Combustion is a chemical reaction between a fuel and an oxidiser (usually oxygen) that produces heat and often light. Chemically, it involves oxidation where the fuel loses electrons and oxygen is reduced. For example, methane combustion:
CH₄ + 2O₂ → CO₂ + 2H₂O + heat.
In this reaction, methane (CH₄) reacts with oxygen to form carbon dioxide and water; energy is released because the products have lower chemical potential energy than reactants. This released energy appears as heat and sometimes light (flame).

Three conditions are essential: a fuel, an oxidiser (oxygen), and sufficient heat (ignition temperature). Fuel provides the chemical substance that will oxidise; oxygen (or another oxidiser) participates in the chemical reaction, accepting electrons; and heat raises the fuel to its ignition temperature to start the reaction. Without any one component the chain reaction cannot be sustained. For example, removing oxygen by smothering prevents combustion even if fuel and heat are present.

Combustion is a rapid chemical oxidation with noticeable heat and usually light (e.g., burning wood). Corrosion is a slow oxidation process, often involving moisture, leading to deterioration of materials (e.g., rusting of iron). Combustion requires ignition temperature and is often reversible only by chemical processing, while corrosion occurs gradually at ambient conditions. Combustion products are typically gases (CO₂, H₂O), while corrosion results in solid oxides or salts (Fe₂O₃ for rust).

Incomplete combustion produces carbon monoxide (CO), soot (particulate carbon), and unburnt hydrocarbons — all harmful pollutants. CO is poisonous, binding haemoglobin and reducing oxygen transport in the blood. Soot and particulates contribute to respiratory illnesses and climate effects. Measures to reduce harm include ensuring complete combustion (properly tuned burners/burners with adequate air), using cleaner fuels (LPG, natural gas), catalytic converters in vehicles to oxidise CO to CO₂, regular maintenance of stoves and chimneys, and introducing emission controls at industrial sources.

Ignition temperature is the minimum temperature at which a substance will catch fire and sustain combustion. The flash point is the lowest temperature at which vapour above a liquid ignites momentarily on exposure to an ignition source. The key difference: ignition temperature is about sustained burning, while flash point is about temporary ignition of vapour. These concepts apply in safety storage of fuels — materials with low flash points are handled carefully, and ignition temperatures guide design of heaters and combustion devices.

Spontaneous combustion can occur when oily rags are left bunched together; oxidation of the oil releases heat, and if heat is not dissipated, temperature rises to ignition point and the rags ignite. For example, linseed oil rags used in wood finishing can self-heat and catch fire in a waste bin. The underlying cause is exothermic oxidation and poor heat dissipation leading to autoignition.

(Students should draw a neat candle flame with three zones.) The inner zone (dark) nearest the wick contains unburnt wax vapour and is relatively cool since insufficient oxygen reaches it. The middle luminous zone contains glowing carbon particles due to partial combustion; it appears yellow and is hotter than inner zone. The outer non-luminous zone is where complete combustion occurs with sufficient oxygen, producing CO₂ and H₂O; it is the hottest and often blue. Temperature gradients arise because oxygen availability and combustion completeness increase from inner to outer zones.

When air supply is sufficient, the fuel mixes well with oxygen resulting in complete combustion; molecular products do not form glowing carbon particles and the flame is blue (hotter). When air supply is restricted, incomplete combustion produces glowing carbon particles that emit yellow light, thus a luminous yellow flame forms. Hence, blue flame = more complete combustion and higher temperature.

When a candle burns, wax near the flame melts and is drawn up the wick by capillary action. The heat vaporises the wax; the vapour mixes with air and burns in the flame. Thus, the wick acts as a transporter of fuel (molten wax) to the combustion zone, sustaining the flame until fuel is exhausted.

Factors include fuel type (higher calorific value fuels give higher temperatures), oxygen supply (more oxygen enables complete combustion and higher temperatures), fuel-air mixing (better mixing increases efficiency), and pressure. Impurities and moisture in fuel reduce temperature by absorbing heat. Optimal air supply and efficient mixing ensure higher flame temperatures and cleaner combustion.

Smaller fuel particles provide larger surface area to volume ratio, allowing faster heat transfer and quicker combustion (e.g., fine wood shavings burn faster than logs). For gases, particle size is irrelevant; mixing and vapourisation matter. In engines and furnaces, smaller/finer fuels enhance combustion rate and completeness.

Turbulence improves mixing of fuel and air, increasing contact between fuel molecules and oxygen, leading to more complete combustion and higher thermal efficiency. Practical burners are designed to introduce controlled turbulence to enhance combustion while avoiding flame instability.

For general hydrocarbon CxHy:
Complete: CxHy + (x + y/4)O₂ → xCO₂ + (y/2)H₂O + energy.
Incomplete (oxygen limited): CxHy + O₂ → CO + (soot) + H₂O + less energy.
Complete combustion converts carbon to CO₂ releasing maximum energy. Incomplete combustion yields CO and carbon (soot) and releases less energy because oxidation is partial.

