Notes For All Chapters – General Science Class 8
1. What is Matter?
- Everything around us is made up of matter.
- Matter is anything that has mass and occupies space.
- Examples: Water, air, soil, wood, metals, plastic, etc.
2. States of Matter
Matter exists in three main states:
| State | Shape | Volume | Particle Arrangement | Example |
|---|---|---|---|---|
| Solid | Fixed | Fixed | Particles are tightly packed | Ice, Wood, Iron |
| Liquid | No fixed | Fixed | Particles are loosely packed | Water, Oil, Milk |
| Gas | No fixed | No fixed | Particles are very far apart | Air, Oxygen, Steam |
Properties of Solids, Liquids, and Gases
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Shape | Fixed | No fixed | No fixed |
| Volume | Fixed | Fixed | No fixed |
| Compressibility | No | Slightly | High |
| Particle Movement | Vibrate in place | Move freely | Move very fast |
3. What is Matter Made of?
- The smallest particles of matter are called atoms and molecules.
- Atoms combine to form molecules.
Intermolecular Force and Movement
- Solids: Particles are very close due to strong intermolecular force.
- Liquids: Particles are loosely packed and can move freely.
- Gases: Particles are far apart and move very fast due to weak intermolecular force.
4. Classification of Matter
Matter is classified based on its chemical composition into:
- Element
- Compound
- Mixture
5. Elements
- An element is made of only one type of atom.
- Cannot be broken down further by physical or chemical means.
- Example: Oxygen (O₂), Hydrogen (H₂), Gold (Au), Silver (Ag)
Types of Elements
| Type | Properties | Examples |
|---|---|---|
| Metals | Shiny, good conductors of heat and electricity | Iron (Fe), Copper (Cu) |
| Non-Metals | Dull, poor conductors | Oxygen (O₂), Carbon (C) |
| Metalloids | Have properties of both metals and non-metals | Silicon (Si), Boron (B) |
6. Compounds
- A compound is a substance formed when two or more elements chemically combine in a fixed ratio.
- Example: Water (H₂O) is made of 2 Hydrogen and 1 Oxygen.
Properties of Compounds
- Elements in a compound are always in a fixed ratio (e.g., H₂O always has 2 Hydrogen and 1 Oxygen).
- Properties of a compound are different from its elements (e.g., Water is a liquid, but Oxygen and Hydrogen are gases).
| Compound | Chemical Formula | Elements Present |
|---|---|---|
| Water | H₂O | Hydrogen, Oxygen |
| Carbon Dioxide | CO₂ | Carbon, Oxygen |
| Sodium Chloride (Salt) | NaCl | Sodium, Chlorine |
7. Mixtures
- A mixture is formed when two or more substances are combined without a chemical reaction.
- The composition of a mixture is not fixed.
- Example: Air, Soil, Sugar in Water.
Types of Mixtures
| Type | Definition | Example |
|---|---|---|
| Homogeneous Mixture | Uniform composition, looks the same | Sugar water, Air |
| Heterogeneous Mixture | Non-uniform composition, different parts visible | Sand and Water, Oil and Water |
Difference between Compound and Mixture
| Property | Compound | Mixture |
|---|---|---|
| Formation | Elements combine chemically | Substances mix physically |
| Composition | Fixed ratio | Variable ratio |
| Properties | New properties form | Original properties remain |
| Example | Water (H₂O) | Air, Sand in Water |
8. Solutions, Suspensions, and Colloids
| Type | Definition | Example |
|---|---|---|
| Solution | A homogeneous mixture where solute dissolves in solvent | Salt in Water, Sugar in Water |
| Suspension | A heterogeneous mixture where particles do not dissolve and settle at the bottom | Mud in Water, Sand in Water |
| Colloid | A heterogeneous mixture where particles are small and do not settle | Milk, Fog, Smoke |
Solution Components
- Solvent – The substance in which another substance dissolves (Example: Water in salt water).
- Solute – The substance that dissolves (Example: Salt in salt water).
9. Molecular Formula and Valency
- Molecular Formula tells us the number of atoms in a compound.
- Valency is the combining capacity of an element.
Example of Molecular Formulas:
| Compound | Molecular Formula | Elements Present |
|---|---|---|
| Water | H₂O | H (2), O (1) |
| Carbon Dioxide | CO₂ | C (1), O (2) |
| Methane | CH₄ | C (1), H (4) |
Valency Chart:
| Element | Symbol | Valency |
|---|---|---|
| Hydrogen | H | 1 |
| Oxygen | O | 2 |
| Carbon | C | 4 |
| Sodium | Na | 1 |
| Calcium | Ca | 2 |
10. Nuclear Reactor and Nuclear Energy
What is a Nuclear Reactor?
- A machine that generates electricity using nuclear fission.
How does a Nuclear Reactor work?
- Uranium-235 or Thorium-232 is used as fuel.
- Neutrons hit uranium atoms, splitting them into smaller atoms and releasing huge energy.
- This energy heats water to produce steam.
- Steam turns turbines to generate electricity.
Parts of a Nuclear Reactor:
- Fuel (Uranium-235): Provides energy.
- Moderator (Graphite/Heavy Water): Slows down neutrons.
- Control Rods (Boron/Cadmium): Absorb extra neutrons to control the reaction.
- Coolant (Water/Carbon dioxide): Removes heat from the reactor.
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