Geography Basics — Physical Features of Earth

Why This Chapter Matters

This chapter sits at the intersection of two sections in the ALP and Technician CBT2 paper: General Science chemistry questions and the trade-relevant material science that shows up in Technician-specific papers. Together, fuels, combustion, materials, corrosion, and lubricants typically account for six to ten marks, and unlike some chemistry topics that need years of laboratory practice to internalise, this material is entirely fact-based and memorisable in a single focused sitting. An ALP spends a working life around diesel and electric traction, and a Technician spends a working life around steel components, welding, and lubrication schedules. This chapter is not abstract classroom chemistry; it is the chemistry of the exact machines you are training to operate and maintain.

The single biggest mistake aspirants make is treating "combustion," "corrosion," and "oxidation" as three unrelated topics instead of recognising that all three are variations of the same underlying chemical idea, a substance reacting with oxygen. Once you see that connection, half the confusion in this chapter disappears. The second common mistake is mixing up alloy compositions, especially which metal goes into stainless steel versus plain carbon steel, and confusing corrosion prevention methods that sound similar (galvanising versus anodising, for instance). This chapter organises facts so those pairs stay distinct in your memory, not blurred together under exam pressure.

1. What Is a Fuel? The Basic Chemistry

A fuel is any substance that releases usable energy, usually in the form of heat, when it undergoes a chemical reaction, most commonly combustion. Fuels store chemical energy in the bonds between their atoms, and burning breaks those bonds, releasing that stored energy as heat and light.

Fuels are classified by their physical state and by their origin.

By Physical State

• Solid fuels: coal, wood, charcoal.
• Liquid fuels: petrol (gasoline), diesel, kerosene.
• Gaseous fuels: natural gas, liquefied petroleum gas (LPG), compressed natural gas (CNG).

By Origin

• Fossil fuels: formed over millions of years from the buried remains of ancient plants and organisms, subjected to heat and pressure underground. Coal, petroleum, and natural gas are the three classic fossil fuels, and they are all non-renewable, meaning nature cannot replace them within a human timescale.
• Biofuels: derived from recently living biological material, such as ethanol from sugarcane or biodiesel from vegetable oil, and considered renewable because the source crop can be regrown within a season or a few years.

Exam trap: Students sometimes assume natural gas is a "clean, renewable" fuel because it burns cleaner than coal. Natural gas is indeed cleaner-burning, but it is still a fossil fuel and therefore non-renewable. Cleaner combustion and renewability are two separate properties; do not let one imply the other.

2. Coal: Formation and Grades

Coal forms from the compressed, heated remains of ancient plant matter buried under sediment over geological time, a process called carbonisation. As carbon content increases and moisture and volatile matter decrease, coal passes through distinct grades, from the least to the most carbon-rich:

Peat → Lignite → Bituminous coal → Anthracite

Grade	Approx. Carbon Content	Character
Peat	Below 60%	Least developed, high moisture, poor fuel
Lignite	60–70%	Brownish, "brown coal," lower heating value
Bituminous	70–85%	Most commonly mined and used industrially
Anthracite	Above 85–90%	Hardest, highest carbon, cleanest burning, least common

Memory hook: Picture coal as a raw recruit rising through ranks over time under constant pressure, "Please Learn Basic Arithmetic" for Peat, Lignite, Bituminous, Anthracite, in that exact order of increasing carbon content and quality.

Exam trap: Anthracite is the highest grade and burns cleanest with the least smoke, while lignite is the lowest usable grade with high moisture content and lower heat output. Papers frequently swap these two in options to test whether you remember which end of the scale each sits on.

3. Petroleum: Formation and Fractional Distillation

Petroleum, commonly called crude oil, forms from the remains of ancient marine organisms buried under layers of sediment over millions of years, subjected to heat and pressure that converts organic matter into hydrocarbons. Crude oil is a complex mixture of many different hydrocarbon compounds, and it needs to be separated before it becomes useful fuel.

This separation happens through fractional distillation, a process carried out in a tall structure called a fractionating column. Crude oil is heated until it vaporises, and the vapour rises through the column, which is hotter at the bottom and progressively cooler toward the top. Different hydrocarbon components, called fractions, condense back into liquid at different heights depending on their boiling points, lighter, smaller molecules with lower boiling points rise higher before condensing, while heavier, larger molecules condense lower down, closer to where they were heated.

