GS Paper III · Science & Technology · Basic Chemistry · NCERT Class 9
🧪 Elements, Compounds & Mixtures
Classification of Matter · Elements (Metals / Non-metals / Metalloids) · Periodic Table · Compounds · Mixtures (Homogeneous / Heterogeneous) · Colloids · Suspensions · Mixtures vs Compounds · PYQs & MCQs
🧱
Classification of Matter — The Big Picture
Pure substance vs Mixture · Elements · Compounds · Homogeneous · Heterogeneous
Classification of Matter — Master Flowchart. All matter (solid, liquid, gas) is first classified as Pure Substance (fixed composition, uniform properties) or Mixture (no fixed composition, variable). Pure substances divide into Elements (cannot be broken down by chemistry — copper, oxygen, iron) and Compounds (fixed composition, broken down only chemically — water, methane, salt). Mixtures divide into Homogeneous (uniform — sugar in water, saltwater) and Heterogeneous (non-uniform — sand & salt, wood, blood, oil in water). (Uploaded image — Legacy IAS)
🧠 Simple Analogy — LEGO City
Think of matter as a LEGO city. Elements are individual LEGO brick types (only red bricks, only blue bricks — each unique and indivisible). Compounds are LEGO structures built by permanently bonding specific bricks in fixed ratios — you can't separate them by just pulling apart; you need force (chemical means). Mixtures are LEGO bricks loosely piled together — easy to separate by hand (physical means), each brick keeps its original identity. A homogeneous mixture is well-shuffled bricks (look uniform throughout); heterogeneous is bricks in separate piles (visible separation).
✅ Pure Substance — Key Facts
• Fixed composition throughout
• Same properties throughout
• Cannot be separated by physical methods
• Two types: Elements and Compounds
• Robert Boyle first used term "element" (1661)
• Lavoisier gave first experimental definition: substance that cannot be broken down by chemical reactions
• 92 naturally occurring elements; 26+ synthetic (manmade)
• Most elements are solids; 11 gaseous; 2 liquid at room temp (Hg, Br)
• Same properties throughout
• Cannot be separated by physical methods
• Two types: Elements and Compounds
• Robert Boyle first used term "element" (1661)
• Lavoisier gave first experimental definition: substance that cannot be broken down by chemical reactions
• 92 naturally occurring elements; 26+ synthetic (manmade)
• Most elements are solids; 11 gaseous; 2 liquid at room temp (Hg, Br)
🔀 Mixture — Key Facts
• Variable composition (no fixed ratio)
• Each component retains its own properties
• Separated by physical methods (filtration, evaporation, distillation, chromatography)
• No new substance is formed
• No chemical reaction occurs
• Two types: Homogeneous (uniform, one phase) and Heterogeneous (non-uniform, visible boundaries)
• Examples: Air (mixture of gases), milk (colloid), seawater (solution), soil (heterogeneous)
• Each component retains its own properties
• Separated by physical methods (filtration, evaporation, distillation, chromatography)
• No new substance is formed
• No chemical reaction occurs
• Two types: Homogeneous (uniform, one phase) and Heterogeneous (non-uniform, visible boundaries)
• Examples: Air (mixture of gases), milk (colloid), seawater (solution), soil (heterogeneous)
⚛
Elements — Metals, Non-metals & Metalloids
118 elements · Properties · Examples · Applications · Lavoisier · IUPAC
📖 Definition
An element is a pure chemical substance consisting of a single type of atom, distinguished by its atomic number (number of protons). It cannot be broken down into simpler substances by any chemical reaction. There are currently 118 recognised elements — ranging from the lightest (hydrogen, Z=1) to the heaviest (oganesson, Z=118). IUPAC (International Union of Pure and Applied Chemistry) approves the names and symbols of elements.
🔴 METALS
Physical properties:
• Lustrous (shiny) — gold, silver
• Malleable (hammered into sheets) — gold, aluminium
• Ductile (drawn into wires) — copper, gold
• Sonorous (rings when struck)
• Good conductors of heat & electricity
• High density and strength
• High melting and boiling points
• Silvery-grey or golden-yellow colour
Chemical properties:
• Reactivity varies: Na (very reactive) → Au (unreactive)
• Prone to corrosion (Fe → rust)
• Form basic oxides
Special facts:
• Mercury: only metal liquid at room temp
• Gallium & Caesium: melt just above room temp
• Sodium & Potassium: very soft (cut with knife)
Applications:
• Construction: Steel (Fe+C alloy)
• Electrical: Copper, Silver wiring
• Aircraft: Aluminium (light + strong)
• Jewellery: Gold, Silver, Platinum
• Lustrous (shiny) — gold, silver
• Malleable (hammered into sheets) — gold, aluminium
• Ductile (drawn into wires) — copper, gold
• Sonorous (rings when struck)
• Good conductors of heat & electricity
• High density and strength
• High melting and boiling points
• Silvery-grey or golden-yellow colour
Chemical properties:
• Reactivity varies: Na (very reactive) → Au (unreactive)
• Prone to corrosion (Fe → rust)
• Form basic oxides
Special facts:
• Mercury: only metal liquid at room temp
• Gallium & Caesium: melt just above room temp
• Sodium & Potassium: very soft (cut with knife)
Applications:
• Construction: Steel (Fe+C alloy)
• Electrical: Copper, Silver wiring
• Aircraft: Aluminium (light + strong)
• Jewellery: Gold, Silver, Platinum
