What is ANFO and why is it classified as a tertiary explosive?

ANFO is ammonium nitrate fuel oil, a mixture of approximately 94% ammonium-nitrate prills and 6% fuel oil by mass. Oxygen balance is tuned to near zero so that detonation releases maximum energy with clean off-gas. VoD is about 3,200 m/s, much slower than military secondaries. ANFO is classified as a tertiary explosive (blasting agent) because it is so insensitive to drop, friction and small flame that it cannot be set off by a normal detonator alone; it needs a secondary booster (a small charge of RDX, PETN-cord or pentolite) to initiate. That insensitivity makes ANFO safe to bulk-handle in mining, which is why it is the most widely used commercial blasting agent worldwide. It has also been used in large vehicle-borne IEDs (Oklahoma City, 1995).

Which Indian statutes govern explosives, and who is the licensing authority?

Two parallel statutes apply. The Explosives Act 1884, read with the Explosives Rules 2008, regulates manufacture, possession, sale, transport, import and use of commercial and industrial explosives, classifying them into seven classes from gunpowder (Class 1) through fireworks (Class 7). The licensing authority is the Petroleum and Explosives Safety Organisation, PESO, headquartered at Nagpur, under DPIIT. The Explosive Substances Act 1908 is the criminal statute aimed at unlawful use: Section 3 carries up to imprisonment for life for causing an explosion likely to endanger life or property, Section 4 covers attempts, Section 5 covers suspicious possession. Trials under this Act have included the 1993 Mumbai blasts, the 2006 Mumbai train bombings, the 26/11 charge sheet, the 2019 Pulwama attack and the Hyderabad and Pune ammonium-nitrate bombings.

Explosives: Definition, Types and Classification

Q: What is the difference between deflagration and detonation for UGC-NET?

Deflagration is subsonic burning of a low explosive or propellant, with the reaction front advancing at millimetres per second to a few metres per second by thermal conduction. Pressure rises gradually. Black powder and smokeless powder deflagrate. Detonation is supersonic decomposition of a high explosive driven by a shock wave that compresses and ignites the next layer; reaction-front velocity is 1,500 to 9,000 m/s and pressure spikes are in the tens of gigapascals. RDX, TNT, PETN and HMX detonate. The same material can transition from deflagration to detonation under confinement, which is why stored stacks of nitrate fuel mix are dangerous in a fire.

Q: How do primary, secondary and tertiary explosives differ in sensitivity and use?

Primary explosives (lead azide, lead styphnate, mercury fulminate, DDNP) are extremely sensitive to heat, shock, friction and spark, and are used in milligram quantities inside detonators and primers to convert a small input into a shock pulse. Secondary explosives (TNT, RDX, HMX, PETN, nitroglycerin, tetryl, picric acid) are insensitive to drop and friction; they need a detonator to set them off and serve as the main charge. Tertiary explosives or blasting agents (ANFO, slurries, emulsions) are so insensitive that they need a secondary booster in addition to the detonator, and are used in bulk mining and civil blasting where cost and safety matter more than brisance.

Q: What is the composition of C-4 and Semtex?

C-4 is a plastic explosive made of approximately 91% RDX bound in a plasticiser-binder system (commonly polyisobutylene, di(2-ethylhexyl) sebacate and a small amount of motor oil). Its VoD is about 8,040 m/s. Semtex is a Czech-origin plastic explosive containing both RDX and PETN bound in a styrene-butadiene matrix; the RDX:PETN ratio varies by grade. VoD is about 7,400 m/s. Semtex was historically notorious for low vapour pressure, which made it hard for sniffer dogs to detect until international taggants were mandated in 1991. The Indian-Army equivalent is PEK1, an RDX-based plastic explosive produced domestically.

UGC-NET Paper 2 Unit VI notes on explosives. Definition, deflagration vs detonation, low/high/plastic classification, VoD, brisance, Indian Explosives Act 1884 and PESO Nagpur.

