📖 Chapter CN08 · NDA Class 11–12 Level🎯 NDA Level : High Priority
This chapter is one of the broadest in NDA Chemistry — it draws chemistry out of the lab and into the real world. Questions appear on reactivity series, alloys used in defence equipment, preparation of important gases, soap vs detergents, cement composition, and environmental issues. Most content is factual and directly memorisable, making this a strong scoring area for average students who revise systematically.
📌 What to expect in NDA (based on 2022–2025 pattern): (1) Metals vs non-metals — physical and chemical properties; reactivity series; (2) Alloys — composition, properties, and defence/industrial applications; (3) Important gases (H₂, O₂, N₂, CO₂) — lab preparation and key properties; (4) Daily-use materials — soap vs detergent; glass types; cement; polymers; (5) Fertilisers, explosives, gunpowder — composition and chemistry; (6) Environmental chemistry — greenhouse gases, ozone depletion, BOD/water pollution.
Low melting and boiling points (many are gases at RT)
Can be solids (S, P, C), liquids (Br₂), or gases (O₂, N₂)
Form acidic oxides (e.g. SO₂, CO₂, NO₂) or neutral (CO, H₂O)
Gain electrons → form anions (e.g. Cl → Cl⁻)
H is a non-metal but behaves like alkali metal in some reactions
📌 NDA Exceptions to Know: Mercury (Hg) is the only liquid metal at RT. Gallium (Ga) melts just above RT (29.8°C). Graphite is a non-metal that conducts electricity. Iodine is a non-metal with lustrous (shiny) appearance. Sodium and Potassium are metals soft enough to cut with a knife. Tungsten (W) has the highest melting point of all metals (3422°C). Bromine is the only liquid non-metal at RT.
1.2
Reactivity Series of Metals
Order of decreasing reactivity — the single most-tested metals concept in NDA
The reactivity series arranges metals from most reactive (top) to least reactive (bottom). It predicts displacement reactions, corrosion behaviour, and extraction methods. A more reactive metal can displace a less reactive metal from its salt solution.
1
K — Potassium
Reacts explosively with cold water; burns violet flame
2
Na — Sodium
Reacts vigorously with cold water; stored in kerosene
3
Ca — Calcium
Reacts with cold water (less vigorously than Na)
4
Mg — Magnesium
Reacts slowly with cold water; briskly with steam
5
Al — Aluminium
Reacts with steam; protected by Al₂O₃ layer (passivation)
6
Zn — Zinc
Reacts with steam and dilute acids; used for galvanising
7
Fe — Iron
Reacts with steam; displaced by Zn, Al, Mg
8
Ni — Nickel
Reacts with dilute H₂SO₄; used in coins, alloys
9
Sn — Tin
Reacts with dilute acids; used in tin plating (food cans)
10
Pb — Lead
Reacts with dilute acids; used in batteries
11
H — (Hydrogen)
Reference point — metals above H displace H from acids
12
Cu — Copper
Does NOT react with dilute acids; resists corrosion
13
Hg — Mercury
Very low reactivity; found free in nature
14
Ag — Silver
Reacts only with conc. HNO₃; used in jewellery, coins
15
Au — Gold
Least reactive; dissolves only in aqua regia (HNO₃ + 3HCl)
💡 Reactivity Series Memory Trick (top to bottom):"Please Stop Calling Me A Zebra In New Schools, Lead Him Copper Mercury Silver Gold" — K, Na, Ca, Mg, Al, Zn, Fe, Ni, Sn, Pb, H, Cu, Hg, Ag, Au. Metals above H in the series displace H₂ from dilute acids. Metals above Cu corrode/rust. Gold resists all ordinary acids — only aqua regia dissolves it.
1.3
Alloys — Composition & Uses
Alloys have properties superior to pure metals — critical for defence applications
An alloy is a homogeneous mixture of two or more metals (or a metal with a non-metal). Alloys are harder, stronger, and more corrosion-resistant than their component metals.
