# Changes Around Us: Physical and Chemical

## Overview

In this chapter, students explore the different types of changes that occur around us, focusing on **physical changes** (where no new substance is formed) and **chemical changes** (where new substances are created). Through daily life examples, activities, and experiments, learners gain a clear understanding of concepts such as combustion, rusting, reversibility of changes, and natural processes like erosion and weathering.

## Key Topics Covered

### 1. Observing Changes Around Us

* Everyday examples include: melting ice, drying wet clothes, boiling water, burning wood, ripening of bananas, and blooming flowers.
* Changes are noticed by observing **size**, **shape**, **smell**, and **other properties** using our senses.

### 2. Grouping Changes

* **Physical Change**: The substance changes in appearance, but remains the same.

* Examples: folding paper, inflating and deflating a balloon, crushing chalk, melting ice.
* No new substance is formed.
* **Chemical Change**: The substance changes both in appearance and nature, resulting in a new substance.

* Examples: blowing air into lime water, mixing vinegar and baking soda.
* Formation of a new substance like carbon dioxide or calcium carbonate proves a chemical reaction has occurred.

### 3. Examples of Chemical Change

* **Rusting**: Formation of iron oxide from iron and oxygen.
* **Combustion**: Burning of magnesium ribbon forms magnesium oxide and releases heat and light.
* **Combustible Substances**: Paper, cotton, wood, kerosene.

### 4. Conditions Needed for Combustion

* Requires three components (Fire Triangle):

1. **Combustible substance** (fuel)
2. **Oxygen**
3. **Heat** (to reach ignition temperature)

### 5. Combined Physical and Chemical Change

* **Burning a Candle**:

* Melting and evaporation of wax: physical change.
* Burning of vapour: chemical change.

### 6. Reversible and Irreversible Changes

* **Reversible**: Ice melting, water boiling and condensing, folding clothes.
* **Irreversible**: Cutting vegetables, making popcorn, rusting.

### 7. Desirable and Undesirable Changes

* **Desirable**: Cooking food, making compost, germination of seeds.
* **Undesirable**: Rusting, rotting food, atmospheric pollution.

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### 8. Natural Changes Over Time

#### A. Weathering of Rocks

* Rocks break down physically due to **temperature variations**, **plant roots**, and **freezing of water** in cracks.
* Chemical weathering changes the rock's composition (e.g., iron in basalt forming red iron oxide).

#### B. Erosion

* Rocks and soil are moved by natural forces like **wind** and **flowing water**.
* Erosion is a **physical change** that smoothens river rocks and forms sediments over time.

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## New Terms and Simple Definitions

| Term | Definition |
| -------------------- | ----------------------------------------------------------------------------- |
| Physical Change | A change where the substance retains its identity; no new substance forms. |
| Chemical Change | A change where one or more new substances are created. |
| Combustion | A chemical reaction involving oxygen, releasing heat and/or light. |
| Ignition Temperature | The lowest temperature at which a substance catches fire. |
| Reversible Change | A change that can be undone to get the original substance back. |
| Irreversible Change | A change that cannot be undone. |
| Rusting | Formation of reddish-brown iron oxide on iron due to reaction with air/water. |
| Weathering | Breaking down of rocks into soil by natural forces over time. |
| Erosion | Movement of soil and rock by water or wind. |
| Chemical Reaction | A process where new substances are formed. |

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## Practice Questions

### Easy (3)

1. **What type of change is melting of ice?**

* Answer: Physical change.
* Explanation: Only the state changes from solid to liquid; no new substance forms.

2. **Name any one chemical change observed in the kitchen.**

* Answer: Cooking food.
* Explanation: It involves the formation of new substances with different taste and smell.

3. **What do you need for combustion to take place?**

* Answer: Fuel, oxygen, and heat.
* Explanation: These three make up the fire triangle required for combustion.

### Medium (2)

4. **Is burning of a candle a physical or chemical change? Explain.**

* Answer: Both.
* Explanation: Wax melting is physical; wax vapor burning is chemical.

5. **What is rusting and why is it considered a chemical change?**

* Answer: Formation of iron oxide on iron.
* Explanation: A new substance (rust) is formed through a chemical reaction with oxygen and moisture.

### Difficult (3)

6. **How can we prove that the gas formed during vinegar and baking soda reaction is carbon dioxide?**

* Answer: Pass the gas through lime water.
* Explanation: Lime water turns milky if carbon dioxide is present.

7. **Why is weathering of rocks a combination of physical and chemical change?**

* Answer: Physical processes break rocks; chemical reactions alter their composition.
* Explanation: Both types of changes contribute to formation of soil.

8. **Explain the role of ignition temperature in starting combustion.**

* Answer: It's the minimum temperature needed to start burning.
* Explanation: Without reaching it, a combustible substance won't ignite even in the presence of oxygen.

### Very Difficult (2)

9. **Can we consider the process of composting a banana peel as both desirable and chemical? Why?**

* Answer: Yes.
* Explanation: It results in useful compost (desirable) and involves decomposition (chemical change).

10. **Is the colour change in a chameleon a physical or chemical change? Justify.**

* Answer: Physical change.
* Explanation: No new substance forms; it's a change in appearance due to light reflection.

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