Last updated: 18 May 2026

Explosives are one of the highest-scoring sub-bullets of UGC-NET Forensic Science Unit VI because every part of the topic has a clean one-line MCQ answer. NTA repeatedly tests the deflagration versus detonation distinction, the low and high explosive split with VoD numbers, the three-tier primary, secondary and tertiary scheme, the chemistry of RDX, PETN, TNT and ANFO, and the Indian statutory frame under the Explosives Act 1884 and the Explosive Substances Act 1908. Get the definitions and the numbers right and this whole bullet collapses into rapid-fire memorisation.

Treat this topic as classification first, chemistry second, statute third. The sibling topic on post-blast residue analysis covers detection and identification of these same substances after a blast; this bullet stays on the upstream side of the workflow, where you define what an explosive is, what families it falls into, and which Indian law governs its possession and use. Read this with the firearms-and-ammunition Unit V topic close to hand, because primer chemistry overlaps with primary-explosive chemistry.

Key terms

Explosive: A chemical or mixture that undergoes rapid exothermic decomposition releasing large volumes of gas and heat when triggered by heat, shock, friction or electric stimulus. The reaction produces a pressure shock wave.
Velocity of Detonation (VoD): The speed at which the detonation wave travels through a high explosive. Reported in metres per second. Low explosives never detonate; high explosives detonate at 1,500 to 9,000 m/s.
Brisance: The shattering effect of an explosive, related to how fast the pressure peak rises. High-brisance secondaries (RDX, HMX, PETN) shatter steel; low-brisance compositions push or heave material.
Sensitivity: How easily an explosive is set off by impact, friction, spark, heat or shock. Primary explosives are highly sensitive; secondary explosives need a detonator; tertiary explosives need a booster.
Oxygen balance: The percentage excess or deficit of oxygen relative to that required to oxidise the carbon, hydrogen and metals in the explosive fully. RDX has oxygen balance about −21.6%, TNT about −74%, ANFO is tuned to about 0%.
Deflagration: Subsonic burning of a propellant or low explosive driven by heat conduction. Reaction front travels at mm/s to a few m/s. Black powder and smokeless powder deflagrate.
Detonation: Supersonic decomposition driven by a shock wave that compresses and ignites the next layer. Reaction front travels at km/s. RDX, TNT, PETN detonate.
Detonator: A small initiating charge of primary explosive (lead azide, lead styphnate, mercury fulminate or DDNP) that converts an electrical, mechanical or pyrotechnic input into a shock pulse strong enough to set off a secondary main charge.

Definition and the deflagration versus detonation split

The single NTA-favourite distinction in this whole topic.

An explosive is a single chemical compound or a mixture that, when initiated by a small input of heat, shock, friction or electric energy, undergoes a very rapid self-sustaining exothermic decomposition. The reaction releases a large volume of hot gas, generates a steep pressure rise, and emits a shock wave that does mechanical work on the surrounding medium. Three features have to be present: rapid reaction rate, large gas volume per unit mass, and a high heat of reaction. Anything short of all three is not an explosive in the forensic sense.

The single most-tested distinction in this bullet is deflagration versus detonation. Both are exothermic decompositions, but the physics is different.

Deflagration is subsonic burning. The reaction front travels through the material by thermal conduction (and convective gas flow), advancing at millimetres per second to a few metres per second. Pressure builds gradually. Black powder, smokeless powder and pyrotechnic compositions deflagrate, which is why they make good propellants: they push a bullet or a rocket without shattering the barrel or the casing.

Detonation is supersonic, shock-driven decomposition. A shock wave compresses and heats the next layer of explosive faster than thermal conduction could, so the wave is self-sustaining. Reaction-front velocities are 1,500 m/s for the slowest secondaries and 9,000 m/s for HMX. Pressure spikes are in the tens of gigapascals. RDX, PETN, TNT and HMX detonate, which is why they shatter steel and concrete rather than push them. A material can deflagrate under one set of conditions and detonate under another (confinement, particle size, density, initiator strength), and this is called the deflagration-to-detonation transition, and it is what makes a stored stack of nitrate-fuel mix dangerous in a fire.

