Government System — President, PM & Parliament

Why This Chapter Matters

This chapter carries some of the most directly "railway-relevant" science content in the entire book. Questions on metals, nonmetals, alloys, and fuels appear regularly in RRB Group D papers, typically 4 to 5 questions, and several past papers have asked specifically about steel, the alloy that literally makes up the rails you'll be working near every day if you clear this exam. This is not disconnected classroom chemistry; it is the material science behind the job itself.

Here you'll build on the last chapter's basics of chemical reactions, going deeper into reaction types, then move into metals versus nonmetals, alloys and their railway applications, fuels and combustion in more depth, and a short, practical introduction to polymers and plastics. The single biggest mistake aspirants make in this chapter: confusing which properties belong to metals and which belong to nonmetals, especially with exceptions like mercury (a liquid metal) and graphite (a nonmetal that conducts electricity) — examiners specifically target these exceptions because they know most students memorise the general rule and forget the outliers. Watch for that trap throughout this chapter.

1. Types of Chemical Reactions

In the previous chapter, you got a quick preview of reaction types. Here is the full, exam-ready breakdown, with real examples you should be able to recall instantly.

Combination Reaction

Two or more substances combine to form a single new product. General form: A + B → AB.

Example: When hydrogen gas burns in oxygen, it forms water: 2H₂ + O₂ → 2H₂O. Another classic example: when quicklime (calcium oxide) is mixed with water, it forms slaked lime, releasing a lot of heat, a reaction you may have seen used at construction sites for whitewashing preparation.

Decomposition Reaction

One compound breaks down into two or more simpler substances, often needing an external input of energy like heat, light, or electricity. General form: AB → A + B.

Example: Heating calcium carbonate (limestone) produces calcium oxide (quicklime) and carbon dioxide gas: CaCO₃ → CaO + CO₂. This same reaction is the backbone of the cement industry. Decomposition triggered by heat is called thermal decomposition; triggered by light, it's called photodecomposition (this is why silver bromide, used in old-style photographic film, breaks down when exposed to light); and triggered by electricity, it's called electrolytic decomposition (like splitting water into hydrogen and oxygen using an electric current, a process called electrolysis).

Exam trap: Combination and decomposition are exact opposites, one joins substances, the other splits them, and students under exam pressure sometimes flip the direction of the arrow in their head when matching an equation to its type. Always check: does the reaction have one product from many reactants (combination), or many products from one reactant (decomposition)?

Displacement Reaction

A more reactive element displaces (pushes out) a less reactive element from its compound. General form: A + BC → AC + B.

Example: When an iron nail is dipped into a copper sulphate solution, iron displaces copper because iron is more reactive than copper: Fe + CuSO₄ → FeSO₄ + Cu. You'll actually see the blue copper sulphate solution turn pale green (iron sulphate colour) and a reddish-brown coating of copper metal deposit on the nail, a well-known school lab demonstration.

This reactivity comparison is governed by the reactivity series (also called the activity series), a ranking of metals from most reactive to least reactive. A commonly taught order, from most to least reactive: potassium, sodium, calcium, magnesium, aluminium, zinc, iron, lead, hydrogen (as reference point), copper, silver, gold.

Memory hook: A simple original line to recall the top portion of this order: "Please Send Cash, My Aunt Zara Is Leaving Home Comfortably, Silently, Gone" — Potassium, Sodium, Calcium, Magnesium, Aluminium, Zinc, Iron, Lead, Hydrogen, Copper, Silver, Gold. Build your own version if this phrasing doesn't stick; the point is linking the first letters into a memorable sentence.

Double Displacement Reaction

Two compounds exchange parts (usually ions) to form two new compounds, often producing an insoluble precipitate. General form: AB + CD → AD + CB.

Example: Silver nitrate reacting with sodium chloride produces silver chloride (an insoluble white precipitate) and sodium nitrate: AgNO₃ + NaCl → AgCl↓ + NaNO₃. Neutralisation reactions between acids and bases, covered in the previous chapter, are also a type of double displacement reaction.

Oxidation and Reduction (Redox Reactions)

Oxidation is the loss of electrons by a substance (often, though not always, involving the gain of oxygen or loss of hydrogen). Reduction is the gain of electrons (often involving the loss of oxygen or gain of hydrogen). These two always happen together in a single reaction, called a redox reaction, because electrons lost by one substance must be gained by another.

Rusting, discussed in the previous chapter, is a clear oxidation reaction: iron loses electrons to oxygen. Combustion is also fundamentally an oxidation process. Exam trap: Do not think oxidation always needs oxygen gas physically present; the modern definition centres on electron transfer, though the "gain of oxygen" version remains a useful shortcut for many common reactions you'll see at this exam level.