CO forms when carbon-containing fuels burn with insufficient oxygen, preventing full oxidation to CO₂. CO binds strongly to haemoglobin, blocking oxygen transport and causing poisoning. Prevention includes ensuring good ventilation, maintaining gas appliances, installing CO detectors, and ensuring chimneys and exhausts are clear so combustion gases leave the house.

Light a candle and place a cold metal spoon or watch glass above the flame; soot deposits on the surface showing carbon particles from incomplete combustion. Alternatively, partially cover the flame with a jar limiting oxygen; a yellow luminous flame appears and soot forms, demonstrating incomplete combustion due to limited oxygen supply.

Fuels are judged by calorific value (energy per unit mass/volume), ease of ignition, combustion cleanliness, and availability. Higher calorific value fuels release more energy on combustion. Clean-burning fuels producing mainly CO₂ and H₂O (e.g., natural gas) are preferred because they are efficient and cause less pollution. Ease of storage and cost also matter in choosing fuels.

Moisture absorbs heat to evaporate, reducing flame temperature and combustion efficiency. Wet fuels burn poorly, produce more smoke and incomplete combustion products. Dry fuels burn hotter and cleaner; hence drying biomass before burning improves efficiency.

Catalytic converters use catalysts (platinum, palladium, rhodium) to promote chemical reactions converting CO and unburnt hydrocarbons into CO₂ and H₂O, and reducing NOx to N₂. They operate at high temperatures and provide surfaces where reactions occur efficiently, reducing pollution from incomplete combustion in vehicle engines.

Fire triangle consists of heat, fuel and oxygen. Removing heat: apply water or cooling agents to lower temperature below ignition point. Removing fuel: create firebreaks or remove nearby combustibles (used in forest fires). Removing oxygen: smother with foam, CO₂ or blankets to suffocate the fire. Each approach interrupts the combustion process and extinguishes the fire.

Water extinguishers cool and are effective on ordinary combustible fires (wood, paper) but unsafe for electrical and oil fires. Foam extinguishers blanket fuels and are suitable for oil/fuel fires. CO₂ extinguishers displace oxygen and are used for electrical and small oil fires; they leave no residue. Dry powder extinguishers interrupt chemical reactions and are versatile for oil, gas and electrical fires. Choosing the right extinguisher avoids worsening a fire (e.g., not using water on oil fires).

Preventive measures: keep flammable cloths away from stove, do not leave cooking unattended, maintain clean burners and exhaust hoods, and store flammable liquids safely. For small oil fires: do not use water — use lids to smother, use foam or dry powder extinguisher, or close gas supply. Always call emergency services for larger fires.

Foam forms a stable blanket over burning oil, preventing oxygen from reaching the fuel and cooling it slightly. The blanket also stops splashing and vapor release, reducing re-ignition risk. Because oil floats on water, water alone can spread oil; foam specifically isolates and suffocates the fuel layer.

Immediately switch off gas supply if safe, avoid flames, switches or electrical devices (which can spark), ventilate by opening doors/windows, evacuate occupants, and inform gas supplier/emergency services. Do not use mobile phones or operate electrical switches in the vicinity until safe.

Sand is useful for extinguishing small oil or electrical fires (if safe) by smothering — it blocks oxygen and absorbs heat. Sand does not conduct electricity, so it can be used cautiously on small electrical fires; however, for large fires professional methods are needed.

In thermal power plants, fuels like coal, oil or gas are burned in furnaces to convert water into steam. High-pressure steam drives turbines connected to generators that produce electricity. Controlled combustion ensures steady heat release, efficient boiler operation, and minimal pollutant release via emission control systems like electrostatic precipitators and flue-gas desulfurisation.

Internal combustion engines burn a fuel-air mixture inside cylinders; expanding gases push pistons producing mechanical work. Complete combustion converts more fuel energy into heat and then into mechanical work, reducing unburnt hydrocarbons and CO emissions, improving fuel efficiency and lowering pollution. Proper air-fuel ratio and spark timing are critical for near-complete combustion.

Industries use filters (electrostatic precipitators) to remove particulates, scrubbers to remove acidic gases, catalytic converters or oxidisers to convert CO and hydrocarbons to CO₂ and H₂O, and adopt cleaner fuels and combustion technologies to lower emissions. Regular monitoring, process optimisation and compliance with emission standards help reduce environmental impact.

Uncontrolled stubble burning contributes substantially to air pollution and particulate matter; it is often done for quick field clearing but harms air quality and soil. Controlled burning under supervision can reduce fuel loads and prevent larger wildfires, but safer alternatives like mulching, mechanical removal, and converting biomass to bioenergy are preferred to reduce pollution and retain soil nutrients.

Protocols include storing flammable liquids in labelled, sealed containers in ventilated cabinets, using spill trays, grounding and bonding during transfer to prevent static sparks, keeping ignition sources away, having fire extinguishers nearby, and training personnel in spill response and emergency evacuation. Safety Data Sheets (SDS) guide handling specifics for each chemical.

Students should: learn clear definitions (combustion, ignition temperature, oxidiser), draw and label flame zones, distinguish complete and incomplete combustion with balanced equations and consequences, remember fire triangle and extinguisher uses, and be able to discuss safety measures and real-life applications. Practice diagrams and model answers under time constraints to ensure clarity and completeness in exam answers.