05. Physical and Chemical Changes Class 7 Science NCERT Notes for Chapter 5 CBSE
Welcome to Physical and Chemical Changes Class 7 Science NCERT Notes for Chapter 5 CBSE
Chapter 5: Physical and Chemical Changes – Study Notes
In this chapter, we will delve deeper into the concepts of physical and chemical changes, which are fundamental in understanding how substances transform in our daily experiences. These comprehensive study notes aim to provide you with a clear understanding of the key concepts explored in this chapter.
Introduction
- Changes in Surroundings: Our daily lives are filled with changes happening around us, often involving various substances. For instance, the process of dissolving sugar in water or curdling milk to make curd are examples of changes that we observe.
- Two Types of Changes: These changes can be broadly categorized into two types: physical changes and chemical changes.
Physical Changes
Activity 5.1: Cutting Paper
- Take a piece of paper and cut it into four square pieces.
- Further, cut each square piece into four smaller squares.
- Arrange these pieces to resemble the original paper.
Physical Change Characteristics:
- Definition: A physical change is a change in which a substance undergoes a transformation in its physical properties, such as size, shape, or state.
- Reversibility: Physical changes are generally reversible, meaning you can usually return the substance to its original state.
- No New Substance: Importantly, no new substances are formed during physical changes. It’s like reshaping or rearranging the same pieces of the puzzle.
Activity 5.2: Chalk Dust and Water
- Collect chalk dust or crush a small piece of chalk into dust.
- Add a little water to the dust to make a paste and shape it into a piece of chalk.
- Allow it to dry.
Question: Did you recover chalk from the dust?
Activity 5.3: Melting Ice
- Take some ice in a glass or plastic tumbler.
- Melt a small portion of ice by placing the tumbler in the sun.
- Then, place the tumbler in a freezing mixture (ice plus common salt).
Question: Does the water become solid ice once again?
Properties of Physical Changes
- Physical changes mainly involve alterations in properties such as shape, size, color, and state of a substance while maintaining its chemical composition.
Chemical Changes
Activity 5.6: Burning Magnesium
- Acquire a small piece of magnesium ribbon.
- Clean its tip with sandpaper.
- Bring the tip near a candle flame.
Question: Does the color of the tip of the blade change?
Chemical Change Characteristics:
- Definition: A chemical change is a change in which one or more entirely new substances are formed through a chemical reaction.
- Irreversible: Unlike physical changes, chemical changes are generally irreversible, meaning you cannot revert the substances to their original form.
- New Substances: The hallmark of a chemical change is the creation of new substances that are distinct from the initial reactants.
Activity 5.7: Copper Sulphate and Iron
- Dissolve copper sulphate in water.
- Add a few drops of dilute sulphuric acid to the solution.
- Submerge a nail or a used shaving blade into the solution.
Question: Observe the color changes in the solution and the nail/blade over time.
Activity 5.8: Vinegar and Baking Soda
- Take vinegar in a test tube and add baking soda.
- Observe the reactions.
Question: What happens to the lime water? Is there any change in the nail/blade?
Conclusion
- Chemical changes play a significant role in our lives as they lead to the formation of entirely new substances, often with unique properties.
- In addition to the creation of new products, chemical changes may result in the release or absorption of heat, light, sound, changes in smell, alterations in color, and even the formation of gases.
Keywords and Definitions
- Physical Change: A physical change is a transformation in which a substance undergoes a change in its physical properties, such as size, shape, or state. These changes are generally reversible, and they do not lead to the creation of new substances.
- Chemical Change: A chemical change is a transformation in which one or more entirely new substances are formed through a chemical reaction. Chemical changes are typically irreversible, and they involve the creation of new substances with distinct properties.
- Reversibility: This term refers to the property of a change, indicating whether it can be undone or reversed.
- New Substance: A substance that is formed as a result of a chemical change, which is distinct from the initial substances involved.
- Properties: Characteristics or attributes that describe the nature or behavior of a substance.
- Irreversible: A change that cannot be undone or reversed.
- Dissolving: The process of a substance mixing uniformly with a solvent to form a solution.
Related Concepts
Difference between physical and chemical change:
| aspect | Physical Change | Chemical Change |
|---|---|---|
| Definition | Alterations in the physical properties of matter without changing its chemical composition. | Results in the formation of new substances with different chemical properties. |
| Chemical Composition | Remains the same. | Changes to form new substances. |
| Reversibility | Often reversible. | Generally irreversible. |
| Example | Melting ice, cutting paper. | Burning wood, digesting food. |
| State of Matter | Can involve changes in state (solid to liquid, etc.). | Involves a change in the chemical identity of matter. |
| Energy Changes | Typically, no significant energy changes. | Often accompanied by energy changes (heat, light, etc.). |
| Observational Indicators | Changes in appearance, state, or size. | Formation of new substances, color change, gas production, energy release. |
Physical changes involve alterations in the physical state or appearance of matter without changing its chemical composition. Examples include melting ice or cutting paper. Chemical changes, on the other hand, result in a new substance with different chemical properties. For instance, when wood burns, it undergoes a chemical change to produce ash and smoke.