Think of fractional distillation like sorting mixed passengers by their destination at a very tall, temperature-graded station: those headed to nearby destinations (lighter fractions, lower boiling points) get off quickly near the bottom, while those headed furthest (needing to rise highest before cooling enough to "get off," in this case the lightest gases) travel to the very top of the column.

Fraction	Approx. Position in Column	Typical Use
Refinery gas (LPG)	Top (lightest)	Cooking gas, heating
Petrol (Gasoline)	Upper	Fuel for petrol engines
Kerosene	Upper-middle	Jet fuel, lighting, stoves
Diesel	Middle	Fuel for diesel engines, including locomotives
Lubricating oil / Wax	Lower-middle	Lubricants, candles, Vaseline
Bitumen/Asphalt	Bottom (heaviest)	Road surfacing, waterproofing

Exam trap: A frequently tested confusion is thinking heavier fractions rise higher because they are "denser and stronger." The opposite is true, heavier fractions have higher boiling points and stay lower in the column since they need much more heat to vaporise and rise. Lighter fractions vaporise easily and rise to the cooler top before condensing.

4. Diesel as a Railway Fuel

Diesel is a middle-weight petroleum fraction, heavier than petrol but lighter than lubricating oil, and it remains a critical fuel for Indian Railways' diesel traction fleet even as electrification expands. Diesel engines work on the compression ignition principle, unlike petrol engines, which use spark ignition.

In a diesel engine, air alone is drawn into the cylinder and compressed to a very high ratio, which heats the air significantly due to the physics of compression (compressing a gas raises its temperature). Diesel fuel is then injected into this hot, compressed air, and it ignites on contact without needing a spark plug. In a petrol engine, by contrast, a fuel-air mixture is drawn in and ignited by a spark plug at the right moment.

Exam trap: This is one of the most commonly tested distinctions in the whole chapter. Diesel engines use compression ignition, no spark plug. Petrol engines use spark ignition, with a spark plug. If a question describes an engine with "no spark plug, fuel ignites due to high compression heat," the answer is always diesel, never petrol.

Key properties tested about diesel:

• Diesel has a higher energy density than petrol, meaning it releases more energy per litre burned, contributing to better fuel efficiency in heavy-duty applications like locomotives and trucks.
• Diesel's cetane number measures its ignition quality, roughly how easily and quickly it ignites under compression; a higher cetane number means quicker, smoother ignition. This is the diesel-world counterpart to petrol's octane number, which measures resistance to premature, uncontrolled ignition (knocking) rather than ease of ignition.
• Exam trap: Do not confuse cetane number and octane number. A higher cetane number is desirable in diesel (faster ignition wanted), while a higher octane number is desirable in petrol (resistance to premature ignition wanted). They measure opposite tendencies for two different types of engines.

5. Electricity as an Energy Source in Railways

Electricity is not a primary fuel in the chemical sense, it does not itself contain stored chemical energy waiting to be burned, but it is the dominant and fastest-growing traction energy source across Indian Railways today, thanks to nationwide electrification. Electric locomotives draw power from an overhead traction line, typically carrying 25 kV AC (25,000 volts, alternating current) on broad gauge routes, through a pantograph, the spring-loaded collector mounted on the locomotive roof that maintains contact with the overhead wire.

Electric traction has clear advantages over diesel traction: no direct on-board combustion emissions, generally lower running and maintenance costs per unit of work done, higher power-to-weight ratios enabling faster acceleration, and no need to carry heavy fuel tanks. The trade-off is the enormous upfront infrastructure cost of electrifying every route, along with dependence on the stability of the wider electricity grid.

Exam trap: Students sometimes assume electric locomotives produce zero pollution overall. They produce zero tailpipe emissions at the point of use, but the electricity itself may still originate from a coal-fired power plant elsewhere on the grid, so total system-wide emissions depend on how that electricity was generated in the first place, not on the locomotive itself.

6. Combustion: The Chemistry Behind Every Fuel

Combustion is a chemical reaction in which a fuel reacts rapidly with oxygen, releasing heat and light. In its simplest, most complete form, combustion of a hydrocarbon fuel produces carbon dioxide and water vapour, along with energy.

Combustion comes in two broad forms, and this pairing is heavily tested:

• Complete combustion occurs when there is sufficient oxygen supply. The fuel burns fully, producing carbon dioxide and water vapour as the main products, along with a clean blue flame, and releasing the maximum possible energy from the fuel.
• Incomplete combustion occurs when oxygen supply is insufficient. The fuel does not burn fully, producing carbon monoxide (CO), a toxic gas, along with soot (unburnt carbon particles) and a yellow, sooty flame, and releasing less energy than complete combustion of the same amount of fuel would.

