Unit 18

Applied Chemistry

Class 12 Chemistry

Chemistry in the Service of Mankind

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NEB/CDC syllabus scope: Unit 18 is a 4-teaching-hour Applied Chemistry chapter covering polymers—addition and condensation polymers, elastomers and fibres, natural and synthetic polymers, and polyethylene, PVC, Teflon, polystyrene, nylon and Bakelite; dyes—introduction and types based on structure and method of application; drugs—characteristics, natural and synthetic drugs, classification of common drugs, habit-forming drugs and drug addiction; and pesticides—introductory study of insecticides, herbicides and fungicides.

1. Chemistry in the Service of Mankind

Applied chemistry connects chemical principles with materials, health, agriculture and everyday technology. In this unit, the focus is not on lengthy mechanisms but on recognizing important classes of useful chemicals, understanding their properties and applications, and appreciating their safe and responsible use.

Diagram 1: Four syllabus areas in Unit 18

2. Polymers

Polymer A polymer is a very large molecule (macromolecule) formed by repeated linking of many small molecules called monomers.

Polymerization The chemical process by which monomer molecules combine to form a polymer is called polymerization.

n Monomer → Polymer

Polymer chains may be approximately linear, branched or cross-linked. Their structure controls properties such as flexibility, strength, elasticity, heat resistance and solubility.

Diagram 2: Basic idea of polymerization

3. Addition and Condensation Polymers

3.1 Addition Polymers

Addition polymers are commonly formed from unsaturated monomers without elimination of a small molecule from each linking step.

n CH₂=CH₂ → [–CH₂–CH₂–]ₙ

Examples: polyethylene, PVC, PTFE (Teflon) and polystyrene.

3.2 Condensation Polymers

Condensation polymers are formed by step-growth reactions between monomers bearing suitable functional groups, often with elimination of small molecules such as water.

Examples: nylon-6,6 and Bakelite.

Feature	Addition polymer	Condensation polymer
Monomers	Usually unsaturated monomers	Usually bi-/polyfunctional monomers
Growth pattern	Chain-growth	Step-growth
Small molecule eliminated	Generally no	Often yes
Examples	PE, PVC, PTFE, polystyrene	Nylon-6,6, Bakelite

Diagram 3: Addition and condensation polymerization

4. Elastomers and Fibres

Elastomers

Polymers capable of substantial reversible stretching. Their chains are flexible and lightly cross-linked.

Example: vulcanized rubber.

Fibres

Polymers with strong intermolecular attractions, high tensile strength and relatively low elasticity.

Examples: nylon and polyester fibres.

Property	Elastomer	Fibre
Elasticity	High	Low to moderate
Chain arrangement	Flexible/coiled, light cross-linking	More aligned chains
Intermolecular force	Moderate	Strong
Main use	Flexible elastic goods	Threads, ropes, fabrics

Diagram 4: Chain-level comparison of elastomers and fibres

5. Natural and Synthetic Polymers

Type	Meaning	Examples
Natural polymers	Produced by living organisms or occurring naturally	Starch, cellulose, proteins, DNA, RNA, natural rubber
Synthetic polymers	Prepared intentionally by chemical synthesis	Polyethylene, PVC, PTFE, polystyrene, nylon, Bakelite

Natural does not automatically mean harmless “Natural” and “synthetic” describe origin, not safety, biodegradability or environmental impact. Properties must be assessed for each material.

6. Important Synthetic Polymers

Polymer	Monomer(s)	Type	Typical uses
Polyethylene (polyethene)	Ethene, CH₂=CH₂	Addition, thermoplastic	Packaging, containers, insulation
PVC	Chloroethene (vinyl chloride), CH₂=CHCl	Addition, thermoplastic	Pipes, cable insulation, profiles
PTFE / Teflon	Tetrafluoroethene, CF₂=CF₂	Addition	Chemical-resistant coatings, seals, insulation
Polystyrene	Styrene, C₆H₅CH=CH₂	Addition	Packaging, insulation, molded articles
Nylon-6,6	Hexane-1,6-diamine + hexanedioic (adipic) acid	Condensation polyamide	Fibres, ropes, engineering materials
Bakelite	Phenol + methanal (formaldehyde)	Condensation, thermosetting	Electrical components, heat-resistant handles

