Chapter 8 • Class 8 Science
🧪 Nature of Matter:
Elements, Compounds & Mixtures
Elements, Compounds & Mixtures
Curiosity — Textbook of Science for Grade 8
📋 Table of Contents
8.1 What Are Mixtures?
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Fig: Poha — a mixture
of many ingredients
of many ingredients
Poha, sprout salad, sugar in water, air — all are mixtures!
- Two or more substances mixed together
- Each substance retains its own properties
- Components do NOT react chemically
- Can be separated by physical methods
📌 Key Definition
Mixture = Two or more substances mixed together where each retains its individual properties and do NOT react chemically.
Components = The individual substances that make up a mixture.
Components = The individual substances that make up a mixture.
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Fig: Sprout salad —
non-uniform mixture
non-uniform mixture
Two Types of Mixtures:
- Non-Uniform — components visible separately (sprout salad, poha, muddy water, sand+water)
- Uniform — components NOT visible separately, evenly distributed (sugar in water, air, seawater, alloys)
Alloys = Uniform Solid Mixtures!
Stainless steel = Iron + Nickel + Chromium + CarbonBrass = Copper + Zinc | Bronze = Copper + Tin
Ancient Indian texts call metal mixtures Mishraloha. Bronze (Kamsya) was used medicinally!
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8.1.1 Air — A Mixture!
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Fig: Lime water turns
milky → CO₂ present!
milky → CO₂ present!
Air is a uniform mixture of gases:
- 🟢 Nitrogen (~78%) — doesn't support combustion
- 🔴 Oxygen (~21%) — supports combustion, needed for life
- 🔵 Argon, CO₂, Water vapour — in small amounts
- 💨 Dust particles — suspended pollutants (not integral part)
📌 Chemical Reaction (Lime Water Test for CO₂)
Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water(Lime water turns milky = CO₂ is present in air!)
📊 Types of Mixtures Table
| Mixture Type | Examples | Uniform/Non-uniform |
|---|---|---|
| Gas + Gas | Air | ✅ Uniform |
| Gas + Liquid | Soda water, O₂ in water | ✅ Uniform |
| Solid + Gas | Carbon particles in air | ❌ Non-uniform |
| Liquid + Liquid | Vinegar (acetic acid + water), Oil + water | Uniform / Non-uniform |
| Solid + Liquid | Sand + water, Seawater | Non-uniform / Uniform |
| Solid + Solid | Baking powder, Alloys | Uniform |
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8.2 What Are Pure Substances?
In science, pure substance = matter that cannot be separated into other kinds of matter by any physical process. All particles are of the same type.
- Common usage "pure" = unadulterated (e.g., Pure Milk, Pure Ghee)
- Science "pure" = only ONE kind of substance, no other substance present
- Even "Pure Milk" is impure in science — it has many substances!
- Pure substance can be: Element OR Compound
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8.3.1 Elements
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Fig: H₂ & O₂ molecules
— both elements
— both elements
- Made of identical particles called atoms
- Cannot be broken into simpler substances
- Building blocks of all matter 🧱
- Examples: Gold, Silver, Sulfur, Carbon, H₂, O₂, Fe, Cu
- Total elements known = 118
⚡ Electrolysis of Water Proves It!
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Electrolysis setup
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H₂: Pop sound 💥
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O₂: Flame brighter 🔥
Electrolysis of Water
Water → Hydrogen + Oxygen
Elements Facts!
Total = 118 elements | Most = Solid state11 elements = Gas at room temp (O₂, N₂, He, etc.)
Only 2 = Liquid at room temp: Mercury (metal) & Bromine (non-metal)
Metals: Au, Ag, Fe, Al, Mg | Non-metals: C, S, H, O | Metalloids: Si, B
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8.3.2 Compounds
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Fig: Water molecule
H₂O (2H + 1O)
H₂O (2H + 1O)
- Formed when elements combine in FIXED ratios chemically
- Properties are completely different from constituent elements
- Cannot be separated by physical methods
- Can only be broken by chemical methods
H₂ = Fuel (burns!) | O₂ = Supports combustion | But H₂O = Extinguishes fire! 😲 Compound properties are totally different!
