🔬 Introduction
In our daily lives, we see many changes happening around us. Some changes are temporary, while others are permanent. Science classifies these changes into two main types:
- Physical changes
- Chemical changes
Let’s understand each of them in detail.
🔹 What is a Physical Change?
A physical change is a change in which only the physical properties of a substance change. These properties include shape, size, state (solid, liquid, gas), and appearance. The chemical composition remains the same.
🔑 Characteristics of Physical Changes:
- No new substance is formed.
- The change is often reversible.
- Only physical properties like state, shape, and size change.
- The substance remains the same, chemically.
📌 Examples of Physical Changes:
- Melting of ice
- Boiling of water
- Breaking a glass
- Dissolving salt in water
- Cutting paper
🔹 What is a Chemical Change?
A chemical change is a change in which a new substance is formed with different properties. These changes usually cannot be reversed easily.
🔑 Characteristics of Chemical Changes:
- New substances are formed.
- The change is often irreversible.
- It involves a chemical reaction.
- Heat, light, or gas may be released.
📌 Examples of Chemical Changes:
- Burning of paper
- Rusting of iron
- Cooking food
- Digesting food
- Souring of milk
🔄 Comparison Table: Physical vs Chemical Changes
| Feature | Physical Change | Chemical Change |
|---|---|---|
| New substance formed | No | Yes |
| Reversible | Usually reversible | Usually irreversible |
| Properties changed | Physical properties only | Both physical and chemical |
| Examples | Melting ice, cutting paper | Burning, rusting, cooking |
| Energy involved | Usually small amounts | Often involves release or absorption of energy |
⚗️ Real-life Examples Explained
1. Melting Ice (Physical Change)
When ice melts, it becomes water. The state changes from solid to liquid, but it’s still H₂O.
2. Burning Wood (Chemical Change)
When wood burns, it turns to ash and releases smoke and gases. A new substance is formed, so it’s a chemical change.
3. Rusting of Iron (Chemical Change)
Iron reacts with oxygen and water to form iron oxide (rust). This is a chemical change.
4. Chopping Vegetables (Physical Change)
The size and shape change, but the vegetable remains the same substance.
Q1. What is a physical change? Give two examples.
Answer:
A physical change is a change in which no new substance is formed, and only physical properties like shape, size, or state change.
Examples:
- Melting of ice
- Boiling of water
Q2. What is a chemical change? Give two examples.
Answer:
A chemical change is a change in which a new substance is formed with new properties.
Examples:
- Rusting of iron
- Burning of wood
Q3. Is melting of wax a physical or chemical change? Why?
Answer:
Melting of wax is a physical change because only its state changes from solid to liquid, and no new substance is formed.
Q4. What happens during the rusting of iron?
Answer:
Iron reacts with oxygen and moisture (water) in the air to form a new substance called iron oxide (rust). It is a chemical change.
Q5. Give one example of a change that is both physical and chemical.
Answer:
Burning of a candle is both a physical and chemical change.
- The melting of wax is a physical change.
- The burning of wax is a chemical change (produces carbon dioxide, water vapor, and soot).
Q6. What are the characteristics of chemical changes?
Answer:
- New substances are formed.
- It is usually irreversible.
- Energy may be released or absorbed.
- Change in color or smell may occur.
Q7. Is dissolving salt in water a physical or chemical change?
Answer:
It is a physical change because no new substance is formed and the salt can be recovered by evaporation.
Q8. Write two differences between physical and chemical changes.
Answer:
| Physical Change | Chemical Change |
|---|---|
| No new substance is formed | New substance is formed |
| Usually reversible | Usually irreversible |
Q9. Why is cooking food a chemical change?
Answer:
Cooking food is a chemical change because new substances are formed, and the original ingredients cannot be recovered in their original form.
Q10. What is the role of heat in chemical changes?
Answer:
Heat can either be absorbed (endothermic reaction) or released (exothermic reaction) during a chemical change. It helps to break or form new chemical bonds.
🌈 Fun Facts
- The change of water into steam is reversible, but the burning of coal is not.
- Fireworks involve chemical changes that release light, sound, and heat.
- Some physical changes like stretching a rubber band can be reversed easily.
✍️ Activity
Try to identify whether the following changes are physical or chemical:
| Change | Physical/Chemical |
|---|---|
| Freezing water | Physical |
| Baking a cake | Chemical |
| Tearing a paper | Physical |
| Lighting a matchstick | Chemical |
| Mixing sugar in water | Physical |
📚 Conclusion
Understanding the difference between physical and chemical changes is important in science. It helps us identify what kind of transformation is happening in materials around us. Physical changes affect only the appearance, while chemical changes form new substances with different properties.
By observing clues like change in color, gas production, temperature change, or formation of a new substance, we can easily tell if a change is chemical or physical.
Leave a Reply