6.1 Polyethylene

n CH₂=CH₂ → [–CH₂–CH₂–]ₙ

6.2 PVC

n CH₂=CHCl → [–CH₂–CHCl–]ₙ

6.3 PTFE (Teflon)

n CF₂=CF₂ → [–CF₂–CF₂–]ₙ

6.4 Polystyrene

n C₆H₅CH=CH₂ → [–CH₂–CH(C₆H₅)–]ₙ

6.5 Nylon-6,6

Nylon-6,6 is formed by condensation between hexane-1,6-diamine and adipic acid, forming amide linkages and eliminating water.

6.6 Bakelite

Bakelite is a highly cross-linked phenol–formaldehyde resin. Once set, it does not soften and reshape like an ordinary thermoplastic.

Diagram 5: Monomer map for the six required synthetic polymers

7. Dyes

Definition Dyes are coloured substances capable of imparting reasonably durable colour to materials such as fibres, fabrics, paper or other substrates.

7.1 Desirable Characteristics of a Dye

It should produce a suitable, sufficiently intense colour.
It should attach or become fixed to the intended material.
Its colour should show useful resistance to washing and normal service conditions.
It should have suitable chemical stability for its intended use.

Why dyes are coloured Many organic dyes contain extended conjugated π-electron systems. Light absorption in the visible region produces the colour observed by the eye. Groups that help generate colour are often called chromophores, while substituents that modify colour or binding may be called auxochromes.

8. Types of Dyes Based on Chemical Structure

The syllabus asks for classification by structure. Common school-level structural examples include:

Structural class	Characteristic feature	Example / note
Azo dyes	Contain –N=N– linkage	p-Hydroxyazobenzene, methyl orange
Nitro dyes	Contain nitro chromophore in a suitable conjugated system	Picric acid is a familiar school example
Phthalein dyes	Phthalein-type conjugated skeleton	Phenolphthalein
Triphenylmethane dyes	Related to a triarylmethane framework	Malachite green
Indigoid dyes	Indigo-type conjugated chromophore	Indigo

8.1 Azo Dyes

Ar–N=N–Ar′

Azo compounds are important because the –N=N– group joins conjugated aromatic systems. Azo dyes can be produced by coupling a diazonium salt with an activated aromatic compound such as phenol or aniline.

Diagram 6: General structural idea of an azo dye

9. Types of Dyes Based on Method of Application

Dye type	How it is applied	Typical use / idea
Direct dyes	Applied directly from solution to fibre	Suitable dyes have direct affinity for fibre
Vat dyes	Converted temporarily to a soluble reduced form, applied, then reoxidized in fibre	Indigo is a classical example
Mordant dyes	Fixed with a metal-ion mordant that helps bind dye to fibre	Alizarin is a classic example
Disperse dyes	Applied as very fine dispersions in water	Used especially with hydrophobic synthetic fibres
Azoic / ingrain dyes	Azo dye is formed within/on the fibre from coupling components	Colour develops on the fabric

Diagram 7: Classification of dyes by application method

10. Drugs

Drug A drug is a chemical substance used to diagnose, prevent, relieve or treat disease, or to modify a physiological function for a medical purpose.

10.1 Desirable Characteristics of a Drug

It should have the intended therapeutic effect.
It should be sufficiently selective for its target or purpose.
Adverse effects should be minimized at medically appropriate use.
It should be chemically and physically stable enough for storage and administration.
Its quality, purity and dose should be controllable.
Benefits should outweigh risks when used appropriately under medical guidance.

Health note Drug classification in this chapter is for academic study. Medicines can have contraindications, interactions and dose-dependent risks; treatment decisions should follow qualified medical guidance rather than self-medication.

