Chapter 15
Matter, Elements & Atmospheric Composition
Unit V · Chemistry. States of matter, atmospheric gas percentages, periodic table essentials.
Full Chapter Notes
Source · FPSC Trap Decoder · CSS MPT Smart Notes (2026 Edition)
MPT Weightage: 3–5 Marks · Difficulty: Medium · Confirmed Past Papers: 2022 · 2023 · 2024
Trend Alert. Nitrogen as the most common gas in air appeared MPT 2024 (repeated twice). Hardness of hammer = metallic bonds appeared MPT 2023 (repeated twice in the same paper). Metal extraction = heating and chemical reduction appeared MPT 2023. Acetylene = gas welding appeared MPT 2024 (repeated twice). Earth's core metallic = density argument appeared MPT 2024. Ammonia in refrigerators appeared MPT 2022.
1. High-Yield Fact Snapshot
| FPSC-Tested Fact | Correct Answer | Year Tested |
|---|---|---|
| Most common gas in air (for breathing) | Nitrogen (78%) | 2024 (Repeated twice) |
| Gas used in gas welding | Acetylene (C₂H₂) | 2024 (Repeated twice) |
| Gas used in refrigerator cooling | Ammonia (NH₃) | 2022 |
| Hardness of hammer due to | Metallic bonds | 2023 (Repeated twice) |
| Metal extraction from ore | Heating and chemical reduction | 2023 |
| Earth's core composition (proof) | Metallic — proven by density of surface less than average density | 2024 |
| Most abundant element in Earth's crust | Oxygen (~46.6%) | High-yield target |
| Most abundant metal in Earth's crust | Aluminum (8.1%) | Iron is #1 for whole Earth |
| Most abundant noble gas in atmosphere | Argon (0.93%) | Not Helium or Neon |
| Nanocomposite in food industry | Protects from heat, fire, floods — All of these | 2024 (Repeated) |
2. Earth's Composition — The Critical Distinctions
The FPSC exploits the difference between the Earth's crust and the whole planet. These two contexts give different answers to the same type of question.
| Category | Rank #1 | FPSC Trap |
|---|---|---|
| Most abundant ELEMENT in crust | Oxygen (~46.6%) | Trap: Students pick Silicon. Silicon is #2. |
| Most abundant ELEMENT in universe | Hydrogen | Trap: Not oxygen — hydrogen fills the universe. |
| Most abundant METAL in crust | Aluminum (8.1%) | Iron Trap: Iron is #1 for the whole planet. Aluminum is #1 for the CRUST only. |
| Most abundant METAL in whole Earth | Iron (dominates the core) | Core = mostly iron and nickel. |
| Most abundant NOBLE GAS in atmosphere | Argon (0.93%) | Trap: Students pick Helium or Neon. Argon is correct. |
| 2nd most abundant element in crust | Silicon (27.7%) | Silicon is a METALLOID — often mistaken for a metal. |
Iron vs Aluminum Trap. Iron is the most abundant metal for the WHOLE EARTH (iron-nickel core). Aluminum is the most abundant metal in the EARTH'S CRUST ONLY. The crust is the thin outer skin — Iron mostly sank to the core during Earth's formation. FPSC asks: 'Most abundant metal in the crust?' → Aluminum. 'Most abundant metal on Earth overall?' → Iron.
Nitrogen vs Oxygen Confusion. 'What is the most common gas in the air we breathe?' → Nitrogen (78%). NOT oxygen. Oxygen is only 21% of the atmosphere. Students assume oxygen is the answer because we breathe it, but nitrogen is nearly 4/5ths of the atmosphere. Appeared MPT 2024 twice.
