Chapter 2: Is Matter Around Us Pure ; class 9th science

 Question 1:

A diamond knife is quite often used for cutting glass. Why?
Answer :Diamond is probably the hardest substance known. Therefore, a knife made from a special type of diamond is used for cutting the glass.

Question 2:
Explain how does soap help in cleaning dirty clothes?
Answer :In dirty clothes, the dust particles are present on oil are present on oil drops sticking to them. Simple water cannot remove these oil drops from the clothes because water and oil as such do not form a stable emulsion. Soap plays the role of emulsifier and helps in forming a stable emulsion between the two. This means that soap helps in removing these oil drops along with the dirt sticking to them. The dirty clothes get washed by soap solution.
 
Question3:
How will you justify that rusting of iron is a chemical change?
Answer : The rust is a brown chemical compound known as hydrated ferric oxide. It cannot be removed from the surface of the metal b any means. Formula of a rust shows that iron has undergone a chemical change.

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