🟢 NON-METALS
Physical properties:
• Variety of colours; not lustrous
• Brittle (not malleable/ductile)
• Poor conductors of heat & electricity (exception: graphite — conducts electricity)
• Not sonorous
• Can be gas, liquid, or solid at room temperature
• Located on right side of periodic table
Chemical properties:
• High electronegativity (tend to gain electrons)
• Can be highly reactive (Fluorine = most reactive)
• Form acidic oxides
Special facts:
• Bromine: only non-metal liquid at room temp
• Carbon: basis of all organic chemistry
• Diamond (carbon): hardest natural substance; conducts heat but not electricity
• Graphite (carbon): soft; conducts electricity
Applications:
• Cl₂ → PVC, water purification
• N₂ → fertilisers (urea, ammonia)
• O₂ → respiration, combustion
• C → fuel, diamonds, graphite electrodes
• Variety of colours; not lustrous
• Brittle (not malleable/ductile)
• Poor conductors of heat & electricity (exception: graphite — conducts electricity)
• Not sonorous
• Can be gas, liquid, or solid at room temperature
• Located on right side of periodic table
Chemical properties:
• High electronegativity (tend to gain electrons)
• Can be highly reactive (Fluorine = most reactive)
• Form acidic oxides
Special facts:
• Bromine: only non-metal liquid at room temp
• Carbon: basis of all organic chemistry
• Diamond (carbon): hardest natural substance; conducts heat but not electricity
• Graphite (carbon): soft; conducts electricity
Applications:
• Cl₂ → PVC, water purification
• N₂ → fertilisers (urea, ammonia)
• O₂ → respiration, combustion
• C → fuel, diamonds, graphite electrodes
🔵 METALLOIDS
Elements with intermediate properties between metals and non-metals. Located along the zig-zag / staircase line in the periodic table.
The 6 Metalloids:
1. Boron (B)
2. Silicon (Si)
3. Germanium (Ge)
4. Arsenic (As)
5. Antimony (Sb)
6. Tellurium (Te)
Properties:
• Act as semiconductors (conduct electricity moderately)
• Brittle, not malleable
• Lack metallic lustre (but some have metallic appearance)
• Intermediate electronegativity
• Intermediate reactivity
Applications:
• Silicon: transistors, solar cells, computer chips
• Germanium: early transistors, infrared optics
• Boron: borosilicate glass (Pyrex), doping silicon
• Arsenic: semiconductors, wood preservative
Mnemonic: "B Si Ge As Sb Te" → "Big Silicon Gets Amazing Semiconductor Technology"
The 6 Metalloids:
1. Boron (B)
2. Silicon (Si)
3. Germanium (Ge)
4. Arsenic (As)
5. Antimony (Sb)
6. Tellurium (Te)
Properties:
• Act as semiconductors (conduct electricity moderately)
• Brittle, not malleable
• Lack metallic lustre (but some have metallic appearance)
• Intermediate electronegativity
• Intermediate reactivity
Applications:
• Silicon: transistors, solar cells, computer chips
• Germanium: early transistors, infrared optics
• Boron: borosilicate glass (Pyrex), doping silicon
• Arsenic: semiconductors, wood preservative
Mnemonic: "B Si Ge As Sb Te" → "Big Silicon Gets Amazing Semiconductor Technology"
| Property | 🔴 Metals | 🟢 Non-metals | 🔵 Metalloids |
|---|---|---|---|
| Lustre | ✅ Shiny | ❌ Not lustrous | ❌ No metallic lustre |
| Malleability | ✅ Malleable | ❌ Brittle | ❌ Brittle |
| Ductility | ✅ Ductile | ❌ Not ductile | ❌ Not ductile |
| Conductivity | ✅ Good conductor | ❌ Poor conductor (graphite: exception) | ⚠ Semiconductor (moderate) |
| Sonorous | ✅ Yes (rings) | ❌ No | ❌ No |
| State at RT | Mostly solid (Hg = liquid) | Gas/liquid/solid (Br = liquid) | Mostly solid |
| Oxide type | Basic oxide | Acidic oxide | Amphoteric (both) |
| Electronegativity | Low | High | Intermediate |
| Examples | Fe, Cu, Au, Na, Al, Zn | O, N, C, Cl, F, H, S, Br | B, Si, Ge, As, Sb, Te |
📊
The Periodic Table — Organisation of Elements
118 elements · Periods · Groups · Blocks · s/p/d/f blocks · Noble gases · Lanthanides · Actinides
The Periodic Table of Elements. 118 elements organised by atomic number (left to right, top to bottom). Periods (rows 1–7): elements in same period have same number of electron shells. Groups (columns 1–18): elements in same group have same valence electrons and similar chemical properties. Colour coding: Pink/Red = Metals (Groups 1–2, most of 3–12). Blue = Transition metals (d-block, Groups 3–12). Yellow = Non-metals (right side, p-block). Green = Lanthanides and Actinides (f-block, bottom two rows). Dashed staircase = Metalloids boundary. Group 18 (far right) = Noble gases (He, Ne, Ar, Kr, Xe, Rn, Og). (Uploaded image — Legacy IAS)
📊 Blocks of the Periodic Table
s-block (Groups 1&2): Alkali metals (Group 1) + Alkaline earth metals (Group 2) + He. Very reactive metals. Na, K, Li, Mg, Ca, Be, He.
p-block (Groups 13–18): Non-metals, metalloids, noble gases. Includes C, N, O, F, Cl, Ne, Ar, B, Si, Al, etc.
d-block (Groups 3–12): Transition metals. Fe, Cu, Zn, Cr, Mn, Co, Ni, Ag, Au, Pt, Hg.
f-block (bottom two rows): Inner transition metals. Lanthanides (58–71, Ce→Lu, rare earth elements) & Actinides (90–103, Th→Lr, radioactive).
p-block (Groups 13–18): Non-metals, metalloids, noble gases. Includes C, N, O, F, Cl, Ne, Ar, B, Si, Al, etc.
d-block (Groups 3–12): Transition metals. Fe, Cu, Zn, Cr, Mn, Co, Ni, Ag, Au, Pt, Hg.
f-block (bottom two rows): Inner transition metals. Lanthanides (58–71, Ce→Lu, rare earth elements) & Actinides (90–103, Th→Lr, radioactive).
