Most changes around us are either physical or chemical. However, some processes involve both types of changes occurring at the same time.
Burning of a Candle - A burning candle is the best example.
Physical Changes:
- Wax near the flame melts.
- Melted wax changes into wax vapour.
- Melted wax flowing away from the flame solidifies again on cooling.
In all these changes,
- No new substance is formed.
- Only the state of wax changes.
Therefore, these are physical changes.
Chemical Change:
The wax vapour burns in the flame.
During burning,
- New substances are formed.
- The change cannot be reversed easily.
Therefore,
Burning of wax vapour is a chemical change.
Michael Faraday:
Michael Faraday explained the Chemical history of a candle.
By carefully observing a burning candle, he showed that
- melting,
- evaporation,
- burning,
all happen together during the same process.
This demonstrated that physical and chemical changes can occur simultaneously.
Are Changes permanent?
Some changes can be reversed, while others cannot.
Reversible and Irreversible changes:
| Description | Reversible changes | Irreversible changec |
| Definition | A reversible change is a change in which the original substance or object can be obtained again. | An irreversible change is a change in which the original substance or object cannot be obtained again. |
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Are all changes desirable?
Some changes are useful and some changes are harmful.
| Description | Desirable changes | Undesirable changes |
| Definition | A desirable change is one that is beneficial or useful. | An undesirable change is one that is harmful or unwanted. |
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Sometimes, the same type of change may be desirable in one situation and undesirable in another.
Example
Food decomposition
In a compost pit \(\to\) desirable
In stored food \(\to\) undesirable
The usefulness of a change depends on its purpose and situation.
Some Slow Natural Changes:
Many natural changes happen very slowly. These changes may take many years to become noticeable.
Weathering:
Weathering is the process by which rocks gradually break down into smaller pieces at the same place. It helps in the formation of soil.
Types of Weathering:
1. Physical Weathering:
The rock breaks into smaller pieces because of
- changes in temperature,
- growth of plant roots,
- freezing of water in rock cracks.
- Only size changes.
- No new substance is formed.
Therefore,
Physical weathering is a physical change.
2. Chemical Weathering:
Some rocks contain iron.
Iron reacts with water, moist air, forming iron oxide. A reddish outer layer appears on the rocks.
Remember:
A new substance (iron oxide) is formed. Therefore, Chemical weathering is a chemical change.
Erosion:
After weathering, broken rock particles are carried away by
- flowing water,
- wind.
This process is called erosion.
Difference Between Weathering and Erosion:
| Weathering | Erosion |
|---|---|
| Breaks rocks into smaller pieces. | Carries the broken rock particles from one place to another. |
| Happens at the same place. | Involves movement of particles |
| Leads to rock breakdown. | Leads to transportation and deposition of sediments. |
Formation of Soil:
Weathering gradually breaks rocks into smaller particles. These particles contribute to the formation of soil, which supports plant growth.