2. Metals and Nonmetals

Properties of Metals

Metals typically show these properties, and this list is a favourite for direct "which of the following is NOT a property of metals" style questions:

• Lustre: metals have a shiny surface when freshly cut or polished.
• Malleability: metals can be hammered into thin sheets without breaking (gold and silver are the most malleable metals known).
• Ductility: metals can be drawn into thin wires (gold is the most ductile metal).
• Conductivity: metals conduct heat and electricity well (silver is the best conductor, as you learned in the physics chapter).
• Sonorous: metals produce a ringing sound when struck, which is why bells are made of metal alloys.
• High melting and boiling points: generally, though there are exceptions.
• Solid at room temperature: with one major exception, mercury, which is liquid at room temperature, a fact that appears repeatedly in exams precisely because it breaks the general rule.
• Good reducing agents: metals tend to lose electrons easily in reactions (they are electropositive).

Properties of Nonmetals

Nonmetals generally show opposite characteristics:

• No lustre (except iodine, which has a shiny, metallic-looking surface despite being a nonmetal, another favourite exception-based question).
• Brittle: nonmetal solids break instead of bending or flattening under a hammer blow.
• Poor conductors of heat and electricity (except graphite, a form of carbon, which conducts electricity well despite being a nonmetal, a genuinely important exception used in pencil leads and as electrodes).
• Low melting and boiling points generally, though diamond (also pure carbon, like graphite) has an extremely high melting point, showing that even within the same element, different structural arrangements (called allotropes) can behave very differently.
• Exist in all three states at room temperature: solids (carbon, sulphur), liquids (bromine), and gases (oxygen, nitrogen, chlorine).

Memory hook: Remember the exception trio with "Metal that flows, nonmetal that shines, nonmetal that conducts" — mercury (liquid metal), iodine (shiny nonmetal), and graphite (conducting nonmetal). These three exceptions are asked more often than the general rules themselves, so do not skip them.

Metalloids

As covered briefly in the previous chapter, metalloids show intermediate properties between metals and nonmetals. Silicon, boron, germanium, arsenic, and antimony are common examples. Silicon's semi-conducting behaviour, again as covered in the physics chapter, makes it the backbone material of the entire electronics and computer chip industry.

3. Alloys and Their Uses

An alloy is a homogeneous mixture of two or more metals, or a metal with a nonmetal, combined usually by melting and cooling together, to produce a material with improved properties compared to the pure metal alone: greater strength, better resistance to corrosion, or a more useful melting point.

Pure iron, on its own, is relatively soft and rusts quickly. This is exactly why railway tracks, wheels, coaches, and structural components are never made from pure iron; they use alloys engineered for strength and durability under constant mechanical stress and weather exposure.

Steel: The Railway Metal

Steel is an alloy of iron with a small, controlled percentage of carbon (usually between 0.2% and 2%), sometimes with other elements added for specific properties. Steel is dramatically stronger and more useful than pure iron, which is why it is THE dominant material in railway construction: rails, wheels, axles, coach bodies, and bridges all rely heavily on various grades of steel.

Stainless steel adds chromium (and often nickel) to the iron-carbon mixture, forming a thin, invisible, self-repairing protective oxide layer on the surface that resists rusting and corrosion far better than ordinary steel. This is exactly why stainless steel is used for railway coach fittings, kitchen utensils, and surgical instruments, anywhere corrosion resistance and hygiene both matter.

Exam trap: Students sometimes think stainless steel never rusts at all under any condition. It is highly rust-resistant, not entirely rust-proof; prolonged exposure to harsh chemicals or salty, humid environments can still eventually cause corrosion, just far more slowly than ordinary steel.

Other Common Alloys

Alloy	Composition	Main use
Bronze	Copper + Tin	Statues, medals, bearings
Brass	Copper + Zinc	Musical instruments, fittings, decorative items
Duralumin	Aluminium + Copper + Magnesium + Manganese	Aircraft bodies, lightweight structures
Solder	Tin + Lead	Joining electrical wires and metal joints
Alnico	Aluminium + Nickel + Cobalt (+ Iron)	Permanent magnets
Amalgam	Mercury + another metal	Dental fillings (historically)

Memory hook: Recall bronze versus brass with "Bro(nze) needs TIN foil, Brass needs ZINC-y energy" — bronze pairs with tin, brass pairs with zinc. A silly line, but it separates two alloys students constantly mix up.