🔑 Key UPSC Facts — Elements
• 118 elements recognised; 92 naturally occurring
• Majority are solids; 11 are gases; 2 liquids (Hg, Br)
• Gallium (Ga) & Caesium (Cs): melt slightly above room temperature
• Most abundant in Earth's crust: Oxygen (46%) → Silicon (28%) → Aluminium (8%) → Iron (5%)
• Most abundant in universe: Hydrogen (75%) → Helium (24%)
• Noble gases (Group 18): He, Ne, Ar, Kr, Xe, Rn — valency 0, inert, full outer shell
• Element symbols: 1st letter CAPITAL, 2nd lowercase. Some from Latin: Fe (Ferrum), Au (Aurum), Na (Natrium), Cu (Cuprum)
• Majority are solids; 11 are gases; 2 liquids (Hg, Br)
• Gallium (Ga) & Caesium (Cs): melt slightly above room temperature
• Most abundant in Earth's crust: Oxygen (46%) → Silicon (28%) → Aluminium (8%) → Iron (5%)
• Most abundant in universe: Hydrogen (75%) → Helium (24%)
• Noble gases (Group 18): He, Ne, Ar, Kr, Xe, Rn — valency 0, inert, full outer shell
• Element symbols: 1st letter CAPITAL, 2nd lowercase. Some from Latin: Fe (Ferrum), Au (Aurum), Na (Natrium), Cu (Cuprum)
🔗
Compounds — Chemically Combined Elements
Fixed ratio · Covalent · Ionic · Organic · Inorganic · Water · Properties differ from constituents
📖 Definition
A compound is a substance composed of two or more elements, chemically combined in a fixed proportion. The properties of a compound are usually very different from those of the constituent elements. A compound can be separated into its elements only by chemical or electrochemical reactions (not by physical means).
Water (H₂O) — A Compound. Water is formed when 2 hydrogen atoms (white spheres in ball-and-stick model) combine with 1 oxygen atom (red sphere). The ratio H:O is always 2:1 by atoms, 1:8 by mass. Water's properties (colourless liquid, neutral, boiling point 100°C) are completely different from hydrogen (flammable gas) and oxygen (supports combustion gas) — this is a hallmark of compounds. (Uploaded image — Legacy IAS)
🔗 Types of Compounds
Ionic compounds: Electrons transferred between atoms → cations and anions form. Ionic bond. High melting points. Conduct electricity when dissolved/molten. Examples: NaCl, MgO, CaCl₂, KBr
Covalent compounds: Electrons shared between atoms. Covalent bond. Lower melting points. Usually don't conduct electricity. Examples: H₂O, CO₂, NH₃, CH₄, C₆H₁₂O₆ (glucose)
Organic compounds: Contain C-H bonds. Found in living organisms. Carbohydrates, proteins, lipids, nucleic acids, petroleum.
Inorganic compounds: No C-H bonds. Salts, metals, minerals, acids, bases. HCl, H₂SO₄, NaOH, NaCl.
Covalent compounds: Electrons shared between atoms. Covalent bond. Lower melting points. Usually don't conduct electricity. Examples: H₂O, CO₂, NH₃, CH₄, C₆H₁₂O₆ (glucose)
Organic compounds: Contain C-H bonds. Found in living organisms. Carbohydrates, proteins, lipids, nucleic acids, petroleum.
Inorganic compounds: No C-H bonds. Salts, metals, minerals, acids, bases. HCl, H₂SO₄, NaOH, NaCl.
✅ Properties of Compounds
Fixed composition: Always same ratio of elements
Different properties: Compound ≠ constituent elements
Homogeneous: Uniform composition throughout
Chemical separation only: Cannot separate by physical methods
Specific melting/boiling points
Energy involved: Formation involves energy change (exo/endothermic)
Classic examples:
• H₂O: H (flammable) + O (supports combustion) → water (extinguishes fire!)
• NaCl: Na (explosive metal) + Cl (toxic gas) → table salt (edible!)
• FeS: Fe (magnetic metal) + S (yellow solid) → FeS (non-magnetic black solid)
Different properties: Compound ≠ constituent elements
Homogeneous: Uniform composition throughout
Chemical separation only: Cannot separate by physical methods
Specific melting/boiling points
Energy involved: Formation involves energy change (exo/endothermic)
Classic examples:
• H₂O: H (flammable) + O (supports combustion) → water (extinguishes fire!)
• NaCl: Na (explosive metal) + Cl (toxic gas) → table salt (edible!)