Alloy
Composition
Key Properties
Defence / Industrial Use
Steel
Fe + C (0.2–2%)
Hard, strong, magnetic
Weapons, ships, bridges, vehicles
Stainless Steel
Fe + Cr (18%) + Ni (8%)
Corrosion-resistant, hard
Surgical instruments, aircraft, cutlery
Brass
Cu (70%) + Zn (30%)
Ductile, corrosion-resistant, golden
Bullet casings, musical instruments, fittings
Bronze
Cu (90%) + Sn (10%)
Hard, wear-resistant, low friction
Coins, medals, bearings, cannons historically
Duralumin
Al (95%) + Cu + Mg + Mn
Light, strong (like steel)
Aircraft fuselage, missiles, spacecraft
German Silver
Cu + Zn + Ni
Silver-like appearance, no real silver
Decorative items, utensils
Solder
Pb (67%) + Sn (33%)
Low MP (~183°C), flows easily
Electrical joints, plumbing
Nichrome
Ni + Cr
High electrical resistance, heat-resistant
Heating elements, toasters, electric furnaces
Magnalium
Al + Mg
Very light, strong
Aircraft parts, lightweight structures
Type Metal
Pb + Sb + Sn
Expands on solidification
Printing type — fills mould sharply
📌 Why alloys are better than pure metals: Pure metals have regular crystalline lattices — planes can slide past each other easily → soft. In alloys, atoms of different sizes disrupt the regular lattice → planes cannot slide → harder. Example: pure iron is soft (nails bend); steel (Fe + C) is hard enough for cutting tools. Pure aluminium is weak; duralumin is strong enough for aircraft.
📝 TOPIC-WISE PYQ
Metals, Reactivity Series & Alloys — NDA Pattern Questions
Q1. Which of the following metals is stored under kerosene oil?
(a) Magnesium (b) Sodium (c) Gold (d) Copper
Answer: (b) Sodium
Sodium is extremely reactive — it reacts vigorously with oxygen and moisture in air, and explosively with water (2Na + 2H₂O → 2NaOH + H₂↑). To prevent this, Na is stored under kerosene (which does not react with Na). Similarly, K is stored under kerosene. Gold and copper are unreactive; magnesium reacts slowly with air but doesn't need kerosene storage.
Q2. The alloy used in making aircraft bodies due to its lightness and strength is:
(a) Steel (b) Brass (c) Duralumin (d) Bronze
Answer: (c) Duralumin
Duralumin = Al (~95%) + Cu + Mg + Mn. It combines aluminium's low density (2.7 g/cm³) with steel-like strength — making it ideal for aircraft fuselage, missiles, and spacecraft where weight is critical. Steel is strong but too heavy. Brass is used for bullet casings. Bronze was historically used in cannons.
Q3. Iron displaces copper from copper sulphate solution. This is because:
(a) Iron is less reactive than copper (b) Iron is more reactive than copper (c) Iron has a higher atomic mass (d) Copper is a non-metal
Answer: (b) Iron is more reactive than copper
Fe + CuSO₄ → FeSO₄ + Cu↓ (copper deposits). Fe is above Cu in the reactivity series → Fe displaces Cu²⁺ from solution. The more reactive metal reduces the less reactive metal's ions. This is why iron nails dipped in CuSO₄ solution turn reddish-brown (copper deposits on iron).
🧠 TRICKY QUESTIONS
Metals & Reactivity — Conceptual Traps
Q. Aluminium is high in the reactivity series, yet aluminium utensils don't corrode. Why?
Answer: Aluminium forms a thin, tough protective oxide layer (passivation).
When Al is exposed to air, it immediately reacts: 4Al + 3O₂ → 2Al₂O₃. This Al₂O₃ layer is only a few nanometres thick but is extremely dense, hard, and chemically inert — it seals the surface from further oxidation. This self-protecting behaviour is called passivation. Iron (Fe) forms a porous rust (Fe₂O₃·xH₂O) that flakes off, exposing fresh iron to further corrosion. Al₂O₃ adheres firmly and does not flake. Anodising (electrochemical thickening of Al₂O₃ layer) is used to make Al even more corrosion-resistant.
Q. German Silver contains no silver at all. Is the name misleading? What is it actually made of?
Answer: Yes — German Silver contains copper, zinc, and nickel. No silver.
German Silver (also called "nickel silver") = Cu + Zn + Ni in various proportions. It has a silver-like appearance due to the nickel content but zero actual Ag. It was developed in Germany in the 19th century as a cheaper silver substitute. Uses: decorative items, cutlery, zippers, musical instrument keys. NDA tests this as a "which alloy contains silver?" trap — the answer is none; only Sterling Silver (92.5% Ag) and coin silver contain actual silver.
2. Important Gases — Preparation & Properties
2.1
H₂, O₂, N₂, CO₂ — Lab Preparation & Key Facts
Each gas has a characteristic test — these are directly tested in NDA
H₂
Hydrogen — "Lightest gas"
Lab prep: Zn + dil. H₂SO₄ → ZnSO₄ + H₂↑
Industrial: Steam reforming of methane: CH₄ + H₂O →(catalyst) CO + 3H₂
Colourless, odourless, tasteless, lightest of all gases
Burns with blue flame: 2H₂ + O₂ → 2H₂O (clean fuel)
Burns with "pop" sound when burning splint applied (test)
Reducing agent: reduces CuO to Cu (CuO + H₂ → Cu + H₂O)
Used in: hydrogenation of oils, fuel cells, rocket fuel, Haber process, balloons (not H₂ — flammable!)
