Class 7 Science - KARNATAKA
Changes Around Us: Physical and Chemical
In this chapter, Class 7 Karnataka (KSEEB) students explore the everyday changes happening around them, classifying them into two main categories: physical and chemical changes. You will learn that physical changes are temporary and do not form new substances, like melting ice or dissolving salt in water. In contrast, chemical changes result in the formation of entirely new substances with different properties, such as rusting of iron, burning of paper, or the setting of curd. Understanding these distinctions is crucial for board exams as it helps build your foundation in chemistry and explains real-world phenomena.
Start Learning FreeKey Concepts
Physical Change
A change in which only the physical properties of a substance like shape, size, color, or state change, without forming any new substance. These changes are mostly reversible.
Chemical Change
A change in which one or more new substances with different chemical properties are formed. These changes are usually permanent and irreversible.
Rusting of Iron
A chemical change where iron reacts with oxygen and moisture present in the air to form a reddish-brown coating called rust (Iron oxide).
Crystallization
A physical process used to obtain pure, large crystals of a substance from its saturated solution, commonly seen in obtaining pure salt from seawater.
Galvanization
A method of preventing the rusting of iron by depositing a thin layer of a non-rusting metal, like zinc, on top of it.
Important Formulas
Board Exam Info
In the Karnataka (KSEEB) Class 7 Science examinations, this chapter typically carries around 6 to 8 marks. Common question types include direct definitions, differentiating between physical and chemical changes with examples, giving reasons for specific reactions (like why rusting happens or why burning is a chemical change), and identifying chemical word equations.
Frequently Asked Questions
Is melting of wax a physical or chemical change?
Melting of wax is generally considered a physical change because the molten wax can solidify back into wax again, and no new substance is formed. However, burning the wick is a chemical change.
Why is photosynthesis considered a chemical change?
Photosynthesis is a chemical change because plants use carbon dioxide and water in the presence of sunlight to produce completely new substances: glucose (food) and oxygen.
How can we prevent iron from rusting?
Iron can be prevented from rusting by painting, greasing, oiling, or applying a layer of zinc (galvanization) to keep it away from oxygen and moisture.
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