For MCQs, the safe one-line rule is this. Low explosives deflagrate at mm/s to a few m/s, are used as propellants and pyrotechnics, and are characterised by burn rate. High explosives detonate at 1,500 to 9,000 m/s, are used as main charges, and are characterised by velocity of detonation (VoD), brisance and oxygen balance.

Key physical properties NTA tests

VoD, brisance, sensitivity, oxygen balance, energy density.

Once you know whether a material deflagrates or detonates, the next layer of MCQs tests the properties used to characterise high explosives.

Velocity of Detonation (VoD). The detonation-wave speed. Measured experimentally by ionisation probes or fibre-optic timers placed along a charge. Approximate values to memorise: ANFO 3,200 m/s, TNT 6,900 m/s, RDX 8,750 m/s, PETN 8,300 m/s, HMX 9,100 m/s, nitroglycerin 7,700 m/s. The bigger the VoD, the more powerful the secondary explosive at the same density.

Brisance. The shattering effect, related to the rate of pressure rise at the detonation front. Measured by the sand-crush test, the lead-block (Trauzl) test or the plate-dent test. RDX, HMX and PETN are high-brisance; ANFO and slurries are low-brisance heaving explosives, which is why ANFO is preferred for mining (heave rock) while RDX is preferred for cutting steel.

Sensitivity. How easily the explosive is initiated. Standardised by drop-hammer impact tests, friction tests, electric-spark tests and minimum initiating charge for shock. Primary explosives are highly sensitive (a static-electric discharge is enough). Secondaries are insensitive to drop and friction; they need a detonator. Tertiaries (ANFO, slurries, emulsions) are so insensitive they need a secondary booster on top of the detonator.

Oxygen balance. Negative oxygen balance means the explosive carries less oxygen than it needs to oxidise all its own carbon and hydrogen, so detonation gives sooty CO-rich gases. TNT (oxygen balance about −74%) is famously oxygen-deficient, which is why TNT smoke is black. ANFO is mixed to about zero oxygen balance (about 94% ammonium nitrate plus 6% fuel oil) for maximum energy release. Mining-grade explosives are engineered to oxygen balance to control fume colour and gallery air quality.

Energy density and crystal density. Energy per unit mass (kJ/g) and packed crystal density (g/cm³) determine how much work the same charge volume can do. HMX is the densest common military explosive (1.91 g/cm³), which is why it goes into compact warheads.

The full classification tree

Low / high (primary, secondary, tertiary) / plastic. Map every name onto a branch.

Low explosives (deflagrating). VoD typically below 1,000 m/s. Burn rather than detonate. The three named families NTA tests are:

Black powder. The original gunpowder, a mechanical mixture of potassium nitrate (about 75%), charcoal (about 15%) and sulphur (about 10%). Used in fuse trains, signal flares, antique muzzle-loaders and pyrotechnics. Hygroscopic and dirty-burning.
Smokeless powder. Nitrocellulose-based propellants, classified by base count. Single base is nitrocellulose (NC) alone. Double base is nitrocellulose plus nitroglycerin (NC + NG). Triple base adds nitroguanidine (NC + NG + NQ) and is used in large-calibre artillery for cool-burning, low-flash performance. Smokeless powder is the propellant in all modern small-arms ammunition; primer chemistry sits next to it in the firearm-injuries and Unit V topics.
Pyrotechnics. Compositions designed for light, heat, smoke or sound rather than mechanical work. Includes signal flares, illumination compositions, smoke generators, firecrackers and incendiary mixtures.

High explosives, primary (initiating). Highly sensitive to heat, shock, friction and spark. Used in milligram quantities inside detonators and primers to convert a small mechanical or electrical input into a shock wave strong enough to set off a secondary. The canonical Indian-syllabus four are lead azide (Pb(N3)2), lead styphnate (lead 2,4,6-trinitroresorcinate), mercury fulminate (Hg(ONC)2) and DDNP (diazodinitrophenol). Lead styphnate plus barium nitrate plus antimony sulphide is also the classical small-arms primer mix that leaves the Pb-Ba-Sb three-component GSR particle profile.