• FeS: Fe (magnetic metal) + S (yellow solid) → FeS (non-magnetic black solid)
| Compound | Formula | Bond Type | Elements | Key Property |
|---|---|---|---|---|
| Water | H₂O | Covalent | H + O | Liquid at RT, neutral pH, H:O = 1:8 by mass |
| Common salt | NaCl | Ionic | Na + Cl | Crystalline solid, conducts electricity in solution |
| Carbon dioxide | CO₂ | Covalent | C + O | Gas at RT, acidic (CO₂ + H₂O → H₂CO₃), greenhouse gas |
| Ammonia | NH₃ | Covalent | N + H | Gas, basic, used in fertilisers |
| Sulphuric acid | H₂SO₄ | Covalent | H + S + O | Strong acid, dense oily liquid |
| Magnesium oxide | MgO | Ionic | Mg + O | White solid, basic oxide, very high melting point |
| Iron sulphide | FeS | Ionic/covalent | Fe + S | Black solid, NOT magnetic (unlike Fe) |
| Glucose | C₆H₁₂O₆ | Covalent (organic) | C + H + O | Organic, sweet, used in cellular respiration |
🌊
Mixtures — Homogeneous & Heterogeneous
Solutions · Alloys · Colloids · Suspensions · Tyndall effect · Separation methods
📖 Definition
A mixture is a combination of two or more substances where each substance retains its own chemical identity and properties. Mixtures are NOT chemically combined. They can be separated by physical methods (filtration, evaporation, distillation, chromatography, centrifugation). The composition of a mixture is variable and not fixed.
🔵 Homogeneous Mixtures (Solutions)
Definition: Uniform composition throughout — cannot see individual components. Single phase. No visible boundaries.
Types:
• Solutions: Solute dissolved in solvent. Salt in water, sugar in water. Clear, transparent.
• Alloys: Solid solutions of metals. Brass (Cu+Zn), Bronze (Cu+Sn), Steel (Fe+C), Solder (Pb+Sn), Stainless steel (Fe+Cr+Ni).
• Gaseous solutions: Air (N₂+O₂+Ar+CO₂+...) — gases mixed uniformly
Properties:
• Uniform (no visible boundaries)
• Components not distinguishable by naked eye
• Stable (don't separate on standing)
• Pass through filter paper
• Do NOT scatter light (Tyndall effect ABSENT)
Examples: Saltwater, brass, vinegar, alcohol in water, air, lemonade, seawater, copper sulphate solution
Types:
• Solutions: Solute dissolved in solvent. Salt in water, sugar in water. Clear, transparent.
• Alloys: Solid solutions of metals. Brass (Cu+Zn), Bronze (Cu+Sn), Steel (Fe+C), Solder (Pb+Sn), Stainless steel (Fe+Cr+Ni).
• Gaseous solutions: Air (N₂+O₂+Ar+CO₂+...) — gases mixed uniformly
Properties:
• Uniform (no visible boundaries)
• Components not distinguishable by naked eye
• Stable (don't separate on standing)
• Pass through filter paper
• Do NOT scatter light (Tyndall effect ABSENT)
Examples: Saltwater, brass, vinegar, alcohol in water, air, lemonade, seawater, copper sulphate solution
🟠 Heterogeneous Mixtures
Definition: Non-uniform composition — visible separation between components. Multiple phases possible.
Types:
• Suspensions: Large particles (>1000 nm) — muddy water, chalk in water. Particles visible; settle on standing; can be filtered; scatter light (Tyndall effect).
• Colloids: Medium particle size (1–1000 nm) — milk, fog, clouds, jelly, blood, smoke, paint. Particles not visible by naked eye; DON'T settle; CANNOT be filtered; show Tyndall effect (scatter light — beam of light visible through colloid).
• Mechanical mixtures: Sand + salt, granite (minerals), soil, rocks.
Examples: Muddy water (suspension), milk (colloid), smoke (colloid), blood (colloid), sand & salt, oil & water, granite, soil, sugar & salt mixed
Types:
• Suspensions: Large particles (>1000 nm) — muddy water, chalk in water. Particles visible; settle on standing; can be filtered; scatter light (Tyndall effect).
• Colloids: Medium particle size (1–1000 nm) — milk, fog, clouds, jelly, blood, smoke, paint. Particles not visible by naked eye; DON'T settle; CANNOT be filtered; show Tyndall effect (scatter light — beam of light visible through colloid).
• Mechanical mixtures: Sand + salt, granite (minerals), soil, rocks.
Examples: Muddy water (suspension), milk (colloid), smoke (colloid), blood (colloid), sand & salt, oil & water, granite, soil, sugar & salt mixed
| Property | Solution (Homogeneous) | Colloid (Heterogeneous) | Suspension (Heterogeneous) |
|---|---|---|---|
| Particle size | <1 nm (molecule/ion level) | 1–1000 nm | >1000 nm (visible) |
| Appearance | Clear, transparent | Translucent or opaque | Opaque, turbid |
| Stability | Very stable (no settling) | Stable (no settling) | Unstable (settles on standing) |
| Filtration | Passes through filter paper | Passes through filter paper | Cannot pass (filtered out) |
| Tyndall effect | ❌ Absent | ✅ Present | ✅ Present |
| Separation | Distillation, evaporation | Centrifugation, coagulation | Filtration, centrifugation |
| Examples | Saltwater, brass, air | Milk, blood, fog, smoke, jelly | Muddy water, chalk water |
🔦 Tyndall Effect — UPSC Favourite
What it is: When a beam of light passes through a colloid, the colloidal particles scatter the light — making the beam visible from the side (like a beam of sunlight through dusty air or fog headlights). Named after physicist John Tyndall.
Where seen: Milk (beam visible), fog (car headlights scatter), smoke, jelly, blood serum, clouds. NOT seen in true solutions (saltwater) — particles too small to scatter light.
Test for colloid vs solution: Shine a torch through the liquid. If beam is visible → colloid. If beam not visible → true solution.