Alloys matter for railways beyond just steel. Overhead electrification wires often use special copper alloys (like cadmium copper or high-conductivity alloys) that balance good electrical conductivity with the mechanical strength needed to withstand tension and weathering across thousands of kilometres of track. Wheel and axle steel is specifically alloyed and heat-treated to resist the immense repeated stress of rolling contact, a very different requirement from, say, the steel in a kitchen knife.

4. Fuels and Combustion

A fuel is any substance that releases usable energy, typically as heat, when it undergoes combustion. Fuels are broadly classified as fossil fuels (coal, petroleum, natural gas, formed over millions of years from decayed organic matter under heat and pressure) and non-fossil or renewable fuels (biogas, biodiesel, hydrogen fuel, and others, which can be replenished on a human timescale).

Coal

Coal formed from ancient plant matter buried and compressed over millions of years, a process called carbonisation. Different stages of coal formation, in order of increasing carbon content and quality, are peat, lignite, bituminous coal, and anthracite (anthracite has the highest carbon content and burns cleanest and hottest of the four). India's railways historically ran on coal-fired steam locomotives, and coal remains central to India's electricity generation even today, with railways themselves transporting enormous volumes of coal from mining regions to power plants across the country, one of the single largest categories of freight the Railways carries.

Petroleum

Petroleum (crude oil) is a mixture of hydrocarbons formed from ancient marine organisms. It is refined in oil refineries through fractional distillation, separating the mixture into components based on differing boiling points: petroleum gas (LPG), petrol (gasoline), kerosene, diesel, and heavier residues like lubricating oil, wax, and bitumen (used in road construction). Diesel is the dominant fuel for a significant share of Indian Railways' locomotive fleet even as electrification expands, alongside the electric traction covered in the physics chapter.

Natural Gas

Natural gas, mainly methane, is often found alongside petroleum deposits and is used as a comparatively cleaner-burning fuel for power generation, cooking (as CNG, compressed natural gas, in vehicles, and PNG, piped natural gas, in homes), and industrial processes. It burns cleaner than coal or petroleum products, producing fewer pollutants per unit of energy released.

Fuel Efficiency and Combustion Quality

The calorific value of a fuel measures how much heat energy it releases per unit mass when burnt completely, usually expressed in kilojoules per kilogram (kJ/kg). Hydrogen has an exceptionally high calorific value, one reason it is being explored as a future clean fuel, though storage and production challenges have kept it from mainstream use so far.

As covered in the previous chapter, combustion needs fuel, oxygen, and heat together (the fire triangle), and incomplete combustion (limited oxygen) produces dangerous carbon monoxide along with soot, while complete combustion produces carbon dioxide and water, releasing more usable energy per unit of fuel.

Exam trap: Do not confuse calorific value with flash point. Flash point is the lowest temperature at which a fuel's vapour can ignite when exposed to a flame; it measures fire risk and handling safety, not energy output. Petrol has a low flash point (highly flammable, ignites easily even at room temperature), while diesel has a comparatively higher flash point (needs more heat before it ignites easily), which is one reason diesel is considered somewhat safer to store and transport than petrol.

5. An Introduction to Polymers and Plastics

A polymer is a large molecule made of many repeating smaller units called monomers, chemically bonded together in long chains, rather like a train made of many identical coaches coupled end to end, each coach being a monomer, and the full train being the polymer.

Natural polymers occur without human synthesis: cotton and cellulose (from plants), natural rubber (from the latex of rubber trees), wool and silk (proteins), and even DNA and starch inside living organisms are natural polymers. Synthetic polymers are manufactured by humans, chemically joining monomers under controlled industrial conditions: plastics, synthetic fibres like nylon and polyester, and synthetic rubber all fall into this category.

Plastics

Plastic is a broad category of synthetic polymers that can be moulded into shape when soft and then set into a durable, useful form. Plastics are classified into two major types based on their behaviour when heated:

• Thermoplastics soften on heating and can be remoulded repeatedly into new shapes; they do not undergo permanent chemical change on heating. Examples include polyethylene (PE), used in bags and bottles; polyvinyl chloride (PVC), used in pipes and cable insulation; and polystyrene, used in packaging and disposable cups.
• Thermosetting plastics (or thermosets) set permanently into shape once heated and moulded the first time; they cannot be melted and remoulded again, because heating triggers permanent chemical bonding between chains. Examples include bakelite (used in electrical switches and fittings, one of the earliest fully synthetic plastics, invented by Leo Baekeland in 1907) and melamine (used in kitchenware and laminate surfaces).

Memory hook: "Thermo-PLASTIC is flexible like elastic" (can be reshaped again and again with heat), while "Thermo-SET is set, no reset" (permanently fixed once moulded). A rhyme built purely to separate these two commonly confused categories.