Think of combustion like a hungry crowd (fuel) trying to board a train (react with oxygen). If there are enough seats for everyone (sufficient oxygen), everyone boards cleanly and the platform clears completely (complete combustion, clean output). If there are too few seats (insufficient oxygen), some passengers are left stranded on the platform in a mess (incomplete combustion, leftover soot and toxic carbon monoxide).

Exam trap: Carbon monoxide poisoning from incomplete combustion is a real and frequently tested danger, especially relevant to enclosed spaces like a running room, a closed garage, or poorly ventilated cabins with a running engine, because CO is colourless and odourless and gives no obvious warning before it becomes dangerous.

The Fire Triangle

Combustion needs three things simultaneously, remembered as the fire triangle: fuel, oxygen (or another oxidiser), and heat (ignition source). Removing any one of the three stops or prevents combustion, which is the entire working principle behind firefighting methods.

• A water-based extinguisher removes heat by cooling the fuel below its ignition temperature.
• A foam or CO2 extinguisher smothers the fire, cutting off its oxygen supply.
• Removing fuel, for example shutting off a gas valve, removes the fire's source of chemical energy entirely.

Memory hook: "FOH" — Fuel, Oxygen, Heat, the three legs of the fire triangle; knock out any one leg and the triangle, and the fire, collapses.

Ignition Temperature and Flash Point

Ignition temperature is the minimum temperature at which a substance catches fire and continues burning on its own, without needing a continuous external flame. Flash point is the lowest temperature at which a liquid gives off enough vapour to form an ignitable mixture with air, though the resulting flame may not sustain itself continuously at that exact temperature. Flash point is always tested as being lower than the temperature needed for sustained, self-supporting combustion.

Exam trap: Flash point and ignition temperature are often confused as the same thing. Flash point is about vapour momentarily catching fire near an ignition source; ignition temperature (also called auto-ignition temperature in some contexts) is the point at which the substance sustains burning on its own.

7. Materials Used in Railways: Steel and Alloys

Steel is an alloy, a mixture of a base metal with one or more other elements, primarily made of iron and carbon, with carbon content generally kept below about 2% by weight. Iron on its own, in its pure form, is relatively soft and prone to rusting; adding a controlled small amount of carbon dramatically increases its hardness and strength, which is why plain iron is rarely used structurally and steel is used almost everywhere instead.

Think of pure iron as unrefined skill, and carbon as the training that toughens it. Too little carbon and the steel stays too soft for demanding structural use; too much carbon and it becomes brittle and prone to cracking under stress. Steel-making is a careful, controlled process of finding the right amount of that toughening ingredient.

Types of Steel by Carbon Content

Type	Approx. Carbon Content	Typical Use
Low-carbon (mild) steel	Up to 0.3%	Structural sections, sheet metal, rails' supporting fixtures
Medium-carbon steel	0.3–0.6%	Axles, gears, stronger structural components
High-carbon steel	0.6–1.5% (roughly)	Cutting tools, springs, wear-resistant parts

Exam trap: Higher carbon content increases hardness and strength but also increases brittleness, reducing ductility (the ability to bend or deform without breaking). Students often assume "more carbon is always better"; in reality, engineers choose the carbon content that balances hardness against the flexibility a specific railway component needs.

Stainless Steel

Stainless steel is an alloy of iron with a significant proportion of chromium, generally at least around 10-11% by weight, along with carbon and often nickel. The chromium reacts with oxygen in the air to form a very thin, tough, self-repairing layer of chromium oxide on the surface, which protects the underlying metal from rusting. If this protective layer is scratched, it reforms on its own as long as oxygen is present, which is exactly why stainless steel resists corrosion so effectively over long periods without needing repeated external protection.

Exam trap: Students often think stainless steel simply "does not rust" because of some inherent property of iron. It resists rusting specifically because of the chromium oxide layer, not because the underlying iron itself has changed its nature. Remove or damage the chromium content and the alloy would rust like ordinary steel.

Other Alloys Relevant to Railways

• Cast iron: an iron-carbon alloy with a much higher carbon content than steel, generally above 2%, making it hard and brittle, good for compressive strength (withstanding crushing loads) but poor under tension (pulling forces) or impact, used for items like brake blocks and certain housing components.
• Bronze: an alloy of copper and tin, valued for its resistance to corrosion, especially from water, and traditionally used in bearings and bushings due to its low-friction, wear-resistant surface.
• Brass: an alloy of copper and zinc, valued for corrosion resistance and machinability, commonly used in fittings and small hardware components.
• Aluminium alloys: valued for their light weight combined with reasonable strength, increasingly used in modern rolling stock bodies to reduce overall train weight and improve energy efficiency.