11. Natural and Synthetic Drugs

Type	Meaning	Examples / context
Natural-origin drug	Active substance obtained from or originally discovered in a natural source	Some antibiotics and plant-derived medicinal molecules
Synthetic drug	Prepared by chemical synthesis	Many modern analgesics, antipyretics and other medicines
Semisynthetic drug	Natural starting substance is chemically modified	Common in modern pharmaceutical development

Exam point Natural and synthetic refer to source or method of production. Neither word alone determines effectiveness or safety.

12. Classification of Some Common Drugs

Class	Main purpose	Typical examples
Antiseptics	Reduce or inhibit microorganisms on living tissues	Iodine-based antiseptics, chloroxylenol formulations
Disinfectants	Reduce microorganisms on inanimate surfaces/materials	Appropriate chlorine- or oxidant-based disinfectants
Antibiotics	Treat susceptible bacterial infections	Penicillin-class drugs, tetracycline-class drugs
Antipyretics	Reduce fever	Paracetamol
Analgesics	Relieve pain	Paracetamol, aspirin in appropriate patients
Anaesthetics	Produce reversible loss/reduction of sensation for medical procedures	Modern agents selected by clinicians
Antacids	Neutralize or reduce effects of excess gastric acid	Mg(OH)₂, Al(OH)₃ preparations
Antihistamines	Block histamine effects in allergic conditions	Various H₁ antihistamines

Diagram 8: Common drug classes and their main roles

12.1 Antiseptic vs Disinfectant

Frequently asked difference An antiseptic is intended for use on living tissue under suitable conditions, whereas a disinfectant is used on inanimate surfaces or objects. A chemical’s safe concentration and formulation matter, so the same substance is not automatically interchangeable between the two uses.

12.2 Antibiotics

Antibiotics act against susceptible bacteria. They do not treat viral infections such as ordinary influenza, and inappropriate use can promote antimicrobial resistance.

Responsible-use point Antibiotics should be used only when medically appropriate and as directed. Misuse and unnecessary use accelerate antimicrobial resistance.

13. Habit-Forming Drugs and Drug Addiction

Drug dependence / addiction Addiction is a health condition characterized by compulsive substance use or drug-seeking despite harmful consequences, often involving impaired control and continued use even when it causes major problems.

Some psychoactive substances can produce tolerance, dependence or addiction. Their effects may involve the central nervous system and can affect judgement, mood, memory, coordination, physical health, relationships and social functioning.

13.1 Important Terms

Tolerance: reduced response after repeated exposure, so the same amount may have less effect.
Dependence: physiological or psychological adaptation in which stopping may cause withdrawal symptoms.
Addiction: compulsive use despite harm and difficulty controlling use.

13.2 Harmful Consequences

Impaired judgement and reduced academic/work performance.
Accidents and risky behaviour.
Mental-health and physical-health complications.
Damage to family, social and financial wellbeing.
Risk of poisoning or overdose with some substances.

Diagram 9: High-level concept of dependence and addiction

Public-health perspective Drug addiction is a medical and social issue. Prevention, counselling, healthcare, family support and evidence-based treatment are more appropriate responses than stigma.

14. Pesticides

Definition Pesticides are substances used to prevent, control or reduce organisms considered harmful in agriculture, public health, stored products or other managed environments.

The Grade 12 syllabus specifically introduces insecticides, herbicides and fungicides.

Type	Target	General examples
Insecticide	Insect pests	Different modern and historical insect-control chemicals
Herbicide	Unwanted plants/weeds	Selective and non-selective weed-control agents
Fungicide	Fungal diseases	Copper-based and other registered fungicidal formulations

14.1 Insecticides

Insecticides are pesticides designed to control insect pests. Historical examples such as DDT are important in chemistry history, while modern regulatory status varies by country because environmental persistence and toxicity are major concerns.

14.2 Herbicides

Herbicides control unwanted plants. Some are selective for particular weeds, whereas others act on a broad range of vegetation.

14.3 Fungicides

Fungicides help prevent or control fungal diseases of plants and materials. Copper-containing preparations are classical examples in agricultural chemistry.

