Welcome to Bodhiclasses – your one-stop destination for deep, engaging, and simplified learning across subjects. At Bodhiclasses, we believe that learning science should not only build your knowledge but also spark your curiosity. Today, we explore a fascinating chapter from Class 7 Science that focuses on the various changes around us, and how to identify whether they are physical or chemical.
What Are the Changes Happening Around Us?
Every day, we witness countless transformations — ice melting, milk turning to curd, or iron rusting. Some of these changes are temporary and reversible, while others are permanent and irreversible. This chapter helps students understand these changes and categorizes them into physical and chemical types.
Physical Changes Around Us – Only Appearance Alters
A physical change affects the form or appearance of a substance without changing its composition. These changes include:
- Melting of ice
- Boiling of water
- Chopping vegetables
- Folding paper or inflating a balloon
In such cases, no new substance is formed. These changes are often reversible, such as freezing water again to form ice.
Chemical Changes Around Us – New Substances Are Born
In contrast, chemical changes result in the formation of one or more new substances. These changes are generally irreversible. Key examples include:
- Rusting of iron
- Burning wood or magnesium
- Mixing vinegar with baking soda (releases CO₂)
- Curdling of milk
- Ripening of fruits
These involve a chemical reaction, often producing heat, light, gas, or precipitate. The reaction between carbon dioxide and lime water (turning it milky) is a classic test for a chemical change.
Combustion – A Special Kind of Chemical Change
Combustion is a chemical reaction in which a substance (fuel) reacts with oxygen, releasing heat and/or light. Substances like paper, kerosene, and wood are combustible.
Key concept: For combustion to occur, three elements are essential:
- A combustible substance
- Oxygen (from air)
- Heat to reach ignition temperature
This is called the fire triangle. Activities in the chapter demonstrate how removing any one of these can stop the fire.
Changes Around Us That Are Both Physical and Chemical
Some changes involve both physical and chemical aspects. A classic example is burning a candle:
- Melting of wax – a physical change
- Burning vapour of wax – a chemical change
This dual nature helps students understand the complexity of real-world phenomena.
Reversible vs Irreversible Changes
- Reversible Changes: Can be undone, e.g., melting and freezing water
- Irreversible Changes: Cannot be undone, e.g., cooking food, rusting, burning paper
The chapter encourages students to experiment and observe which changes can or cannot be reversed.
Desirable and Undesirable Changes
- Desirable Changes: Cooking food, ripening fruits, composting, fermentation
- Undesirable Changes: Food spoilage, rusting, pollution from combustion
Interestingly, some changes can be both—for instance, food decomposition is undesirable in storage but desirable in composting.
Slow Natural Changes – Weathering and Erosion
Over long periods, rocks break down due to physical and chemical forces like:
- Temperature fluctuations
- Root growth
- Water and wind erosion
This leads to weathering (formation of soil) and erosion (movement of sediments). These are slow yet powerful changes that shape the Earth.
Final Thoughts – Observing and Understanding Our World
This chapter beautifully integrates science with everyday life, helping students become more observant and analytical. From a melting ice cube to the formation of rust, students learn to classify and understand the science behind it.