Memory hook: Remember copper's two famous alloy partners with "Tin makes Bronze, Zinc makes Brass", a simple pairing that eliminates the common mix-up between these two copper alloys.

8. Corrosion: Chemistry's Slow Attack on Metal

Corrosion is the gradual destruction of a metal caused by chemical reaction with substances in its environment, most commonly oxygen and moisture. Rusting specifically refers to the corrosion of iron and its alloys, forming hydrated iron oxide (Fe2O3.xH2O), the reddish-brown flaky substance familiar from any exposed, unprotected iron surface, an old railway fitting left out in monsoon rain being a very familiar example.

Rusting is fundamentally an oxidation reaction: iron loses electrons to oxygen in the presence of water, forming iron oxide. This is the same broad chemical family as combustion, both are reactions with oxygen, though rusting happens slowly at ordinary temperatures while combustion happens rapidly, usually releasing visible heat and light.

Conditions that accelerate corrosion:

• Moisture: water is essential for the rusting reaction to proceed at a meaningful rate; perfectly dry iron rusts extremely slowly.
• Salt: dissolved salts, such as those in coastal air or on roads treated for snow, dramatically speed up rusting by improving the electrical conductivity of the water film on the metal surface, accelerating the electrochemical reaction.
• Acidic environments: industrial pollution and acid rain increase corrosion rates on exposed metal structures.
• Higher temperatures: generally speed up most chemical reactions, including corrosion, up to a point.

Exam trap: A classic exam distractor claims that "dry rusting" happens quickly without any water present. In reality, both oxygen and moisture are required together for rusting to proceed at a meaningfully fast rate; extremely dry environments, like a desert, show very slow rusting even with oxygen freely available.

Methods of Corrosion Prevention

Railways depends heavily on effective corrosion prevention because so much of its infrastructure, rails, coach bodies, bridges, and fittings, sits exposed to weather for decades.

• Painting: a physical barrier coating that keeps moisture and oxygen away from the metal surface; the most common and cheapest method for large structures like bridges and coach exteriors.
• Galvanising: coating iron or steel with a layer of zinc, either by dipping the item in molten zinc (hot-dip galvanising) or through electroplating. Zinc protects in two ways: it forms a physical barrier, and even where the coating is scratched, zinc corrodes preferentially before the underlying iron does, because zinc is more reactive than iron. This second mechanism is called sacrificial protection.
• Anodising: an electrochemical process, mainly used on aluminium, that deliberately thickens the naturally occurring oxide layer on the metal's surface, making it more durable and corrosion-resistant. Note that anodising is specific to metals like aluminium, unlike galvanising, which specifically applies to iron and steel using zinc.
• Alloying: creating alloys like stainless steel specifically to resist corrosion through their own chemical composition, rather than relying on an external coating at all.
• Cathodic protection: attaching a more reactive "sacrificial" metal, often magnesium or zinc, to a structure like a buried pipeline or a ship's hull; the sacrificial metal corrodes instead of the protected structure, following the exact same underlying principle as galvanising's sacrificial protection, just applied at a larger structural scale.

Exam trap: Galvanising and anodising are frequently confused because both involve a protective surface layer. Galvanising specifically means coating with zinc, typically applied to iron and steel. Anodising specifically means electrochemically thickening the metal's own natural oxide layer, typically applied to aluminium. If a question mentions zinc coating, the answer is galvanising; if it mentions an oxide layer treatment on aluminium, the answer is anodising.

9. Lubricants: Reducing Friction, Reducing Wear

A lubricant is a substance introduced between two moving surfaces to reduce friction and wear between them. Without lubrication, moving metal parts in constant contact, an axle bearing, a gear, a piston, would generate enormous heat from friction, wear down rapidly, and eventually seize entirely.

Think of a lubricant as the smooth announcement and clear signalling that keeps trains moving through a junction without grinding to a halt against each other. Just as good signalling prevents costly collisions and delays, a good lubricant film prevents costly metal-on-metal wear and heat buildup.