Where seen: Milk (beam visible), fog (car headlights scatter), smoke, jelly, blood serum, clouds. NOT seen in true solutions (saltwater) — particles too small to scatter light.
Test for colloid vs solution: Shine a torch through the liquid. If beam is visible → colloid. If beam not visible → true solution.
⚙ Important Alloys — UPSC Favourite
Steel: Fe + C (0.2–2.1%) → Strong, used in construction, vehicles, tools
Stainless Steel: Fe + Cr (10–30%) + Ni → Rust-resistant, cutlery, surgical instruments
Cast Iron: Fe + C (>2%) → Brittle, engine blocks, pipes
Stainless Steel: Fe + Cr (10–30%) + Ni → Rust-resistant, cutlery, surgical instruments
Cast Iron: Fe + C (>2%) → Brittle, engine blocks, pipes
Brass: Cu + Zn → Musical instruments, locks, taps
Bronze: Cu + Sn → Statues, medals, bearings
Solder: Pb + Sn → Joining metals in electronics
Duralumin: Al + Cu + Mg + Mn → Aircraft, spacecraft
Bronze: Cu + Sn → Statues, medals, bearings
Solder: Pb + Sn → Joining metals in electronics
Duralumin: Al + Cu + Mg + Mn → Aircraft, spacecraft
Amalgam: Mercury (Hg) + another metal → Dental fillings (Hg + Ag + Sn + Cu)
Nichrome: Ni + Cr → Heating elements (toasters, geysers)
Magnalium: Mg + Al → Aircraft, scientific instruments
18-carat gold: 75% Au + 25% Cu/Ag → Jewellery (pure gold too soft)
Nichrome: Ni + Cr → Heating elements (toasters, geysers)
Magnalium: Mg + Al → Aircraft, scientific instruments
18-carat gold: 75% Au + 25% Cu/Ag → Jewellery (pure gold too soft)
⚖
Compounds vs Mixtures — Master Comparison High Yield
Fixed vs variable composition · Chemical vs physical separation · Properties · Formation
| Feature | 🔗 Compound | 🌊 Mixture |
|---|---|---|
| Formation | Elements react chemically to form new compound | Components just mix physically — no chemical reaction |
| Composition | Fixed (always same ratio) | Variable (any proportion) |
| Properties | Entirely different from constituent elements | Shows properties of constituent substances |
| Separation | Only by chemical/electrochemical means | By physical methods (filtration, distillation etc.) |
| Energy change | ✅ Energy released or absorbed during formation | ❌ No significant energy change |
| Homogeneity | Always uniform (pure substance) | May be uniform (homogeneous) or non-uniform (heterogeneous) |
| Pure substance? | ✅ Yes — always pure | ❌ No — not a pure substance |
| Classic example | FeS (iron sulphide) — black, non-magnetic | Fe + S mixture — silver + yellow, magnetic (Fe still present) |
| Example 2 | Water (H₂O) — liquid, neutral | H₂ + O₂ mixture — explosive gas mixture |
| New substance formed? | ✅ Yes — entirely new substance | ❌ No — original substances unchanged |
🧠 Key Test — Fe and S Experiment (NCERT Classic)
Mix iron filings + sulphur powder → Mixture: you can separate Fe with a magnet; S dissolves in CS₂; Fe is still magnetic; both retain original colour and properties.
Heat iron filings + sulphur strongly → Compound (FeS): black solid forms; NOT magnetic (Fe's property gone); doesn't dissolve in CS₂ separately; properties completely different from Fe or S alone. This is the most direct UPSC-tested contrast between compound and mixture.
Heat iron filings + sulphur strongly → Compound (FeS): black solid forms; NOT magnetic (Fe's property gone); doesn't dissolve in CS₂ separately; properties completely different from Fe or S alone. This is the most direct UPSC-tested contrast between compound and mixture.
🔬 Methods of Separation of Mixtures
🧲
Magnetic Separation
Separate magnetic components. Example: Iron from sand or sulphur (magnet attracts Fe, leaves S). Used in scrap metal yards.
🫧
Filtration
Separate insoluble solid from liquid. Example: Sand from water. Filter paper retains large particles; liquid (filtrate) passes through.
💧
Evaporation
Remove liquid to obtain dissolved solid. Example: Salt from saltwater. Salt pans use solar evaporation. Used in salt production in India (Gujarat, Rajasthan).
🌀
Distillation
Separate liquids with different boiling points. Simple: one liquid + solid (alcohol from water). Fractional: liquids with close boiling points (petroleum refining into petrol, kerosene, diesel).
🎨
Chromatography
Separate components with different affinities for a stationary phase and mobile phase. Separates dyes in ink, pigments in plants. Used in forensics, drug testing, food quality control.
🌀
Centrifugation
Rapid spinning separates components by density. Blood into plasma and cells; milk into cream and skimmed milk; uranium isotope separation (gas centrifuge). Used in medical labs, dairy industry.
📜
PYQs & Practice MCQs
Elements · Compounds · Mixtures · Colloids · Metalloids · Alloys
📜 UPSC Pattern — Elements, Compounds & Mixtures Statements
Pattern Q
Q. Consider the following statements regarding the distinction between a compound and a mixture:
- A compound has a fixed composition whereas a mixture has a variable composition.
- The properties of a compound are similar to those of its constituent elements, while a mixture shows the properties of each component.
- A compound can be separated into its constituent elements only by chemical or electrochemical methods, while a mixture can be separated by physical methods.
- No energy change occurs during the formation of a compound.