3. Atmospheric Composition — Complete Reference
| Gas | % in Atmosphere | Role | FPSC Strategic Note |
|---|---|---|---|
| Nitrogen (N₂) | ~78% | Dilutes oxygen; prevents rapid combustion | Most abundant gas. 2024: Most common gas in air = Nitrogen. |
| Oxygen (O₂) | ~21% | Necessary for respiration and combustion | Second most abundant. NOT the most abundant. |
| Argon (Ar) | ~0.93% | Chemically inert; used in light bulbs | Most abundant NOBLE GAS. More abundant than CO₂. |
| Carbon Dioxide (CO₂) | ~0.04% | Photosynthesis; primary greenhouse gas | LESS abundant than Argon — a common trap. |
| Other noble gases | Trace | Neon, Helium, Krypton, Xenon | All far less abundant than Argon. |
4. Industrial Gases — Confirmed Past Paper Facts
| Gas | Industrial Use | FPSC Year & Note |
|---|---|---|
| Acetylene (C₂H₂) | Gas welding — mixed with oxygen produces extreme heat flame | 2024 — Repeated twice. 'Which gas is used in gas welding?' → Acetylene. |
| Ammonia (NH₃) | Refrigerant (especially older/industrial systems) | 2022. 'Gas in refrigerator' → Ammonia (not methane, not nitrogen). |
| Nitrogen (N₂) | Food packaging (chip packets) — prevents oxidation/rancidity | Nitrogen is inert — prevents food from going stale. |
| Helium (He) | Balloons and airships — light AND non-flammable | Preferred over Hydrogen. Hydrogen is lighter but highly flammable. |
| Oxygen (O₂) | Medical use; supports combustion in welding (paired with Acetylene) | Oxy-acetylene welding uses both oxygen AND acetylene. |
| Carbon Dioxide (CO₂) | Fire extinguishers; carbonated drinks; dry ice | Dry ice = solid CO₂ — sublimates directly without liquid stage. |
Acetylene Welding Trap — Confirmed MPT 2024 (appeared twice). 'Which gas is used in gas welding?' Options: (A) Hydrogen (B) Oxygen (C) Ethylene (D) Acetylene. Answer = D) Acetylene. Acetylene (C₂H₂) mixed with oxygen produces an oxyacetylene flame reaching ~3,500°C — hot enough to cut and weld metals. Hydrogen, Oxygen, and Ethylene are all distractors.
Ammonia Refrigerator Trap — Confirmed MPT 2022. 'Which gas is used in a refrigerator for cooling?' Options include Methane, Ammonia, Nitrogen. Answer = Ammonia (NH₃). Ammonia is a common refrigerant, especially in older and industrial refrigeration systems. Modern domestic refrigerators use HFCs, but Ammonia remains the standard MPT answer.
5. Metal Extraction & Material Science
| Metal | Principal Ore | Extraction Method | FPSC Note |
|---|---|---|---|
| Aluminum | Bauxite (Al₂O₃·2H₂O) | Electrolysis (Hall-Héroult process) | Bauxite → Aluminum. Ruby & Sapphire also = Aluminum Oxide. |
| Iron | Hematite (Fe₂O₃) | Heating and chemical reduction | Hematite → Iron. 2023: metal extraction = heating + chemical reduction. |
| Mercury (Hg) | Cinnabar (HgS) | Heating (cinnabar decomposes) | Only LIQUID metal at room temperature. Cinnabar = mercury sulfide. |
| Copper | Chalcopyrite (CuFeS₂) | Smelting + purification | Hematite is iron ore — NOT copper. Common trap. |
| Gold | Found free in nature | Physical separation/amalgamation | Dissolves in Aqua Regia (1 HNO₃ : 3 HCl). |
Metal extraction from ore = Heating + Chemical Reduction. This appeared directly in MPT 2023. The process involves heating the ore and using a reducing agent (usually carbon/coke) to strip oxygen from the metal oxide, leaving pure metal.