Types of Lubricants

• Liquid lubricants (oils): mineral oils derived from petroleum, or synthetic oils engineered for specific performance needs, used widely in engines, gearboxes, and hydraulic systems where continuous flow and cooling are needed.
• Semi-solid lubricants (greases): oil combined with a thickening agent, staying in place better than plain oil, commonly used in bearings and joints where the lubricant needs to resist being flung out by motion or gravity.
• Solid lubricants: substances like graphite and molybdenum disulphide, used where liquid lubricants are impractical, for example at very high temperatures where oil would break down, or in situations needing a dry, clean lubricating film.

Exam trap: Graphite, a form of carbon, is often mistakenly assumed to be a poor conductor since coal and most carbon-related materials taught earlier in school chemistry are non-conductors. Graphite is actually a good conductor of electricity, one of very few non-metals with this property, which is exactly why it is used both as a lubricant and as the material in dry-cell battery electrodes and pencil "lead."

Key Properties of a Good Lubricant

• Viscosity: a measure of a fluid's resistance to flow; too thin and the lubricant film breaks down under load, too thick and it creates excess drag and resistance.
• Viscosity index: measures how much a lubricant's viscosity changes with temperature; a higher viscosity index means more stable performance across a wide temperature range, valuable for engines and machinery that must work reliably from a cold morning start to sustained high-temperature running.
• Flash point: a lubricant's flash point should be well above its expected operating temperature, to avoid any fire risk during use.

Quick Revision — One-Line Facts

• Fossil fuels, coal, petroleum, and natural gas, are all non-renewable.
• Biofuels are renewable because their source crops regrow within a short timescale.
• Coal grades rise in quality as: Peat, Lignite, Bituminous, Anthracite.
• Anthracite has the highest carbon content and burns cleanest among coal grades.
• Petroleum is separated into fractions through fractional distillation.
• Lighter petroleum fractions rise to the top of the fractionating column; heavier ones stay near the bottom.
• Diesel engines use compression ignition and have no spark plug.
• Petrol engines use spark ignition with a spark plug.
• Cetane number measures diesel ignition quality; octane number measures petrol's resistance to premature ignition.
• Electric locomotives collect power via a pantograph from an overhead line, typically 25 kV AC on broad gauge.
• Complete combustion produces carbon dioxide and water with sufficient oxygen supply.
• Incomplete combustion produces toxic carbon monoxide and soot due to insufficient oxygen.
• The fire triangle consists of fuel, oxygen, and heat; removing any one stops combustion.
• Flash point is lower than the ignition (auto-ignition) temperature of a substance.
• Steel is an alloy of iron and carbon, with carbon content generally below about 2%.
• Higher carbon content increases hardness but reduces ductility in steel.
• Stainless steel resists rusting due to a protective chromium oxide layer.
• Cast iron has higher carbon content than steel and is hard but brittle.
• Bronze is an alloy of copper and tin; brass is an alloy of copper and zinc.
• Rusting is the corrosion of iron, forming hydrated iron oxide.
• Rusting requires both oxygen and moisture together to proceed at a meaningful rate.
• Salt accelerates rusting by increasing the conductivity of the moisture film on metal.
• Galvanising coats iron or steel with zinc for sacrificial and barrier protection.
• Anodising electrochemically thickens the natural oxide layer, mainly on aluminium.
• Cathodic protection uses a sacrificial, more reactive metal to protect a structure from corrosion.
• Lubricants reduce friction and wear between moving surfaces.
• Grease is oil combined with a thickening agent for better retention in place.
• Graphite and molybdenum disulphide are common solid lubricants.
• Graphite conducts electricity despite being a non-metal, unusual among non-metals.
• Viscosity index measures how stable a lubricant's viscosity stays across temperature changes.

Memory Tables

Table 1: Diesel vs Petrol Engine Fundamentals

Feature	Diesel Engine	Petrol Engine
Ignition method	Compression ignition	Spark ignition
Spark plug needed	No	Yes
Fuel introduced	Injected into hot compressed air	Mixed with air before compression
Quality measure	Cetane number (higher = better)	Octane number (higher = better)

Table 2: Corrosion Prevention Methods

Method	What It Applies	Mechanism
Painting	Any exposed metal surface	Physical barrier against moisture and oxygen
Galvanising	Iron and steel	Zinc coating; barrier plus sacrificial protection
Anodising	Aluminium	Electrochemically thickened natural oxide layer
Alloying	Steel (e.g., stainless steel)	Chromium forms a self-repairing oxide layer
Cathodic protection	Large structures (pipelines, hulls)	Sacrificial reactive metal corrodes first

