirely man-made. Earlier 'plastics' like celluloid were based on natural materials (cellulose). Bakelite opened the era of synthetic plastics.