- a) 1 and 2 only
- b) 1 and 3 only ✓
- c) 1, 3 and 4 only
- d) 2, 3 and 4 only
Statement 1 CORRECT: A compound always has a fixed (constant) composition by mass — water is always H:O = 1:8, NaCl is always Na:Cl = 23:35.5 by mass. A mixture, by contrast, has a variable composition — saltwater can be 1% or 50% salt, seawater is ~3.5% salt etc. This is based on the Law of Definite Proportions.
Statement 2 WRONG: This is the exact reverse of the truth. The properties of a compound are different from its constituent elements — water (liquid, neutral) differs entirely from hydrogen (flammable gas) and oxygen (gas that supports combustion). A mixture shows the properties of its constituent substances (iron + sulphur mixture: the iron part is still magnetic, the sulphur part still dissolves in CS₂).
Statement 3 CORRECT: Compounds require chemical/electrochemical reactions to break apart — water is split by electrolysis; NaCl is separated by electrolysis of brine. Mixtures are separated by physical means — filtration, distillation, evaporation, chromatography, magnetic separation.
Statement 4 WRONG: Energy changes DO occur during compound formation. Exothermic reactions release heat (burning of iron + sulphur to form FeS releases energy). Endothermic reactions absorb heat. This is a fundamental property of chemical reactions — mixtures, by contrast, involve no significant energy change.
Statement 2 WRONG: This is the exact reverse of the truth. The properties of a compound are different from its constituent elements — water (liquid, neutral) differs entirely from hydrogen (flammable gas) and oxygen (gas that supports combustion). A mixture shows the properties of its constituent substances (iron + sulphur mixture: the iron part is still magnetic, the sulphur part still dissolves in CS₂).
Statement 3 CORRECT: Compounds require chemical/electrochemical reactions to break apart — water is split by electrolysis; NaCl is separated by electrolysis of brine. Mixtures are separated by physical means — filtration, distillation, evaporation, chromatography, magnetic separation.
Statement 4 WRONG: Energy changes DO occur during compound formation. Exothermic reactions release heat (burning of iron + sulphur to form FeS releases energy). Endothermic reactions absorb heat. This is a fundamental property of chemical reactions — mixtures, by contrast, involve no significant energy change.
🧪 Practice MCQs — Elements, Compounds & Mixtures (Click to attempt)
Q1. Milk is classified as a colloid. Which of the following best explains why milk is a colloid and NOT a true solution?
- (a) Milk is a colloid because it contains water as the solvent and fat as the solute, and fat dissolves completely in water to form a transparent solution that scatters light due to the high refractive index of dissolved fat molecules
- (b) Milk is a colloid because it can be completely separated into fat and water by simple filtration through ordinary filter paper — true solutions cannot be filtered, but colloids always can
- (c) Milk is a colloid because fat is dispersed as tiny droplets (1–1000 nm) that are too large to dissolve but too small to settle — giving it characteristic properties: Tyndall effect (beam of light visible through milk), stable (doesn't separate on standing), and passes through ordinary filter paper but not through semi-permeable membranes
- (d) Milk is a colloid because it is a heterogeneous mixture where fat visibly floats on top of water — since you can see the fat layer clearly separated from the water, it satisfies the definition of a colloid as a visually separable mixture
A colloid (or colloidal solution) is a type of heterogeneous mixture where dispersed particles (1–1000 nm) are suspended in a continuous medium. These particles are larger than those in a true solution (<1 nm) but smaller than those in a suspension (>1000 nm). Milk is a colloid (specifically an emulsion) where fat droplets (colloidal size) are dispersed in water. Key properties of a colloid: (1) Particles are 1–1000 nm — too small to see with naked eye, too large to dissolve. (2) Stable — particles don't settle under gravity (unlike suspensions). (3) Pass through ordinary filter paper (unlike suspensions). (4) Cannot pass through semi-permeable membranes (unlike true solutions — this is the basis of dialysis). (5) Show Tyndall effect — colloidal particles scatter light, making a light beam visible when passed through milk (solution particles are too small to scatter). Option (a) is wrong: fat does NOT dissolve in water — it is dispersed as droplets. Option (b) is wrong: colloids DO pass through ordinary filter paper (suspensions don't). Option (d) is wrong: in fresh milk, the fat is NOT visibly separated — it appears uniform (though cream does separate in raw milk on standing).
Q2. Silicon is classified as a metalloid and is used extensively in electronic devices. Which of the following correctly explains the significance of silicon's metalloid classification for electronics?
- (a) Silicon is used in electronics because it is a metal with high electrical conductivity — its conductivity is similar to copper and silver, making it ideal for wiring in computer chips and replacing expensive metals
- (b) Silicon's metalloid classification is significant because it acts as a semiconductor — it can conduct electricity under certain conditions (when doped with small amounts of other elements like phosphorus or boron) but acts as an insulator under others — this controllable conductivity is the foundation of all transistors, diodes, and integrated circuits (computer chips)
- (c) Silicon is classified as a metalloid because it is a liquid at room temperature — its fluid state at room temperature allows it to be easily shaped into circuits before solidifying into the required semiconductor configuration
- (d) Silicon's metalloid nature means it has the same electrical conductivity as a typical metal but the brittleness of a non-metal — it is used in electronics specifically for its ability to be hammered into extremely thin sheets (1 atom thick) without breaking
Metalloids (boron, silicon, germanium, arsenic, antimony, tellurium) have intermediate properties between metals and non-metals. Their most important property for technology is semiconductor behaviour — they conduct electricity much better than insulators (like rubber or glass) but much less well than metals (like copper). Silicon's conductivity can be precisely controlled by "doping" — adding tiny amounts (1 in 10 million atoms) of other elements: N-type doping (phosphorus/arsenic: adds extra electrons → conducts better). P-type doping (boron/gallium: removes electrons → creates "holes"). By combining N-type and P-type silicon, engineers create: transistors (switch and amplify signals), diodes (allow current in one direction), integrated circuits (chips containing billions of transistors). This is why silicon is the backbone of all modern electronics — smartphones, computers, satellites, solar cells. The "Silicon Valley" in California is named after this element. Germanium (another metalloid) was used in early transistors before silicon replaced it. India has a growing semiconductor manufacturing sector — the Semiconductor Mission 2022 (₹76,000 crore) aims to build domestic chip manufacturing (ISMC project Mysuru, Micron-Tata SPS in Gujarat).