Diagram 10: Insecticide, herbicide and fungicide

14.4 Benefits and Risks

Potential benefit	Potential concern
Protect crops from damaging pests	Toxicity to non-target organisms
Reduce disease transmission in specific public-health uses	Residues and environmental contamination
Improve agricultural yield and quality when appropriately used	Development of pesticide resistance
Protect stored products and materials	Persistence or bioaccumulation for some chemicals

Responsible-use principle Pesticides should only be used according to current local regulations and product labels by trained or informed users. More pesticide is not automatically more effective; unnecessary exposure increases risks to people, wildlife and the environment.

Diagram 11: Benefits and risks in pesticide use

15. High-Yield Comparison Tables

15.1 Polymer Quick Comparison

Category	Key idea	Example
Addition polymer	No small-molecule elimination in chain addition	Polyethylene
Condensation polymer	Step-growth, often eliminates small molecule	Nylon-6,6
Elastomer	High reversible elasticity	Vulcanized rubber
Fibre	High tensile strength	Nylon fibre
Natural polymer	Produced naturally	Cellulose
Synthetic polymer	Chemically manufactured	PVC

15.2 Dye Quick Comparison

Classification basis	Examples of types
Chemical structure	Azo, nitro, phthalein, triphenylmethane, indigoid
Method of application	Direct, vat, mordant, disperse, azoic/ingrain

15.3 Drug and Pesticide Quick Comparison

Term	Main context
Drug	Medical/physiological purpose
Antiseptic	Microorganism control on living tissue
Disinfectant	Microorganism control on inanimate surfaces
Insecticide	Insect control
Herbicide	Weed control
Fungicide	Fungal control

16. Common Exam Mistakes

Calling a monomer a polymer or vice versa.
Writing nylon-6,6 as an addition polymer. It is a condensation polyamide.
Writing Bakelite as a thermoplastic. It is a cross-linked thermosetting polymer.
Forgetting that PE, PVC, PTFE and polystyrene are addition polymers.
Confusing an elastomer with a fibre; elasticity and tensile strength are different properties.
Assuming all natural polymers are biodegradable under all conditions or all synthetic polymers are non-biodegradable.
Mixing dye classification by structure with classification by method of application.
Forgetting the –N=N– linkage in an azo dye.
Calling a disinfectant automatically safe for living tissue. Antiseptic and disinfectant uses are not interchangeable.
Writing antibiotics as treatments for viral infections.
Describing natural drugs as automatically safer than synthetic drugs.
Treating habit formation, dependence and addiction as exactly the same concept.
Defining all pesticides as insecticides. Insecticides are only one pesticide class.
Confusing herbicides with fungicides.
Ignoring environmental and non-target effects of pesticides.

17. Worked Examples

Worked Example 1: Classify PVC

Monomer: CH₂=CHCl, an unsaturated vinyl monomer. Polymerization joins C=C monomers without elimination of a small molecule.

Answer: PVC is a synthetic addition polymer and thermoplastic.

Worked Example 2: Classify nylon-6,6

It is produced from a diamine and a dicarboxylic acid by forming amide linkages.

Answer: synthetic condensation polymer; also a fibre-forming polyamide.

Worked Example 3: Structure vs application of a dye

A dye contains –N=N– and is fixed to a fabric using a mordant.

Structural class: azo dye. Application class: mordant dye. These classifications answer different questions.

Worked Example 4: Antiseptic or disinfectant?

A chemical formulation is intended for decontaminating a laboratory bench.

Classification: disinfectant use, because the target surface is inanimate.

Worked Example 5: Classify pesticide by target

A registered product is intended to control a fungal disease of crops.

Answer: fungicide.

18. Important Exam Questions

Short-Answer Questions

Define polymer, monomer and polymerization.
Differentiate addition and condensation polymers.
What is an elastomer? Give one example.
What is a fibre-forming polymer?
Differentiate natural and synthetic polymers.
Name the monomer of polyethylene.
Name the monomer of PVC.
Name the monomer of PTFE.
Name the monomer of polystyrene.
Name the monomers of nylon-6,6.
Name the monomers of Bakelite.
Why is Bakelite called a thermosetting polymer?
Define dye and state desirable properties of a dye.
What is an azo dye?
Differentiate structural and application-based classification of dyes.
What is a vat dye?
What is a mordant dye?
What is a disperse dye?
Define a drug and state desirable characteristics.
Differentiate natural and synthetic drugs.
Differentiate antiseptics and disinfectants.
What is an antibiotic?
What are antipyretics and analgesics?
What is drug dependence/addiction?
Define pesticide.
Differentiate insecticide, herbicide and fungicide.

