Class 7 Science - CBSE
Changes Around Us: Physical and Chemical
The Class 7 Science chapter 'Changes Around Us: Physical and Chemical' explores how various substances undergo transformations daily. Students learn to classify these changes into two main categories: physical changes, where no new substances are formed and properties like shape or state change temporarily, and chemical changes, where new substances with entirely different properties are created permanently. Understanding this distinction is crucial for CBSE board exams, as it forms the foundation of chemistry and helps students explain everyday phenomena like rusting, digestion, and burning, frequently appearing in conceptual and application-based questions.
Start Learning FreeKey Concepts
Physical Change
A change in which only the physical properties of a substance, such as shape, size, color, or state, change, and no new substance is formed. These changes are mostly reversible.
Chemical Change
A change in which one or more new substances with different properties are formed. These changes are usually permanent and irreversible.
Rusting of Iron
A chemical process where iron reacts with oxygen and moisture in the air to form a reddish-brown flaky layer called rust, which weakens iron structures.
Crystallization
A physical process used to obtain pure crystals of a substance from its solution by evaporating the solvent, such as getting pure salt from seawater.
Galvanization
A method of protecting iron from rusting by coating it with a thin layer of a more reactive metal like zinc.
Important Formulas
Board Exam Info
In CBSE Class 7 Science examinations, this chapter typically carries around 6 to 8 marks. Questions usually include direct definitions, differentiating between physical and chemical changes with examples, explaining the conditions necessary for rusting, and identifying chemical changes based on observations like color change or gas evolution.
Frequently Asked Questions
Is melting of wax a physical or chemical change?
Melting of wax is generally considered a physical change because only the state of the wax changes from solid to liquid, and it can solidify again on cooling. However, burning the wick of a candle is a chemical change.
Why is ripening of fruit considered a chemical change?
Ripening of fruit is a chemical change because it involves complex chemical reactions that alter the taste, color, texture, and composition of the fruit, creating entirely new substances that cannot be turned back into the raw fruit.
How does galvanization prevent rusting?
Galvanization prevents rusting by applying a protective layer of zinc metal over the iron surface. Since zinc does not easily corrode, it blocks air and moisture from coming into direct contact with the iron.
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