Nylon is a well-known synthetic polymer used in ropes, parachutes, clothing, and, relevant to railways, in various durable fittings and fabric components. Polyester is another widely used synthetic fibre, common in railway upholstery and uniform fabrics because of its durability and resistance to wrinkling and shrinking.

Rubber

Natural rubber comes from the latex sap of the rubber tree (Hevea brasiliensis), but raw natural rubber is too soft, sticky in heat, and brittle in cold to be practically useful on its own. Charles Goodyear discovered vulcanisation in 1839, a process of heating rubber with sulphur, which cross-links the rubber's polymer chains, making it stronger, more elastic, and far more resistant to temperature extremes. Vulcanised rubber is what makes modern tyres, conveyor belts, railway buffer pads, and rubber gaskets in train couplings possible; without vulcanisation, none of these applications would survive real-world temperature swings and mechanical stress.

Exam trap: Vulcanisation is specifically the sulphur cross-linking process for rubber, not a general term for "hardening" any material. Keep this term tied precisely to rubber and Charles Goodyear in your memory.

Environmental Note

Most common plastics are non-biodegradable, meaning natural microbial processes cannot break them down within any reasonable human timescale, leading to long-term accumulation in landfills, soil, and water bodies. This single property, durability that outlives its usefulness, is exactly why plastic waste management, recycling, and gradually shifting toward biodegradable alternatives has become such a major public policy and environmental concern across India and globally, an area you'll likely also see referenced in the environment-related portions of your General Awareness preparation.

Quick Revision — One-Line Facts

1. A combination reaction joins two or more substances into one product.
2. A decomposition reaction breaks one compound into two or more simpler substances.
3. Thermal, photodecomposition, and electrolytic decomposition are triggered by heat, light, and electricity respectively.
4. A displacement reaction happens when a more reactive metal pushes out a less reactive one from its compound.
5. The reactivity series ranks metals from most to least reactive, with potassium near the top and gold near the bottom.
6. A double displacement reaction exchanges ions between two compounds, often forming a precipitate.
7. Oxidation is loss of electrons; reduction is gain of electrons; together they form a redox reaction.
8. Gold is the most malleable and most ductile metal known.
9. Mercury is the only metal that is liquid at room temperature.
10. Graphite is a nonmetal that conducts electricity, unlike most other nonmetals.
11. Iodine is a nonmetal that shows metallic lustre, an exception to the general nonmetal rule.
12. Diamond and graphite are both allotropes of pure carbon with very different properties.
13. An alloy is a mixture of two or more metals, or a metal with a nonmetal, made for improved properties.
14. Steel is an alloy of iron and carbon, the dominant material in railway tracks, wheels, and coaches.
15. Stainless steel adds chromium (and often nickel) to resist rusting far better than ordinary steel.
16. Bronze is copper plus tin; brass is copper plus zinc.
17. Duralumin (aluminium-based alloy) is widely used in aircraft bodies for its light weight and strength.
18. Coal formation stages, in order of increasing quality, are peat, lignite, bituminous coal, and anthracite.
19. Petroleum is refined through fractional distillation based on differing boiling points of its components.
20. LPG, petrol, kerosene, and diesel are all fractions obtained from crude oil refining.
21. Natural gas, mainly methane, burns cleaner than coal or petroleum products.
22. Calorific value measures the heat energy released per unit mass of fuel when burnt completely.
23. Flash point measures how easily a fuel's vapour ignites, indicating fire risk, not energy output.
24. A polymer is a large molecule made of many repeating monomer units bonded in chains.
25. Thermoplastics can be remoulded repeatedly on heating; thermosetting plastics set permanently after first moulding.
26. Bakelite, invented by Leo Baekeland, is one of the earliest fully synthetic thermosetting plastics.
27. Charles Goodyear discovered vulcanisation in 1839, heating rubber with sulphur to strengthen it.
28. Vulcanised rubber is used in tyres, conveyor belts, and railway buffer and gasket components.
29. Most common plastics are non-biodegradable, causing long-term environmental accumulation.
30. Nylon and polyester are widely used synthetic fibres, including in railway upholstery and uniforms.