6. Hardness & Metallic Bonds
The hardness of metals (like a hammer) is due to METALLIC BONDS. Metallic bonding = a lattice of positive metal ions surrounded by a 'sea' of delocalised electrons. The strength of these bonds determines hardness. This appeared in MPT 2023 and was repeated twice in the same paper.
| Bond Type | Nature | Hardness Contribution |
|---|---|---|
| Metallic Bond | Metal ions in sea of electrons | Gives metals their hardness, conductivity, and malleability. 2023 answer for 'hardness of hammer.' |
| Covalent Bond | Shared electrons between atoms | Found in diamonds — gives extreme hardness. Distractor in 2023. |
| Ionic Bond | Electrostatic attraction between ions | Gives ionic crystals their brittleness — not metals. |
7. Dry Ice & Phase Changes
Solid Carbon Dioxide (CO₂). Unlike regular ice, dry ice does NOT melt into liquid at normal atmospheric pressure. It SUBLIMATES — converts directly from solid to gas. This property makes it ideal for shipping frozen goods (no liquid residue). Sublimation point: –78.5°C.
The process by which a solid converts directly into a gas without passing through the liquid phase. Dry ice → CO₂ gas is the classic FPSC example. The reverse (gas to solid) is called deposition.
8. Earth's Core — The Density Argument
Scientists proved Earth's core is metallic using DENSITY EVIDENCE. The density of Earth's surface rocks (~2.7 g/cm³) is much LESS than Earth's average density (~5.5 g/cm³). Since surface rocks are lighter, the interior must contain something far denser. Only metals (especially iron and nickel) can account for this density difference. This appeared directly in MPT 2024.
9. Battle Card — 5-Minute Revision
| Fact | Answer |
|---|---|
| Most common gas in air | Nitrogen (78%) — NOT oxygen (MPT 2024) |
| Gas used in gas welding | Acetylene (C₂H₂) — MPT 2024 (repeated twice) |
| Gas used in refrigerators | Ammonia (NH₃) — MPT 2022 |
| Hardness of hammer due to | Metallic bonds — MPT 2023 (repeated twice) |
| Metal extraction from ore | Heating and chemical reduction — MPT 2023 |
| Earth's core = metallic (proof) | Density of surface less than average density of Earth (MPT 2024) |
| Most abundant element in crust | Oxygen (~46.6%) |
| Most abundant metal in crust | Aluminum (8.1%) — NOT iron |
| Most abundant metal in whole Earth | Iron (dominates the core) |
| 2nd most abundant element in crust | Silicon (27.7%) — metalloid, not metal |
| Most abundant noble gas in air | Argon (0.93%) — NOT helium or neon |
| Argon vs CO₂ | Argon (0.93%) greater than CO₂ (0.04%) |
| Dry Ice | Solid CO₂ — sublimates (no liquid stage) |
| Bauxite | Ore of Aluminum |
| Cinnabar | Ore of Mercury (only liquid metal at room temp) |
| Hematite | Ore of Iron |
| Helium vs Hydrogen in balloons | Helium preferred — non-flammable (Hydrogen is flammable) |
| Nanocomposite in food industry | Protects from heat, fire, and floods — All of these (MPT 2024) |
10. Practice MCQs (FPSC Level)
Part A — Basic Recall (Q1–Q4)
Direct recall on confirmed past-paper facts.
The most common gas found in the air we breathe is:
Show explanation
Nitrogen makes up approximately 78% of the atmosphere — nearly four-fifths. Oxygen is only 21%. Nitrogen acts as a chemical buffer, diluting oxygen and preventing rapid combustion.
Trap: Students assume oxygen (which we breathe) is the most abundant — wrong.
MPT 2024 (Repeated twice)
Which gas is used in gas welding?
Show explanation
Acetylene (C₂H₂), mixed with oxygen, produces an oxyacetylene flame reaching approximately 3,500°C — the highest temperature achievable through combustion. This is hot enough to cut and weld metals.
Trap: Hydrogen, Oxygen, and Ethylene all appeared as distractors.
