Notes For All Chapters – General Science Class 8
1. Introduction
Matter can undergo two types of changes:
Physical change – No new substance is formed.
Chemical change – A new substance is formed with different properties.
Chemical changes happen in daily life, such as rusting of iron, cooking food, burning, and respiration.
2. Physical and Chemical Changes
(A) What is a Physical Change?
In a physical change, the substance remains the same, but its shape, size, or state may change.
No new substance is formed, and the change is reversible.
(B) Examples of Physical Change
Melting of ice – Ice changes to water but remains the same substance.
Boiling of water – Water turns into steam but remains H₂O.
Breaking of glass – The shape changes, but the glass remains the same.
(C) What is a Chemical Change?
In a chemical change, the original substance is changed into a new substance with different properties.
These changes are irreversible.
(D) Examples of Chemical Change
Rusting of iron – Iron reacts with oxygen and water to form rust.
Burning of paper – Paper burns to form ash and smoke.
Digestion of food – Food is broken down into new substances for energy.
3. Signs of a Chemical Change
Change in color – Example: Copper sulphate solution turns green when iron is added.
Change in temperature – Example: Heat is produced when fuel burns.
Formation of gas – Example: Carbon dioxide gas is produced when baking soda reacts with lemon juice.
Formation of precipitate – Example: A solid is formed when two solutions mix.
4. Chemical Reactions and Equations
(A) What is a Chemical Reaction?
A chemical reaction is a process in which one or more substances (reactants) react to form new substances (products).
(B) Word Equations for Some Reactions
Rusting of iron:
Iron + Oxygen + Water → Iron Oxide (Rust)Burning of fuel:
Fuel + Oxygen → Carbon dioxide + Water + HeatReaction of vinegar and baking soda:
Acetic acid + Sodium bicarbonate → Carbon dioxide + Water + Salt
5. Chemical Bonds
(A) What is a Chemical Bond?
A chemical bond is the force that holds atoms together in a molecule.
(B) Types of Chemical Bonds
| Type | Definition | Example |
|---|---|---|
| Ionic Bond | Formed when one atom transfers electrons to another. | Sodium chloride (NaCl) |
| Covalent Bond | Formed when two atoms share electrons. | Water (H₂O) |
6. Ionic and Covalent Compounds
(A) What are Ionic Compounds?
Formed when electrons are transferred from one atom to another.
Strong bonds with high melting and boiling points.
Dissolve in water and conduct electricity.
Example: Sodium chloride (NaCl).
(B) What are Covalent Compounds?
Formed when atoms share electrons.
Have weaker bonds than ionic compounds.
Usually do not conduct electricity.
Example: Water (H₂O), Oxygen (O₂), Carbon dioxide (CO₂).
7. Importance of Chemical Changes in Daily Life
| Chemical Change | Use |
|---|---|
| Rusting of iron | Weakens iron structures. |
| Burning of fuels | Produces energy for vehicles and cooking. |
| Photosynthesis | Plants make food using sunlight. |
| Digestion of food | Converts food into energy. |
| Respiration | Provides oxygen and energy to the body. |
8. Examples of Common Chemical Reactions
| Reaction | Chemical Equation |
|---|---|
| Photosynthesis | Carbon dioxide + Water → Glucose + Oxygen |
| Respiration | Glucose + Oxygen → Carbon dioxide + Water + Energy |
| Rusting of iron | Iron + Oxygen + Water → Iron oxide |
| Neutralization | Acid + Base → Salt + Water |
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