High explosives, secondary (main charge). Insensitive to drop, friction and small flame. Need a detonator. VoD 4,000 to 9,000 m/s. The names every NET aspirant must be able to identify by acronym and chemistry:

TNT (2,4,6-trinitrotoluene). Nitroaromatic. Melt-cast pourable, oxygen balance −74%, VoD 6,900 m/s. Long the military standard.

VoD comparison chart and oxygen-balance numbers

The numbers NTA loves to put in distractors.

The numbers vary by a few hundred m/s across textbook sources because VoD depends on packing density, particle size and confinement. For NET MCQs, learn the order (BP and smokeless < ANFO < TNT < NG < PETN < RDX < HMX) and one representative value for each. Oxygen balance approximations: RDX −21.6%, HMX −21.6%, PETN −10%, TNT −74%, NG +3.5%, ANFO close to 0%.

Alternative classification axes

By use, by chemistry, by UN transport hazard class.

The primary-secondary-tertiary tree is the one NTA tests first, but a question can also frame the same set of substances along three other axes.

By use. Military (RDX, HMX, TNT, C-4, Semtex, PEK1, PE-3A) versus commercial (ANFO, slurries, emulsions, dynamite, detonating cord) versus improvised (homemade nitrate-mix, urea nitrate, TATP, HMTD, fertiliser bombs). Indian IED casework crosses the commercial-improvised line: a stolen mining-grade slurry stick triggered by a commercial detonator and packed into a tiffin box is the prototypical Maoist or terrorist device, and the sibling NET topic on country-made bombs and IEDs picks this up in detail.

By chemistry. Three broad chemical families and a fourth catch-all. Nitroaromatics are toluene or phenol rings with nitro groups (TNT, picric acid, tetryl). Nitramines have N–NO2 bonds (RDX, HMX, tetryl, nitroguanidine). Nitrate esters have O–NO2 bonds (PETN, NG, NC, ethylene glycol dinitrate). Inorganic and primary explosives are the rest (lead azide, lead styphnate, mercury fulminate, ammonium nitrate, peroxides like TATP and HMTD).

By UN transport hazard class. The ICAO/IATA Dangerous Goods regulations and the UN Yellow Book classify explosives under Class 1, with six divisions. Class 1.1 explosives have a mass-explosion hazard (most military secondaries); Class 1.2 have a projection hazard but not mass explosion; Class 1.3 have a fire hazard plus minor blast (most propellants); Class 1.4 have minor hazard (commercial fireworks); Class 1.5 are very insensitive substances with a mass-explosion hazard (ANFO and blasting agents); Class 1.6 are extremely insensitive detonating articles. NET sometimes drops a one-liner on UN 1.1 versus 1.5: remember that ANFO is 1.5, RDX-based main charges are 1.1.

The Indian statutory frame

Explosives Act 1884, Explosives Rules 2008, Explosive Substances Act 1908, PESO Nagpur.

Indian law treats explosives under two parallel statutes. The Explosives Act 1884 (with the Explosives Rules 2008) regulates manufacture, possession, sale, transport, import and use of commercial and industrial explosives. The Explosive Substances Act 1908 is a criminal statute aimed at unlawful use, attempted use and conspiracy with explosives.

Explosives Act 1884 + Explosives Rules 2008. The Rules classify explosives into seven classes for the purpose of licensing and storage.

Class 1, gunpowder and similar deflagrating mixtures.
Class 2, nitrate mixtures (ANFO and similar).
Class 3, nitro-compounds (TNT, RDX, picric acid, PETN, NG and other nitrate esters and nitramines).
Class 4, chlorate mixtures.
Class 5, fulminates (lead azide, mercury fulminate, primary initiators).
Class 6, ammunition (loaded cartridges, shells).
Class 7, fireworks and similar pyrotechnics.

The licensing authority is the Petroleum and Explosives Safety Organisation (PESO) at Nagpur, under the Department for Promotion of Industry and Internal Trade (DPIIT), Ministry of Commerce and Industry. PESO issues manufacture, import, possession, sale and transport licences and inspects explosives factories and magazines.