Highly flammable; forms explosive mixtures with air (4–75%)
O₂
Oxygen — "Supporter of combustion"
Lab prep: 2KMnO₄ →(heat) K₂MnO₄ + MnO₂ + O₂↑
Also: 2H₂O₂ →(MnO₂ catalyst) 2H₂O + O₂↑
Industrial: Fractional distillation of liquid air
Colourless, odourless, slightly denser than air (32 vs 29 average)
Itself does NOT burn — but supports combustion (rekindling glowing splint test)
Dissolves in water (supports aquatic life)
Used in: steel making, welding, medical, respiration
Liquid O₂ (LOX) used as oxidiser in rocket propellants
N₂
Nitrogen — "Inert atmospheric gas"
Lab prep: NH₄Cl + NaNO₂ →(heat) N₂↑ + 2H₂O + NaCl
Industrial: Fractional distillation of liquid air (BP −196°C)
Makes up 78% of atmosphere; colourless, odourless
Very inert at room temperature (strong N≡N triple bond)
Does NOT support combustion or burn (extinguishes flame)
Liquid N₂ used for: cryogenic storage of biological samples, food preservation, freeze-branding
Used in: Haber process (NH₃), explosives, fertilisers, inert atmosphere for food packaging
Test: gas that extinguishes burning splint and does NOT give cloudy limewater
Answer: (c) Carbon dioxide
CaCO₃ (marble) + 2HCl → CaCl₂ + H₂O + CO₂↑. The CO₂ produced turns limewater milky (Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O). This is the standard lab preparation of CO₂ and also the standard test — both appear in NDA. The same reaction explains why limestone buildings erode in acid rain.
Q2. A glowing splint is placed in a jar of gas and it relights. The gas is most likely:
Answer: (d) Oxygen
The "glowing splint rekindles" test is the standard confirmatory test for oxygen. O₂ supports combustion — a glowing (not fully lit) splint bursts back into flame in O₂-rich atmosphere. H₂ burns with a "pop" sound when burning splint applied. CO₂ extinguishes burning splints. N₂ extinguishes burning splints and gives no reaction with limewater.
3. Materials in Daily Use
3.1
Soap vs Detergents — The Most-Tested Pair
Different chemistry, same job — but with important practical differences
🧻 Soap
Composition: Sodium salt of fatty acid (sodium stearate CH₃(CH₂)₁₆COONa)
Also has hydrophobic tail + hydrophilic –SO₃⁻Na⁺ head
Works in hard water: Ca/Mg sulphonates are soluble (no scum)
Effective in acidic, alkaline, and hard water conditions
More versatile than soap; used in shampoos, dishwashing liquids
Problem: Non-biodegradable (older types) — cause water pollution (foam on rivers)
Modern detergents are now biodegradable (linear alkyl chains)
⚛ Saponification — Soap Making Reaction
Fat (Triester of glycerol) + 3NaOH → Soap (3 fatty acid sodium salts) + Glycerol
Example:
(C₁₇H₃₅COO)₃C₃H₅ + 3NaOH → 3C₁₇H₃₅COONa + C₃H₅(OH)₃
Tristearin (fat) NaOH Sodium stearate (soap) Glycerol
Micelle formation:
Soap molecules arrange with hydrophobic tails pointing inward (into grease)
and hydrophilic heads pointing outward (into water)
→ spherical "micelle" traps grease and is carried away by water
Hard water:
2C₁₇H₃₅COONa + CaCl₂ → (C₁₇H₃₅COO)₂Ca↓ + 2NaCl
Sodium stearate (hard water) Calcium stearate (scum) NaCl
Glycerol (glycerin) from saponification is a valuable by-product used in cosmetics, medicines, and food. "Hard water" contains dissolved Ca²⁺ and Mg²⁺ ions (from limestone rock). Soft water has low Ca²⁺/Mg²⁺ and lathers easily with soap.
