Understanding Chemical Reactions

📚 Key Concepts

🔹 Real-Life Example

When you cook an egg, the clear liquid egg white turns solid and white. This change cannot be reversed – you can’t turn a cooked egg back into a raw egg! Similarly, when wood burns to form ash, the original wood is gone forever. These irreversible changes where new substances form are chemical reactions, unlike physical changes such as melting ice that can be easily reversed.

Chemical Reaction: Chemical reactions are the processes in which new substances with new properties are formed. During a chemical reaction, atoms of one element do not change into those of another element. Only a rearrangement of atoms takes place in a chemical reaction.

🧪 Important Concepts

🔸 Key Characteristics of Chemical Reactions

• New substances are formed with different properties
• Energy changes occur (heat, light, or sound may be produced)
• The change is usually irreversible
• Atomic rearrangement takes place
• Mass is conserved (Law of Conservation of Mass)

🔹 Physical vs Chemical Changes

Physical Changes:

• Easily reversible
• No new substance formed
• Only physical properties change
• Examples: melting ice, dissolving sugar

Chemical Changes:

• Difficult to reverse
• New substances formed
• Chemical properties change
• Examples: burning paper, rusting iron

🔍 Advanced: Signs of Chemical Reactions

Chemical reactions can be identified by observing:

• Evolution of gas: Bubbles formation
• Formation of precipitate: Solid formation in liquid
• Change in color: Different color appearance
• Change in temperature: Heat absorption or release
• Change in state: Solid to liquid, etc.