Explosive Substances Act 1908. Section 3 punishes causing an explosion likely to endanger life or property with up to imprisonment for life. Section 4 covers attempts. Section 5 covers possession of explosive substances under suspicious circumstances. Section 6 covers abetment. Trials under this Act in India have prosecuted RDX-based bombings including the 1993 Mumbai serial blasts, the 2006 Mumbai suburban train bombings, the 26/11 charge sheet and the 2019 Pulwama attack (which used a vehicle-borne IED based on approximately 350 kg of RDX-mixed explosive). Ammonium-nitrate-based attacks including the 2007 Hyderabad twin bombings and the 2010 German Bakery, Pune bombing were also tried under this Act.

MCQ takeaways

Deflagration is subsonic (mm/s to a few m/s, thermal conduction); detonation is supersonic (1,500 to 9,000 m/s, shock-driven).Low explosives include black powder (KNO3 + C + S, ~75/15/10), smokeless powder (NC single, NC+NG double, NC+NG+NQ triple base) and pyrotechnics. Used as propellants.Primary explosives are lead azide, lead styphnate, mercury fulminate and DDNP. Used in detonators and primers.Secondary explosives are TNT, RDX, HMX, PETN, nitroglycerin, tetryl and picric acid. Need a detonator to set off.Tertiary explosives are ANFO, slurries and emulsions. Need a booster plus a detonator.RDX = cyclotrimethylenetrinitramine = hexogen = cyclonite. VoD ~8,750 m/s. The canonical Indian terrorist-use explosive.PETN = pentaerythritol tetranitrate. Nitrate ester. VoD ~8,300 m/s. Component of Semtex and detonating cord.C-4 = ~91% RDX + binder. Semtex = RDX + PETN + binder.ANFO = 94% ammonium nitrate + 6% fuel oil. Tertiary blasting agent, oxygen balance close to zero, VoD ~3,200 m/s.TNT has oxygen balance about −74% (very negative), which is why TNT smoke is black.PESO Nagpur is the licensing authority under the Explosives Act 1884 and Explosives Rules 2008.Section 3 of the Explosive Substances Act 1908 carries up to imprisonment for life.UN Class 1.1 = mass-explosion hazard; UN Class 1.5 = very insensitive (ANFO).

What is the difference between deflagration and detonation for UGC-NET?

Deflagration is subsonic burning of a low explosive or propellant, with the reaction front advancing at millimetres per second to a few metres per second by thermal conduction. Pressure rises gradually. Black powder and smokeless powder deflagrate. Detonation is supersonic decomposition of a high explosive driven by a shock wave that compresses and ignites the next layer; reaction-front velocity is 1,500 to 9,000 m/s and pressure spikes are in the tens of gigapascals. RDX, TNT, PETN and HMX detonate. The same material can transition from deflagration to detonation under confinement, which is why stored stacks of nitrate fuel mix are dangerous in a fire.

How do primary, secondary and tertiary explosives differ in sensitivity and use?

Primary explosives (lead azide, lead styphnate, mercury fulminate, DDNP) are extremely sensitive to heat, shock, friction and spark, and are used in milligram quantities inside detonators and primers to convert a small input into a shock pulse. Secondary explosives (TNT, RDX, HMX, PETN, nitroglycerin, tetryl, picric acid) are insensitive to drop and friction; they need a detonator to set them off and serve as the main charge. Tertiary explosives or blasting agents (ANFO, slurries, emulsions) are so insensitive that they need a secondary booster in addition to the detonator, and are used in bulk mining and civil blasting where cost and safety matter more than brisance.

What is the composition of C-4 and Semtex?

C-4 is a plastic explosive made of approximately 91% RDX bound in a plasticiser-binder system (commonly polyisobutylene, di(2-ethylhexyl) sebacate and a small amount of motor oil). Its VoD is about 8,040 m/s. Semtex is a Czech-origin plastic explosive containing both RDX and PETN bound in a styrene-butadiene matrix; the RDX:PETN ratio varies by grade. VoD is about 7,400 m/s. Semtex was historically notorious for low vapour pressure, which made it hard for sniffer dogs to detect until international taggants were mandated in 1991. The Indian-Army equivalent is PEK1, an RDX-based plastic explosive produced domestically.

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