Physical and chemical changes:
Physical changes are those in which the substance’s chemical composition remains the same, but its physical properties may change. Chemical changes, however, involve a transformation into a new substance with different chemical properties. An example of a physical change is when water freezes into ice, while a chemical change is when hydrogen and oxygen combine to form water.
Physical change and chemical change: Physical changes are alterations in the form or state of matter without changing its chemical identity. For example, when you crush a can, its shape changes, but it’s still the same material. Chemical changes involve the formation of new substances with different properties. When iron rusts, it undergoes a chemical change, turning into iron oxide.
Physical and chemical changes class 7: In class 7, students learn about the distinction between physical and chemical changes. Physical changes are about alterations in the appearance or state of matter, like cutting paper. Chemical changes involve the creation of new substances with different properties, such as the burning of a piece of paper to produce ash.
Difference between physical change and chemical change:
Physical changes affect the physical properties of a substance without altering its chemical composition. For example, when you melt chocolate, it changes from a solid to a liquid but remains chocolate. Chemical changes involve a transformation into a new substance with different chemical properties. When milk sours, it becomes curdled and acidic.
Physical and chemical changes examples: Examples of physical changes include boiling water, which changes its state from liquid to gas, and crushing a soda can, which alters its shape. Chemical changes are exemplified by the burning of gasoline in a car’s engine, where it transforms into various gases and releases energy.
Example of physical change and chemical change:
A classic example of a physical change is when ice melts into water. The chemical composition remains H2O. In contrast, when iron reacts with oxygen to form rust, it’s a chemical change. Iron combines with oxygen to create iron oxide, altering its chemical structure.
What is physical and chemical change?
Physical changes involve alterations in the physical properties of a substance, like size, shape, or state, without changing its chemical composition. Chemical changes, however, result in the formation of new substances with different chemical properties. For instance, baking a cake involves both physical changes (mixing ingredients) and chemical changes (baking to create a new substance).
Physical change and chemical changes: In the context of class 7, students explore physical and chemical changes. Physical changes involve alterations in the state or appearance of matter, like freezing water into ice. Chemical changes, on the other hand, result in new substances with different properties, such as the digestion of food in the body.
Frequently Asked Questions (FAQs)
Q1: What are physical changes, and how can you identify them?
A1: Physical changes are transformations in a substance’s physical properties, such as size, shape, or state, without the formation of new substances. They are often reversible, and no new substances are created during these changes. You can identify physical changes by observing alterations in the substance’s appearance or state that can be undone.
Q2: Is melting ice a physical or chemical change?
A2: Melting ice is a physical change. It involves a change in the state of water from solid (ice) to liquid (water) without forming new substances.
Q3: What defines a chemical change, and can you provide examples?
A3: A chemical change is a transformation in which one or more entirely new substances are formed through a chemical reaction. Examples include burning magnesium, rusting of iron, and the reaction between vinegar and baking soda.
Q4: Are chemical changes reversible?
A4: Chemical changes are generally irreversible. Once new substances are formed through a chemical reaction, it is challenging or impossible to return the substances to their original state.
Q5: Why is it important to distinguish between physical and chemical changes?
A5: Distinguishing between physical and chemical changes helps us understand how substances behave and transform. It is crucial in various fields, including chemistry, cooking, and manufacturing, as it influences the processes and reactions we use to create or modify substances.
Q6: How can I prevent rusting of iron objects?
A6: You can prevent rusting of iron objects by applying protective coatings such as paint or grease, which act as barriers to oxygen and water. Another method is galvanization, where a layer of a protective metal like zinc is deposited on the iron surface.
Q7: Can you explain the process of crystallization mentioned in the chapter?
A7: Crystallization is a process in which large crystals of a pure substance are formed from a solution. It involves dissolving a substance in a solvent and allowing it to slowly cool or evaporate, leading to the formation of well-defined crystal structures.
Q8: What are some real-life applications of chemical changes?
A8: Chemical changes are essential in various aspects of daily life. Examples include digestion of food in our bodies, ripening of fruits, fermentation of grapes to make wine, and the production of medicines and useful materials like plastics through chemical reactions.
Q9: How do chemical changes differ from physical changes in terms of properties and reversibility?
A9: Chemical changes involve the formation of new substances and often result in changes in properties such as color, odor, and the release or absorption of energy. They are generally irreversible. Physical changes, on the other hand, involve alterations in physical properties and are typically reversible.
Q10: Can you provide more examples of chemical changes in our surroundings?
A10: Certainly! Additional examples of chemical changes include the rusting of iron objects, the combustion of fuels like wood or gasoline, the baking of bread, the souring of milk, and the process of photosynthesis in plants.
This was the end of Physical and Chemical Changes Class 7 Science NCERT Notes for Chapter 5 CBSE
For more information, refer to NCERT SOLUTIONS, NCERT NOTES, and NCERT BOOKS .
