Important Questions for Class 7 Chapter 6 - Physical and Chemical Changes

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Important questions based on NCERT syllabus forChapter 6 - Physical and Chemical Changes:

Question-1: Define Rusting and how can we prevent rusting?

Solution: When a piece of iron is left in open for some time, it acquires a film of brownish powdery substance called rust. The process is known as rusting.
We can prevent rusting by following methods:
(1) By painting or greasing: - Apply a coat of paint or grease regularly on iron articles to prevent from rusting.
(2) By galvanizing:-Deposit a layer of zinc or chromium on iron. The process is known as galvanization.

Question-2: Explain the following:
a) Galvanization
b) Crystallization

Solution: 1) Depositing a layer of zinc on iron to prevent it from rusting is known as galvanization. It prevents direct contact of iron with air and oxygen and thus prevents it from rusting.
2) The process of formation of solid crystals from a homogeneous solution is called crystallization and it is a physical change. Ex: Formation of Copper sulphate crystals.

Question-3: Explain in detail what happens when magnesium oxide is dissolved in water. Support your answer with help of a reaction.

Solution: When magnesium oxide is dissolved in water, magnesium hydroxide is formed. The equation for the
reaction is as follow:
MgO +H2O --> Mg (OH)2