Practice MCQs

Q1. Which of the following is classified as a fossil fuel? (a) Ethanol (b) Biodiesel (c) Natural gas (d) Firewood

Q2. Arrange the coal grades in correct order of increasing carbon content. (a) Anthracite, Bituminous, Lignite, Peat (b) Peat, Lignite, Bituminous, Anthracite (c) Lignite, Peat, Anthracite, Bituminous (d) Bituminous, Anthracite, Peat, Lignite

Q3. In fractional distillation of crude oil, which fraction typically condenses nearest the top of the fractionating column? (a) Bitumen (b) Diesel (c) Refinery gas/LPG (d) Lubricating oil

Q4. Which type of ignition does a diesel engine use? (a) Spark ignition (b) Compression ignition (c) Electric arc ignition (d) Catalytic ignition

Q5. A higher cetane number in diesel fuel indicates: (a) Better resistance to knocking (b) Faster, smoother ignition under compression (c) Lower energy content (d) Higher sulphur content

Q6. Electric locomotives typically collect power from the overhead line through which component? (a) Bogie (b) Pantograph (c) Buffer (d) Coupler

Q7. Incomplete combustion of a fuel due to insufficient oxygen primarily produces which toxic gas? (a) Carbon dioxide (b) Carbon monoxide (c) Sulphur dioxide (d) Nitrogen dioxide

Q8. Which three elements make up the fire triangle? (a) Fuel, water, heat (b) Fuel, oxygen, heat (c) Oxygen, water, pressure (d) Carbon, oxygen, nitrogen

Q9. Steel is primarily an alloy of iron with which element? (a) Zinc (b) Carbon (c) Tin (d) Chromium

Q10. Stainless steel resists rusting mainly due to the presence of which element forming a protective oxide layer? (a) Nickel (b) Zinc (c) Chromium (d) Carbon

Q11. Bronze is an alloy formed by combining copper with which metal? (a) Zinc (b) Tin (c) Aluminium (d) Nickel

Q12. Rusting of iron requires the simultaneous presence of: (a) Heat and carbon dioxide only (b) Oxygen and moisture (c) Nitrogen and pressure (d) Sunlight only

Q13. Coating iron or steel with a layer of zinc to prevent corrosion is called: (a) Anodising (b) Galvanising (c) Alloying (d) Cathodic protection

Q14. Anodising is an electrochemical corrosion-prevention process mainly applied to which metal? (a) Iron (b) Aluminium (c) Copper (d) Lead

Q15. Which of the following is classified as a solid lubricant? (a) Mineral oil (b) Grease (c) Graphite (d) Synthetic hydraulic fluid

Answer Key

Q	Answer	Reason
Q1	(c)	Natural gas forms from ancient buried organisms like coal and petroleum, unlike ethanol, biodiesel, or firewood, which are renewable biofuels.
Q2	(b)	Coal quality and carbon content rise in the order Peat, Lignite, Bituminous, Anthracite, formed through progressive carbonisation.
Q3	(c)	Lightest fractions with the lowest boiling points rise highest and condense near the top of the column, unlike heavy bitumen at the bottom.
Q4	(b)	Diesel engines ignite fuel purely from the heat of high compression, with no spark plug involved, unlike petrol engines.
Q5	(b)	Cetane number measures how readily diesel ignites under compression; higher means faster, smoother ignition.
Q6	(b)	The pantograph is the roof-mounted collector that maintains contact with the overhead traction wire.
Q7	(b)	Insufficient oxygen during combustion leaves carbon partly unburnt, forming toxic carbon monoxide instead of carbon dioxide.
Q8	(b)	Fuel, oxygen, and heat must all be present together for combustion; removing any one stops the fire.
Q9	(b)	Steel is fundamentally an iron-carbon alloy, with carbon content generally kept below about 2%.
Q10	(c)	Chromium reacts with oxygen to form a thin, self-repairing chromium oxide layer that protects the underlying steel.
Q11	(b)	Bronze is copper alloyed with tin, distinct from brass, which is copper alloyed with zinc.
Q12	(b)	Rusting is an oxidation reaction that proceeds at a meaningful rate only when both oxygen and moisture are present together.
Q13	(b)	Galvanising specifically refers to applying a protective zinc coating to iron or steel.
Q14	(b)	Anodising electrochemically thickens the natural oxide layer on aluminium, unlike galvanising, which applies to iron and steel.
Q15	(c)	Graphite is a solid lubricant used where liquid oils or greases are impractical, unlike mineral oil, grease, or hydraulic fluid, which are liquid or semi-solid.