नोट: इस लेख का हिंदी संस्करण भी उपलब्ध है।
🔰 Introduction
In our daily life, we observe many changes around us—ice melting, milk turning into curd, iron rusting, paper burning, etc. These changes may look simple, but scientifically they are very important.
In Class 7 Science Chapter 5 (NCERT), we study Physical and Chemical Changes, which helps us understand:
- What kind of changes are happening
- Why they occur
- Whether they can be reversed or not
This chapter builds a strong foundation for higher-level science concepts.
📘 What is a Change?
🔹 Definition of Change
A change is any process in which a substance undergoes a difference in its shape, size, state, or properties.
🔹 Examples of Changes
- Ice melting into water
- Tearing of paper
- Milk converting into curd
- Burning of wood
🔹 Types of Changes
Changes are mainly divided into two categories:
1. Physical Change
2. Chemical Change
🧊 Physical Change
🔹 Definition
A physical change is a change in which only the physical properties (such as shape, size, or state) of a substance change, but its chemical composition remains the same.
🔹 Examples of Physical Changes
✔️ 1. Melting of Ice
Ice → Water (only state changes)
✔️ 2. Boiling of Water
Water → Steam (liquid to gas)
✔️ 3. Cutting or Tearing Paper
Only shape changes, not the substance
🔹 Characteristics of Physical Changes
⭐ 1. No new substance is formed
The substance remains the same
⭐ 2. Usually reversible
The original substance can often be obtained again
⭐ 3. Only physical properties change
Like shape, size, and state
🔹 Types of Physical Changes
🔸 (A) Reversible Changes
Changes that can be undone
Examples:
- Ice → Water → Ice
- Melting of wax
🔸 (B) Irreversible Changes
Changes that cannot be reversed
Examples:
- Tearing paper
- Breaking glass
🔥 Chemical Change
🔹 Definition
A chemical change is a change in which a substance undergoes a chemical reaction and forms a new substance with different properties.
🔹 Examples of Chemical Changes
✔️ 1. Rusting of Iron
Iron reacts with oxygen and water to form rust
✔️ 2. Burning of Paper
Paper turns into ash (new substance)
✔️ 3. Milk Turning into Curd
Chemical composition changes
🔹 Characteristics of Chemical Changes
⭐ 1. New substance is formed
With completely different properties
⭐ 2. Irreversible in nature
Cannot be undone easily
⭐ 3. Energy change occurs
Heat or light is released or absorbed
⭐ 4. Change in color or smell
Often observed
⚖️ Difference Between Physical and Chemical Changes
| Basis | Physical Change | Chemical Change |
|---|---|---|
| New Substance | Not formed | Formed |
| Properties | Only physical properties change | Chemical properties change |
| Nature | Usually reversible | Irreversible |
| Example | Melting of ice | Burning of paper |
🧪 Indicators of Chemical Change
A chemical change can be identified by the following signs:
🔹 1. Evolution of Gas
Example: Bubbles in soda
🔹 2. Change in Color
Example: Iron turning reddish-brown (rust)
🔹 3. Change in Temperature
Heat is released or absorbed
🔹 4. Formation of Precipitate
A solid forms in a solution
🔩 Rusting of Iron
🔹 What is Rusting?
Rusting is a chemical change in which iron reacts with oxygen and moisture to form rust (iron oxide).
🔹 Chemical Reaction
Iron + Oxygen + Water → Rust (Iron Oxide)
🔹 Conditions Necessary for Rusting
✔️ Presence of Oxygen (Air)
✔️ Presence of Water (Moisture)
Both are essential
🔹 Harmful Effects of Rusting
- Weakens iron objects
- Damages bridges, vehicles, machines
- Causes economic loss
🔹 Prevention of Rusting
🔸 1. Painting
Prevents contact with air and water
🔸 2. Oiling or Greasing
Forms a protective layer
🔸 3. Galvanization
Coating iron with zinc
🔥 Combustion (Burning)
🔹 What is Combustion?
Combustion is a chemical process in which a substance reacts with oxygen and burns, producing heat and light.
🔹 Examples
- Burning of wood
- Burning of candle
🔹 Key Features
- Produces heat and light
- Forms new substances (ash, gases)
🧴 Crystallization
🔹 What is Crystallization?
Crystallization is a process used to obtain pure solid crystals from a solution.
🔹 Example
- Formation of copper sulphate crystals
🔹 Importance
- Used for purification of substances
- Widely used in laboratories
🌍 Physical and Chemical Changes in Daily Life
🔹 Physical Changes Around Us
- Cutting vegetables
- Boiling water
- Folding clothes
🔹 Chemical Changes Around Us
- Cooking food
- Digestion of food
- Milk turning into curd
- Rusting of iron
📊 Important NCERT Questions
🔸 Q1. What is a physical change?
Answer: A change in which only physical properties change and no new substance is formed.
🔸 Q2. What is a chemical change?
Answer: A change in which a new substance is formed with different properties.
🔸 Q3. Why does rusting occur?
Answer: Due to the presence of oxygen and moisture.
🔸 Q4. Are all chemical changes irreversible?
Answer: Yes, most chemical changes are irreversible.
🔸 Q5. Are all physical changes reversible?
Answer: No, some physical changes are irreversible (like tearing paper).
🎯 Key Exam Points
- Physical change → No new substance
- Chemical change → New substance formed
- Rusting is a chemical change
- Crystallization is a physical process
- Combustion is a chemical change
📚 Conclusion
The chapter “Physical and Chemical Changes” helps us understand the scientific nature of changes happening around us.
- Physical changes help us understand changes in form and state
- Chemical changes explain how new substances are formed
This chapter is very important as it builds the base for future topics in chemistry and science.
✨ Pro Study Tips
👉 Always relate concepts with real-life examples
👉 Practice differences between physical and chemical changes
👉 Revise topics like rusting, combustion, and crystallization regularly
Class 7 Science : Physical and Chemical Changes Free Practice Quiz
Only physical properties change; no new substance is formed.
Burning forms new substances like ash and gases.
Melted wax can solidify again.
Both oxygen and moisture are necessary for rusting.
Only shape changes, not the substance.
Chemical changes always produce a new substance.
Gas formation is a sign of chemical change.
It helps in obtaining pure crystals.
Burnt paper cannot be converted back.
Combustion is a reaction with oxygen producing heat and light.
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