🧪 Key Compounds with Ratios
| Compound | Elements | Ratio | Nature |
|---|---|---|---|
| 💧 Water (H₂O) | Hydrogen + Oxygen | 2:1 | Compound |
| 🧂 Common Salt (NaCl) | Sodium + Chlorine | 1:1 | Compound |
| 🍬 Sugar (C₁₂H₂₂O₁₁) | Carbon + Hydrogen + Oxygen | Fixed | Compound |
| ⚫ Iron Sulfide (FeS) | Iron + Sulfur | Fixed | Compound |
⚗️ Iron + Sulfur = Mixture vs Compound Experiment
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Iron filings powder
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Sample A (mix) vs B (compound)
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Magnet test A vs B
| Test | Sample A (Mixture) | Sample B (Compound = Iron Sulfide) |
|---|---|---|
| Appearance | Black + Yellow particles visible | Uniformly black throughout |
| Magnet test | ✅ Iron attracted to magnet | ❌ Not attracted (new substance!) |
| + HCl gas | Hydrogen gas (no smell, pop sound) | Hydrogen sulfide gas (rotten egg smell) |
| Separated? | ✅ Yes, by magnet | ❌ Cannot be separated physically |
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Elements vs Compounds vs Mixtures
ELEMENT
- ✅ Pure substance
- 🔬 Identical atoms
- ❌ Can't be broken further
- 📦 Fixed composition
- ⚡ Building block
- Examples: Au, O₂, Fe, C
COMPOUND
- ✅ Pure substance
- 🔗 Elements in fixed ratio
- ❌ Not separated physically
- 🆕 New properties
- ⚗️ Chemical combination
- Examples: H₂O, NaCl, FeS
MIXTURE
- ❌ Not pure
- 🔀 Variable ratio
- ✅ Separated physically
- 🔄 Retains properties
- 🚫 No chemical reaction
- Examples: Air, Alloys, Seawater
| Property | ⚛️ Element | 🧬 Compound | 🥗 Mixture |
|---|---|---|---|
| Pure Substance? | ✅ Yes | ✅ Yes | ❌ No |
| Composition | Fixed | Fixed ratio | Variable |
| Separation | Cannot break | Only chemical | Physical methods |
| New properties? | — | ✅ Yes, different | ❌ Retains own |
| Examples | Fe, Au, O₂, C | H₂O, NaCl, CO₂ | Air, Brass, Mud water |
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8.5 Minerals
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Fig: Minerals — Quartz,
Calcite, Mica, Talc
Calcite, Mica, Talc
Minerals = Natural, solid substances in rocks with fixed chemical composition.
- Most minerals = compounds (made of 2+ elements)
- Some = native minerals (pure elements like Gold, Silver, Sulfur)
- Examples: Quartz, Calcite, Mica, Pyroxene, Olivine, Talc
- Cement = from Calcite + Quartz + Alumina + Iron oxide
- Talcum powder = from mineral Talc
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Fig: Dhokra Art
(Bihar & Odisha)
(Bihar & Odisha)
Dhokra Art — Science in Culture!
Ancient craft from Bihar & Odisha using brass/bronze (mixtures of metals). Beeswax model → clay mould → molten brass poured in → beautiful figures! Shows use of compounds & mixtures in Indian art. 🎨
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📸 Quick Revision Snapshots
Mixture = 2+ substances mixed, each retains properties, no chemical reaction, can be separated physically.
Air = uniform mixture of N₂(78%), O₂(21%), Ar, CO₂, water vapour. Lime water test proves CO₂ in air.
Pure Substance (science) = only one type of particle. Can be element OR compound.
Element = simplest pure substance, made of identical atoms, cannot be broken further. 118 elements known. Building blocks of matter.
Compound = elements combined chemically in FIXED ratio. New properties. Cannot be separated physically. (H₂O, NaCl, sugar)
Electrolysis of water → H₂ (pop sound) + O₂ (flame brighter). Proves water is a compound of H & O.
Iron + Sulfur mixture (A) → magnet attracts iron. Heated (B = Iron Sulfide compound) → magnet has NO effect. New substance formed!
Minerals = natural solid substances with fixed composition. Most are compounds. Some native minerals are pure elements (Gold, Silver).
✨ Notes by @edugrown — Happy Learning! ✨