Class 7 Science - PUNJAB
Changes Around Us: Physical and Chemical
The Class 7 Science chapter 'Changes Around Us: Physical and Chemical' explores how various substances undergo transformations in our daily lives. Students learn to distinguish between reversible and irreversible changes, and specifically classify them into physical changes, where no new substances are formed, and chemical changes, where new substances with different properties are created. Topics like crystallization, rusting of iron, and magnesium ribbon burning are covered in detail. This chapter is very important for Punjab (PSEB) board exams as it forms the fundamental base for higher chemistry concepts, frequently appearing in both objective and short-answer 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 usually reversible.
Chemical Change
A change in which one or more new substances with entirely different chemical properties are formed. These changes are generally permanent and irreversible.
Rusting of Iron
A chemical process where iron reacts with oxygen and moisture present in the air to form a brown flaky layer of hydrated iron oxide, commonly known as rust.
Galvanization
A method of protecting iron and steel from rusting by coating them with a thin layer of a more reactive metal like zinc.
Crystallization
A physical process used to obtain pure crystals of a dissolved solid from its saturated solution by cooling, such as obtaining sugar or salt crystals.
Important Formulas
Board Exam Info
In the Punjab (PSEB) Class 7 Science examination, this chapter typically carries around 4 to 6 marks. Questions usually include objective type fill-in-the-blanks, true/false, differentiating between physical and chemical changes, and short-reasoning questions such as why rusting happens or why cutting wood is a physical change.
Frequently Asked Questions
Is melting of wax a physical or chemical change?
Melting of wax is primarily a physical change because only the state of the wax changes from solid to liquid, and it solidifies back on cooling. However, burning of the candle wick is a chemical change.
Why does lime water turn milky when carbon dioxide gas is passed through it?
Carbon dioxide reacts with lime water [calcium hydroxide] to form a white precipitate of calcium carbonate, which makes the solution appear milky.
How can we prevent iron from rusting?
Iron can be prevented from rusting by painting, greasing, oiling, or by applying a coat of zinc metal through a process called galvanization.
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