MPT 2024 (Repeated twice)
The most abundant element in the Earth's crust by mass is:
Show explanation
Oxygen constitutes approximately 46.6% of the Earth's crust by mass — making it the most abundant element. Silicon is second (27.7%), then Aluminum (8.1%), then Iron (5%).
Trap: Students confuse this with the most abundant element in the universe (Hydrogen) or the most abundant metal (Aluminum).
The hardness of a hammer is primarily due to:
Show explanation
The hardness of metals is due to METALLIC BONDS — a lattice of positive metal ions held together by a delocalised 'sea' of electrons. The strength and density of these bonds determine hardness, conductivity, and malleability.
MPT 2023 (Repeated)
Part B — Trap-Based (Q5–Q8)
FPSC traps drawn from confirmed past-paper distractor patterns.
In terms of metallic abundance in the Earth's crust, Aluminum ranks as the:
Show explanation
Aluminum (8.1% of crust) is the most abundant metal in the Earth's CRUST. Iron ranks second among metals in the crust (~5%).
Trap: For the WHOLE EARTH (including the iron-nickel core), Iron is the most abundant metal. FPSC exploits this crust vs. whole-Earth distinction.
Which gas is used in a refrigerator for cooling?
Show explanation
Ammonia (NH₃) is the gas used in refrigerators for cooling — especially in older and industrial refrigeration systems. Modern domestic fridges use HFCs, but Ammonia remains the standard MPT answer.
Trap: Methane and nitrogen are not standard refrigerants.
MPT 2022
The most abundant noble gas present in the atmosphere is:
Show explanation
Argon (0.93%) is the most abundant noble gas in the atmosphere — far exceeding Helium (0.0005%) and Neon (0.0018%).
Trap: FPSC consistently offers Helium as the attractive distractor. Radon is present only in trace amounts and is radioactive.
The process of extracting metal from its ore primarily involves:
Show explanation
Metal extraction from ore involves heating the ore and using a reducing agent (typically carbon/coke) to remove oxygen from the metal oxide — a process called smelting. This chemical reduction leaves pure metal.
MPT 2023
Part C — Elite Simulation (Q9–Q12)
Multi-statement and cause-and-effect logic at full FPSC difficulty.
How do scientists know the Earth's core is made of metal?
Show explanation
Scientists determined Earth's core is metallic by comparing densities: surface rocks have a density of ~2.7 g/cm³ while Earth's average density is ~5.5 g/cm³. Since the surface is lighter, the interior must be much denser — only metals (iron and nickel) explain this difference.
MPT 2024
Consider atmospheric composition: (1) Nitrogen constitutes nearly four-fifths of the atmosphere. (2) Oxygen supports combustion but is not the most abundant gas. (3) Carbon dioxide is more abundant than Argon. Which are correct?
Show explanation
Statements 1 and 2 are correct: Nitrogen = ~78% (four-fifths); Oxygen supports combustion but is only 21%. Statement 3 is FALSE: Argon is more abundant (0.93%) than CO₂ (0.04%).
Trap: CO₂ is far less abundant than Argon despite being widely discussed as a greenhouse gas.
Nanocomposite materials are useful in the food industry as they prevent packaged food from damage due to:
Show explanation
Nanocomposite materials in food packaging create enhanced barrier properties that protect against multiple environmental threats simultaneously — heat, fire, and moisture/floods.
MPT 2024 (Repeated)
Which metal has the property of being liquid at room temperature?
Show explanation
Mercury (Hg) is the only metal that exists in liquid state at room temperature (melting point: –38.8°C). Its principal ore is Cinnabar (HgS — mercury sulfide). Mercury is extracted by simply heating cinnabar, which decomposes to release mercury vapour.
11. Answer Key & Explanations
Matter, Elements & Atmosphere — Q1–Q12
| Q | Correct | Type | Primary Trap | Why Others Fail |
|---|