Long-Answer Questions

Classify polymers and differentiate addition and condensation polymerization with examples.
Explain elastomers and fibres with their structural features.
Compare natural and synthetic polymers.
Write monomers, polymerization type and uses of polyethylene, PVC, PTFE, polystyrene, nylon-6,6 and Bakelite.
Classify dyes on the basis of chemical structure and method of application.
Explain azo dyes and the importance of conjugation in colour.
Describe direct, vat, mordant, disperse and azoic dyes.
Define drugs and discuss their desirable characteristics.
Classify common drugs according to their therapeutic action.
Differentiate antiseptics, disinfectants and antibiotics.
Write a short note on habit-forming drugs and drug addiction.
Define pesticides and explain insecticides, herbicides and fungicides.
Discuss benefits and environmental concerns associated with pesticides.

Diagram / Flowchart Questions

Draw monomer → polymer formation.
Draw a comparison of addition and condensation polymerization.
Draw elastomer vs fibre chain arrangement.
Draw the monomer map of six important synthetic polymers.
Draw the general structure of an azo dye.
Draw classification of dyes by application.
Draw common drug classes and their actions.
Draw a concept map for habit formation and addiction.
Draw classification of pesticides.
Draw a benefits-vs-risks pesticide management diagram.

Exam Strategy This 4-hour unit is mainly classification and application based. Memorize the six required synthetic polymers and their monomers, keep the two dye-classification systems separate, learn drug classes by main action, and classify pesticides by target organism.

19. One-Minute Revision

A polymer is a macromolecule made from repeating monomer units.
Addition polymers form without routine loss of a small molecule during chain addition.
Condensation polymers form by step-growth and often eliminate a small molecule.
Elastomers are highly elastic; fibres have high tensile strength.
Natural polymers include cellulose, starch, proteins and natural rubber.
Polyethylene monomer = ethene.
PVC monomer = chloroethene.
PTFE monomer = tetrafluoroethene.
Polystyrene monomer = styrene.
Nylon-6,6 monomers = hexane-1,6-diamine + adipic acid.
Bakelite monomers = phenol + formaldehyde.
PE, PVC, PTFE and polystyrene are addition polymers.
Nylon-6,6 and Bakelite are condensation polymers.
Dyes impart durable colour to suitable materials.
Azo dyes contain –N=N–.
Dyes may be classified by structure or by application method.
Application classes include direct, vat, mordant, disperse and azoic dyes.
Drugs are chemicals used for medical or physiological purposes.
Antiseptics are for living tissues; disinfectants are for inanimate surfaces.
Antibiotics act against susceptible bacteria, not ordinary viral infections.
Antipyretics reduce fever; analgesics reduce pain.
Addiction involves compulsive use despite harm.
Pesticides control harmful organisms.
Insecticides target insects.
Herbicides target unwanted plants.
Fungicides target fungi.
Safe use, regulation and environmental effects matter for both medicines and pesticides.

20. Diagram Practice

Students should practice these labelled diagrams for the NEB examination:

Four syllabus areas of Chemistry in the Service of Mankind.
Monomer-to-polymer chain formation.
Addition vs condensation polymerization.
Elastomer vs fibre molecular arrangement.
Six important synthetic polymers and their monomers.
General azo-dye structure.
Dye classification by method of application.
Common drug classes by action.
Addiction concept map.
Classification of pesticides.
Responsible pesticide management: benefits and risks.

Source handling: The original Nepal eNotes PDF remains embedded above. The typed section follows the verified NEB/CDC syllabus and is designed as a searchable, responsive study companion. Where the PDF viewer does not expose handwritten page text, the typed section is a syllabus-aligned reconstruction and is not claimed to be a word-for-word transcription.