Q3. A student mixes iron filings with sulphur powder in a test tube and heats the mixture strongly. After heating, a new black substance forms. Which of the following observations would confirm that a compound (iron sulphide, FeS) has been formed rather than just a mixture remaining?
- (a) The black substance is lighter than the original iron+sulphur mixture — compounds always have lower mass than the sum of their constituent elements due to the energy released during bond formation
- (b) The black substance dissolves completely in water, forming a clear blue solution similar to copper sulphate — iron sulphide's high solubility in water proves it is a compound rather than a physical mixture
- (c) The black substance can be separated back into iron and sulphur by heating it gently — if heating reverses the combination, it proves the substances were only physically mixed and can be separated, confirming compound formation was incomplete
- (d) The black substance is NOT attracted to a magnet (unlike iron), cannot be separated by dissolving in carbon disulphide (which would dissolve sulphur but leave iron in a mixture), and has completely different properties from either iron or sulphur — confirming the elements have reacted to form a new substance with new properties, not just mixed together
The iron + sulphur experiment is the classic NCERT distinction between a mixture and a compound. Before heating (mixture): Iron filings are silvery-grey; sulphur is yellow. A magnet can attract the iron filings from the mixture. Sulphur dissolves in carbon disulphide (CS₂) — the iron doesn't. Each component retains its original properties. After heating (compound FeS formed): A black solid forms (iron sulphide, FeS). The black substance is NOT attracted to a magnet — iron's magnetic property is gone (the iron atoms are now chemically bonded to sulphur). The black substance does NOT dissolve in CS₂ as sulphur would — sulphur's properties are gone. When FeS is treated with dilute H₂SO₄: Fe+S mixture would fizz and smell of rotten eggs (H₂S from sulphur) while iron dissolves separately. FeS compound: both elements react together to produce H₂S + FeSO₄ — a single chemical reaction. The properties of FeS (black, non-magnetic, specific melting point) are completely different from Fe (silvery, magnetic) and S (yellow, dissolves in CS₂). This is the hallmark of compound formation: new substance with entirely new properties. Option (a) is wrong: mass is conserved (Law of Conservation of Mass). Option (c) is exactly wrong: if gentle heating reverses it, it means a compound formed (you need chemical energy to break it apart).
⚡ Quick Revision — Elements, Compounds & Mixtures
| Topic | Key Facts |
|---|---|
| Classification | Matter → Pure Substance (Elements + Compounds) + Mixtures (Homogeneous + Heterogeneous). Robert Boyle coined "element" (1661). Lavoisier: element cannot be broken down by chemical reactions. |
| Elements | 118 recognised; 92 natural; mostly solids; 11 gases; 2 liquids (Hg, Br). Cannot be broken down chemically. Atomic number = number of protons = unique identity. IUPAC: 1st letter capital, 2nd lowercase. Latin symbols: Fe, Au, Na, Cu, Ag. |
| Metals | Lustrous, malleable, ductile, sonorous, good conductors. Form basic oxides. Mercury = only liquid metal at RT. Sodium/Potassium = soft, reactive. Gold = unreactive. Applications: steel (construction), copper (wiring), aluminium (aircraft). |
| Non-metals | Brittle, poor conductors (exception: graphite conducts electricity). High electronegativity (gain electrons). Form acidic oxides. Bromine = only liquid non-metal at RT. Fluorine = most reactive non-metal. Diamond (hardest) and graphite are both carbon. |
| Metalloids | 6: Boron, Silicon, Germanium, Arsenic, Antimony, Tellurium. Semiconductors. Along zig-zag line in periodic table. Silicon = chips/transistors. Germanium = early transistors. Boron = borosilicate glass. Mnemonic: B Si Ge As Sb Te. |
| Compounds | Fixed composition. Properties different from elements. Separated only by chemical/electrochemical means. Types: Ionic (NaCl, MgO), Covalent (H₂O, CO₂), Organic (C-H bonds), Inorganic (no C-H). H:O in water = 1:8 by mass always. |
| Mixtures | Variable composition. Components retain properties. Separated by physical means. Homogeneous (uniform, one phase): solutions, alloys, air. Heterogeneous (non-uniform): suspensions (>1000nm, settle, filtered), colloids (1–1000nm, stable, Tyndall effect), mechanical mixtures. |
| Tyndall Effect | Scattering of light by colloidal particles. Seen in: milk, fog, blood, smoke, jelly. NOT seen in: true solutions (saltwater, CuSO₄). Test: torch beam visible → colloid. Not visible → solution. |
| Key Alloys | Steel (Fe+C), Brass (Cu+Zn), Bronze (Cu+Sn), Duralumin (Al+Cu+Mg+Mn), Solder (Pb+Sn), Stainless steel (Fe+Cr+Ni), Nichrome (Ni+Cr, heating elements), Amalgam (Hg+metals, dental fillings). |
| Compound vs Mixture | Compound: fixed composition, new properties, energy involved, chemical separation only. Mixture: variable, retains properties, no energy change, physical separation. FeS experiment: heated = compound (non-magnetic); unheated = mixture (magnetic Fe still present). |
🚨 5 UPSC Traps — Elements, Compounds & Mixtures:
Trap 1 — "Properties of a compound are similar to its constituent elements" → WRONG! Compounds have properties COMPLETELY DIFFERENT from their constituent elements — this is their defining characteristic. Na (explosive metal) + Cl (toxic gas) = NaCl (edible salt). H₂ (flammable) + O₂ (supports combustion) = H₂O (extinguishes fire). Fe (magnetic silver metal) + S (yellow solid) = FeS (black, non-magnetic solid). The properties of MIXTURES show those of constituents; compound properties are new and different. Pattern Q Statement 2 directly tests this.