📌 Gunpowder — NDA often asks composition: Black powder (gunpowder) = 75% KNO₃ (potassium nitrate/saltpetre) + 15% C (charcoal) + 10% S (sulphur). KNO₃ is the oxidiser (provides oxygen); C and S are fuels. Invented in China (~9th century). First used militarily in the 11th century. The oxygen from KNO₃ allows rapid combustion even without atmospheric oxygen — critical for use in firearms and rockets. 2KNO₃ + S + 3C → K₂S + N₂ + 3CO₂
📝 TOPIC-WISE PYQ
Materials in Daily Use — NDA Pattern Questions
Q1. Soaps are ineffective in hard water because:
(a) Soap is a weak acid (b) Soap reacts with Ca²⁺/Mg²⁺ to form insoluble scum (c) Hard water dissolves soap (d) Soap does not form micelles
Answer: (b) Soap reacts with Ca²⁺/Mg²⁺ to form insoluble scum
2RCOONa + CaCl₂ → (RCOO)₂Ca↓ + 2NaCl. The insoluble calcium (or magnesium) stearate forms a grey scum that settles on clothes and bathtubs, wasting soap without cleaning anything. Detergents work in hard water because their Ca/Mg sulphonate salts are soluble.
Q2. The main component of gunpowder that acts as the oxidising agent is:
Answer: (c) Potassium nitrate (KNO₃)
KNO₃ (saltpetre) provides oxygen for rapid combustion of C and S even in confined spaces (gun barrel) where atmospheric O₂ is absent. Charcoal (C) and sulphur (S) are fuels that react with the oxygen provided by KNO₃. The products (K₂S, N₂, CO₂) are produced rapidly, generating the expanding gas pressure that propels a bullet.
Q3. Borosilicate glass (Pyrex) is preferred for laboratory glassware because:
(a) It is transparent (b) It has a very low coefficient of thermal expansion — resists thermal shock (c) It is cheaper (d) It dissolves in water
Answer: (b) Very low coefficient of thermal expansion — resists thermal shock
Borosilicate glass (SiO₂ + 12–15% B₂O₃) expands very little when heated (coefficient ~3.3 × 10⁻⁶/°C vs 9 × 10⁻⁶/°C for soda-lime glass). This means it doesn't crack when hot liquid is poured into it or when heated with a Bunsen burner. Soda-lime glass (used for windows) shatters under thermal shock. Pyrex beakers, flasks, and test tubes are all borosilicate.
🧠 TRICKY QUESTIONS
Materials — Common Misconceptions
Q. Glass is described as an "amorphous solid" or a "supercooled liquid." What does this mean?
Answer: Glass has no regular crystalline arrangement — its atoms are randomly arranged like a liquid, but it flows so slowly it behaves as a rigid solid.
Crystalline solids (like NaCl, quartz) have long-range order — atoms in a repeating pattern with a sharp melting point. Glass is made by rapidly cooling molten SiO₂ + additives, which "freezes" the random liquid arrangement before crystals can form. This is why glass has no sharp melting point (it softens gradually over a range). The old claim that "old glass is thicker at the bottom because glass flows over centuries" is a myth — glass is rigid at room temperature. It is an amorphous solid, not truly a flowing liquid.
Q. NH₄NO₃ (ammonium nitrate) is used as a fertiliser but is also explosive. Why is this?
Answer: NH₄NO₃ contains both a reducing agent (NH₄⁺) and an oxidising agent (NO₃⁻) in the same molecule — making it self-reactive under shock or heat.
As a fertiliser: NH₄NO₃ provides 35% nitrogen (both ammoniacal-N and nitrate-N) which plants absorb directly. As an explosive: at high temperature or shock, it decomposes violently — 2NH₄NO₃ → 2N₂ + 4H₂O + O₂ (or → N₂O + 2H₂O) releasing large volumes of gas in microseconds. ANFO (ammonium nitrate + fuel oil) is the most widely used commercial explosive for mining. The 2020 Beirut explosion involved 2750 tonnes of ammonium nitrate. NDA tests: "Which fertiliser is also an explosive?" → NH₄NO₃.
4. Environmental Chemistry
4.1
Air Pollution, Water Pollution & Environmental Issues
Real-world consequences of chemistry — increasingly tested in NDA GS+Chemistry overlap
Methane (CH₄) is 25× more potent as a greenhouse gas than CO₂ over 100 years (Global Warming Potential, GWP). Sources: paddy fields, cattle, landfills, natural gas leaks. CFC alternatives: HFCs (hydrofluorocarbons) — no Cl, no ozone damage, but still greenhouse gases.
Answer: (b) Chlorofluorocarbons (CFCs)
CFCs (used in refrigerators, air conditioners, aerosols) release Cl· radicals in the stratosphere under UV radiation. These Cl· radicals catalytically destroy ozone: Cl· + O₃ → ClO + O₂, then ClO + O· → Cl· + O₂. One Cl atom can destroy ~100,000 O₃ molecules. The Montreal Protocol (1987) phased out CFCs globally. CO₂ is the main greenhouse gas (causes warming, not ozone depletion — different problem).
