Physical and Chemical Changes Ncert Notes

Introduction

In our daily lives, we observe many changes around us—some are temporary, while others are permanent. For example, ice melts into water, and paper burns into ash. These changes are classified into two types: physical changes and chemical changes. This chapter explains the differences between these changes, their characteristics, and examples to help us understand how matter behaves.

1. What are Physical Changes?

A physical change is a change in the physical properties of a substance, such as its shape, size, color, or state (solid, liquid, gas), without altering its chemical composition. In simpler terms, the substance remains the same even after the change.

Characteristics of Physical Changes:

• No new substance is formed.
• The change is usually reversible (can be undone).
• Only the physical appearance or state changes, not the chemical nature.
• No significant energy (like heat or light) is produced or absorbed in most cases.

Examples of Physical Changes:

1. Melting of Ice: When ice melts into water, it changes from a solid to a liquid. If you freeze the water again, it turns back into ice. The substance (H₂O) remains the same.
2. Cutting Paper: When you cut a sheet of paper into pieces, the shape and size change, but it is still paper.
3. Dissolving Sugar in Water: Sugar disappears when mixed with water, but you can get it back by evaporating the water. The sugar doesn’t change into a new substance.
4. Stretching a Rubber Band: The rubber band changes shape but returns to its original form when released.

Activity Example:Physical and Chemical Changes

Take a piece of paper and fold it into a boat. Now, unfold it. The paper returns to its original shape. This is a physical change because no new substance is formed.

2. What are Chemical Changes?

A chemical change occurs when a substance undergoes a transformation that results in the formation of one or more new substances with different chemical properties. This happens because the original substance reacts chemically, and its composition changes.

Characteristics of Chemical Changes:

• A new substance is formed.
• The change is usually irreversible (cannot be undone easily).
• The chemical composition of the substance changes.
• Energy changes (heat, light, or sound) are often involved.
• Sometimes, a gas is produced, or a color change occurs.

Examples of Chemical Changes:Physical and Chemical Changes

1. Rusting of Iron: When iron reacts with oxygen and moisture, it forms rust (iron oxide), a new substance. Rust cannot turn back into iron easily.
2. Burning of Paper: When paper burns, it turns into ash, carbon dioxide, and water vapor. The original paper is gone, and new substances are formed.
3. Cooking Food: When you cook rice or bake a cake, the raw materials change into new edible substances with different properties.
4. Souring of Milk: Milk turns into curd due to the action of bacteria, forming lactic acid. This is a chemical change because curd is a new substance.

Activity Example:Physical and Chemical Changes

Burn a small piece of magnesium ribbon (with adult supervision). It produces a bright white light and turns into a white powder (magnesium oxide). This is a chemical change because magnesium oxide is a new substance, different from the original magnesium.

3. Comparing Physical and Chemical Changes

4. Some Common Examples Explained

• Boiling Water: Water turns into steam (gas), but it’s still H₂O. This is a physical change because the substance doesn’t change chemically.
• Burning a Candle: This involves both physical and chemical changes. The wax melts (physical change), but it also burns to produce carbon dioxide and water vapor (chemical change).
• Mixing Baking Soda and Vinegar: When baking soda (sodium bicarbonate) reacts with vinegar (acetic acid), it fizzes and produces carbon dioxide gas, water, and a salt. This is a chemical change because new substances are formed.

5. Rusting of Iron – A Detailed Look

Rusting is an important example of a chemical change. It happens when iron reacts with oxygen (from the air) and water (moisture) to form a reddish-brown substance called rust (iron oxide).

Conditions for Rusting:

• Presence of oxygen.
• Presence of water (moisture).

Prevention of Rusting:

1. Painting: Coating iron with paint prevents air and water from reaching it.
2. Galvanization: Coating iron with zinc protects it from rusting.
3. Oiling or Greasing: Applying oil or grease keeps moisture away.

Activity Example:Physical and Chemical Changes

Take two iron nails. Keep one nail in a dry place and the other in a wet place (like a test tube with water). After a few days, the nail in the wet place will show rust, while the dry one won’t. This shows that moisture is necessary for rusting.

6. Crystallization – A Special Case

Crystallization is the process of forming solid crystals from a solution, like when salt is obtained from seawater by evaporation. It is considered a physical change because:

• No new substance is formed (salt remains salt).
• Only the state changes (from dissolved in water to solid crystals).

Activity Example:Physical and Chemical Changes

Dissolve some salt in water. Heat the solution until the water evaporates. You’ll see salt crystals forming. This is a physical change because the salt’s chemical composition doesn’t change.

7. Importance of Understanding Changes

• In Daily Life: Knowing the difference helps us preserve things (e.g., preventing rust) or use changes to our advantage (e.g., cooking food).
• In Industries: Physical changes like cutting and shaping are used in manufacturing, while chemical changes like burning fuel provide energy.

8. Key Terms to Remember

• Physical Properties: Characteristics like shape, size, or state that can be observed without changing the substance (e.g., color of paper).
• Chemical Properties: Characteristics that describe how a substance reacts with others to form new substances (e.g., iron’s ability to rust).
• Reversible Change: A change that can be undone (e.g., melting ice).
• Irreversible Change: A change that cannot be undone easily (e.g., burning wood).

9. Questions to Test Understanding

1. Is folding a shirt a physical or chemical change? Why?
2. Why does food spoil? Is it a physical or chemical change?
3. How can you prevent an iron gate from rusting?
4. What happens when you mix lemon juice with baking soda? Is it a physical or chemical change?

10. Conclusion

Physical and chemical changes are all around us. Physical changes affect how a substance looks or feels without changing what it is, while chemical changes create entirely new substances. By studying these changes, we learn how materials behave and how we can control or use them in our lives. Whether it’s melting ice or rusting iron, understanding these concepts helps us make sense of the world.

Ch 6 Physical And Chemical Changes