Trap 2 — "Graphite is a non-metal so it cannot conduct electricity" → WRONG! Graphite is a non-metal (carbon) that CONDUCTS electricity — it is the standard exception to the rule that non-metals are poor conductors. Graphite's layered structure has delocalised electrons that carry current. This is why graphite is used as electrodes in electrolysis and in dry cells (batteries). Diamond (also pure carbon) does NOT conduct electricity but conducts heat better than any metal. Both diamond and graphite are non-metals, but they have opposite electrical properties.
Trap 3 — "Colloids can be separated by ordinary filtration" → WRONG! Colloids CANNOT be separated by ordinary filter paper — their particles (1–1000 nm) are too small to be caught by filter paper pores. Only suspensions (>1000 nm) can be filtered through ordinary filter paper. Colloids can be separated by centrifugation, coagulation, dialysis (semi-permeable membrane), or electrophoresis. This is often tested by comparing milk (colloid) vs muddy water (suspension) — muddy water can be filtered, milk cannot.
Trap 4 — "Mercury is a non-metal because it is liquid at room temperature" → WRONG! Mercury (Hg, Z=80) is a METAL — the only metal that is liquid at room temperature. It is lustrous, conducts electricity, and forms amalgams with other metals. Bromine (Br) is the only non-metal liquid at room temperature. Common confusion: both Hg and Br are liquids at RT, but Hg = metal and Br = non-metal. Gallium and Caesium are metals that are solid at RT but melt just above room temperature.
Trap 5 — "Air is a compound of nitrogen, oxygen, and other gases" → WRONG! Air is a MIXTURE — not a compound. Its composition varies (more CO₂ in cities, more water vapour near oceans), each component retains its own properties (N₂ remains N₂, O₂ remains O₂), and the components can be separated by physical means (fractional distillation of liquid air separates O₂, N₂, Ar). If air were a compound, it would have a fixed composition, the gases would have reacted to form new substances, and you'd need chemical means to separate them. Similarly, seawater = mixture (not compound), alloys = mixtures (not compounds).
Trap 1 — "Properties of a compound are similar to its constituent elements" → WRONG! Compounds have properties COMPLETELY DIFFERENT from their constituent elements — this is their defining characteristic. Na (explosive metal) + Cl (toxic gas) = NaCl (edible salt). H₂ (flammable) + O₂ (supports combustion) = H₂O (extinguishes fire). Fe (magnetic silver metal) + S (yellow solid) = FeS (black, non-magnetic solid). The properties of MIXTURES show those of constituents; compound properties are new and different. Pattern Q Statement 2 directly tests this.
Trap 2 — "Graphite is a non-metal so it cannot conduct electricity" → WRONG! Graphite is a non-metal (carbon) that CONDUCTS electricity — it is the standard exception to the rule that non-metals are poor conductors. Graphite's layered structure has delocalised electrons that carry current. This is why graphite is used as electrodes in electrolysis and in dry cells (batteries). Diamond (also pure carbon) does NOT conduct electricity but conducts heat better than any metal. Both diamond and graphite are non-metals, but they have opposite electrical properties.
Trap 3 — "Colloids can be separated by ordinary filtration" → WRONG! Colloids CANNOT be separated by ordinary filter paper — their particles (1–1000 nm) are too small to be caught by filter paper pores. Only suspensions (>1000 nm) can be filtered through ordinary filter paper. Colloids can be separated by centrifugation, coagulation, dialysis (semi-permeable membrane), or electrophoresis. This is often tested by comparing milk (colloid) vs muddy water (suspension) — muddy water can be filtered, milk cannot.
Trap 4 — "Mercury is a non-metal because it is liquid at room temperature" → WRONG! Mercury (Hg, Z=80) is a METAL — the only metal that is liquid at room temperature. It is lustrous, conducts electricity, and forms amalgams with other metals. Bromine (Br) is the only non-metal liquid at room temperature. Common confusion: both Hg and Br are liquids at RT, but Hg = metal and Br = non-metal. Gallium and Caesium are metals that are solid at RT but melt just above room temperature.
Trap 5 — "Air is a compound of nitrogen, oxygen, and other gases" → WRONG! Air is a MIXTURE — not a compound. Its composition varies (more CO₂ in cities, more water vapour near oceans), each component retains its own properties (N₂ remains N₂, O₂ remains O₂), and the components can be separated by physical means (fractional distillation of liquid air separates O₂, N₂, Ar). If air were a compound, it would have a fixed composition, the gases would have reacted to form new substances, and you'd need chemical means to separate them. Similarly, seawater = mixture (not compound), alloys = mixtures (not compounds).
