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7. Explain why burning of wood and cutting it into small pieces are considered as two different types of changes. ~
Answer: Burning of wood is a chemical change because in burning new substances are formed as
Wood Oxygen → Charcoal Carbon dioxide Heat Light
But cutting it into small pieces is physical change because no new substance is formed. We can only reduce the size of wood.
8. Describe how crystals of copper sulphate are prepared.
Answer: Take a cupful of water in a beaker and add a few drops of dilute sulphuric acid. Heat the water. When it starts boiling, add copper sulphate powder slowly. Continue to add copper sulphate powder till no more powder can be dissolved. .During this process continuously stir the solution. Filter the solution. Leave it for cooling. Look it after some time, you can see the crystals of copper sulphate.
9. Explain how painting of an iron gate prevents it from rusting?
Answer: It is known that for rusting the presence of oxygen and moisture is essential. Painting prevents the iron gate from coming in contact with oxygen and moisture.