Memory Tables

Table 1: Reaction Types at a Glance

Reaction type	General form	Real example
Combination	A + B → AB	Hydrogen + Oxygen → Water
Decomposition	AB → A + B	Limestone → Quicklime + CO₂
Displacement	A + BC → AC + B	Iron + Copper sulphate → Iron sulphate + Copper
Double displacement	AB + CD → AD + CB	Silver nitrate + Sodium chloride → Silver chloride + Sodium nitrate

Table 2: Metals vs Nonmetals — Properties and Key Exceptions

Property	Metals (typical)	Nonmetals (typical)	Notable exception
State at room temperature	Solid	Solid, liquid, or gas	Mercury (liquid metal)
Lustre	Shiny	Dull	Iodine (shiny nonmetal)
Electrical conductivity	Good	Poor	Graphite (conducting nonmetal)
Malleability/ductility	Yes	No, brittle	—

Table 3: Alloys, Composition, and Railway/Industry Relevance

Alloy	Composition	Where it's used
Steel	Iron + Carbon	Rails, wheels, axles, coach bodies
Stainless steel	Iron + Carbon + Chromium (+ Nickel)	Coach fittings, cutlery, surgical tools
Bronze	Copper + Tin	Statues, bearings, medals
Brass	Copper + Zinc	Musical instruments, fittings
Duralumin	Aluminium + Copper + Magnesium + Manganese	Aircraft bodies

Practice MCQs

Q1. In which type of reaction do two substances combine to form a single product? (a) Decomposition (b) Combination (c) Displacement (d) Double displacement

Q2. Heating limestone (calcium carbonate) to form calcium oxide and carbon dioxide is an example of: (a) Combination reaction (b) Decomposition reaction (c) Displacement reaction (d) Redox reaction only

Q3. In the reactivity series, which of these metals is the most reactive? (a) Gold (b) Copper (c) Potassium (d) Iron

Q4. Which of the following metals is liquid at room temperature? (a) Iron (b) Mercury (c) Sodium (d) Zinc

Q5. Which nonmetal is known to conduct electricity? (a) Sulphur (b) Graphite (c) Iodine (d) Phosphorus

Q6. Steel is primarily an alloy of iron with: (a) Zinc (b) Carbon (c) Tin (d) Chromium only

Q7. Stainless steel resists rusting mainly due to the addition of: (a) Zinc (b) Carbon (c) Chromium (d) Lead

Q8. Bronze is an alloy made of: (a) Copper and Zinc (b) Copper and Tin (c) Iron and Carbon (d) Aluminium and Copper

Q9. The highest quality coal, with the highest carbon content, is: (a) Peat (b) Lignite (c) Bituminous coal (d) Anthracite

Q10. Petroleum is separated into useful products like petrol, diesel, and kerosene through: (a) Filtration (b) Fractional distillation (c) Evaporation (d) Sublimation

Q11. Which term describes the lowest temperature at which a fuel's vapour can catch fire? (a) Calorific value (b) Boiling point (c) Flash point (d) Melting point

Q12. A large molecule made of many repeating monomer units is called a: (a) Alloy (b) Polymer (c) Isotope (d) Compound

Q13. Which type of plastic can be softened and remoulded repeatedly on heating? (a) Thermosetting plastic (b) Thermoplastic (c) Bakelite (d) Melamine

Q14. Who discovered the process of vulcanisation of rubber? (a) Leo Baekeland (b) Charles Goodyear (c) John Dalton (d) Michael Faraday

Q15. Which of the following is a natural polymer? (a) Nylon (b) PVC (c) Natural rubber (d) Polystyrene

Answer Key

Q	Answer	Reason
Q1	(b)	Combination reactions merge two or more reactants into a single new product.
Q2	(b)	Decomposition breaks one compound into simpler substances; here heat splits limestone into two products.
Q3	(c)	Potassium sits near the very top of the reactivity series, making it highly reactive among common metals.
Q4	(b)	Mercury is the sole metal that stays liquid at normal room temperature.
Q5	(b)	Graphite, a carbon allotrope, conducts electricity, an exception among nonmetals.
Q6	(b)	Steel is fundamentally an iron-carbon alloy, with carbon typically between 0.2% and 2%.
Q7	(c)	Chromium forms a protective oxide layer on the surface, giving stainless steel its rust resistance.
Q8	(b)	Bronze pairs copper with tin, distinct from brass, which pairs copper with zinc.
Q9	(d)	Anthracite has undergone the most carbonisation, giving it the highest carbon content and cleanest burn.
Q10	(b)	Fractional distillation separates crude oil components based on their differing boiling points.
Q11	(c)	Flash point measures ignition risk from vapour, distinct from calorific value, which measures energy released.
Q12	(b)	A polymer is built from many repeating monomer units joined into long chains.
Q13	(b)	Thermoplastics soften on heating and can be reshaped multiple times without permanent chemical change.
Q14	(b)	Charles Goodyear discovered vulcanisation in 1839, heating rubber with sulphur to strengthen it.
Q15	(c)	Natural rubber comes directly from tree latex, unlike nylon, PVC, and polystyrene, which are synthetic.