Q2. Acid rain has a pH:
(a) Greater than 7 (b) Equal to 7 (c) Less than 5.6 (d) Equal to 5.6
Answer: (c) Less than 5.6
Normal (clean) rain already has pH ~5.6 because CO₂ dissolves to form carbonic acid (H₂CO₃). Acid rain forms when SO₂ and NOₓ (from burning coal, vehicle exhaust) dissolve to form H₂SO₄ and HNO₃ — lowering pH below 5.6. Effects: erosion of marble/limestone buildings (CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂), damage to forests, acidification of lakes.
Q3. Biochemical Oxygen Demand (BOD) is a measure of:
(a) Amount of oxygen in water (b) Level of organic pollution in water (c) Temperature of water (d) Amount of CO₂ dissolved
Answer: (b) Level of organic pollution in water
BOD = amount of dissolved oxygen (DO) needed by microorganisms to decompose organic matter in water over 5 days at 20°C. Clean water: BOD < 1 mg/L. Polluted river: BOD 5–10 mg/L. Sewage: BOD > 200 mg/L. High BOD = high organic pollution = bacteria consume all O₂ = fish die (oxygen depletion). BOD is the most commonly used measure of water pollution by organic waste.
🧠 TRICKY QUESTIONS
Environmental Chemistry — Conceptual Traps
Q. Is the ozone (O₃) in photochemical smog (ground level) the same as the ozone that protects us in the stratosphere?
Answer: Same molecule (O₃) — but in different locations with opposite effects. Stratospheric ozone (good): 15–35 km up; naturally formed; absorbs harmful UV-B and UV-C radiation before it reaches Earth. Depletion by CFCs is the ozone hole problem. Tropospheric ozone (bad): Ground level; formed from NO₂ + sunlight + VOCs (vehicle exhaust). Part of photochemical smog. Causes: eye irritation, respiratory damage, crop damage. It does NOT protect from UV.
NDA trap: "We need ozone" — True (stratospheric). "Ozone is a pollutant" — Also True (tropospheric). Context is everything. Ozone at ground level is a health hazard; ozone at high altitude is a life-saver.
Q. CO₂ is described as the main greenhouse gas. But methane (CH₄) is 25 times more potent. Why is CO₂ more important for climate change?
Answer: Because CO₂ is produced in vastly larger quantities than CH₄.
Global Warming Potential (GWP) over 100 years: CO₂ = 1 (reference); CH₄ = 25; N₂O = 298; SF₆ = 23,900. CH₄ is far more potent per molecule, but global CO₂ emissions (~37 billion tonnes/year) dwarf CH₄ emissions. Combined effect makes CO₂ the largest contributor to total radiative forcing. Also, CO₂ persists in the atmosphere for 300–1000 years while CH₄ breaks down in ~12 years. So CO₂ is the primary driver of long-term climate change despite lower GWP. NDA often tests: "Which is the most abundant greenhouse gas?" → CO₂ (by volume of human emissions); "Which is most potent?" → SF₆ (industrial) or N₂O/CH₄ (natural+agricultural).
📄 CN08 Formula & Fact Sheet — Quick Reference
🔌 Reactivity Series (top→bottom)
K > Na > Ca > Mg > Al > Zn > Fe > Ni > Sn > Pb
H (reference) > Cu > Hg > Ag > Au
Metals above H displace H₂ from dilute acids
More reactive metal displaces less reactive from salt solution
Fe + CuSO₄ → FeSO₄ + Cu (Fe above Cu)
⚛ Key Alloys
Steel: Fe + C; Stainless: Fe + Cr + Ni
Brass: Cu + Zn (bullet casings); Bronze: Cu + Sn
Duralumin: Al + Cu + Mg + Mn (aircraft)
Solder: Pb + Sn (electrical joints)
Nichrome: Ni + Cr (heating elements)
💨 Gas Tests
H₂: "pop" when burning splint applied
O₂: relights glowing splint
CO₂: milky limewater (CaCO₃↓)
N₂: extinguishes splint; limewater stays clear
Lab prep CO₂: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
🧻 Soap vs Detergent
Soap: Na salt of fatty acid; fails in hard water (scum)
Saponification: fat + NaOH → soap + glycerol
Detergent: Na alkyl sulphonate; works in hard water
Micelle: hydrophobic tail inward, hydrophilic head outward
This material is for personal NDA exam preparation only.
Unauthorised reproduction or distribution is prohibited.
All rights reserved. ·
ODEA.classes@gmail.com
·
olivedefence.com