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NCERT 7th Class (CBSE) Science: Chemical and Chemical Changes

Chemical Reaction & Equations: 10 Science

Question: Which one of the following is a chemical change? Give reason also.
(a) Burning of wax    (b) Melting of wax

Answer: (a) Burning of wax: Because candle wax is a hydrocarbon which burns to produce CO2 and HO2 and it is an irreversible change.

Question: Which one is a chemical change – Fermentation of fruit juice or diluting fruit juice?

Answer: Fermentation of fruit juice.

Question: Which one a chemical change – Rusting of iron or melting of iron?

Answer: Rusting of iron.

Question: State one basic difference between a physical change and chemical change.

Answer: In a physical change, no new substance is formed while in a chemical change, a new substance is formed.

Question: A magnesium ribbon should be cleaned before burning it in air. Why?

Answer: It should be cleaned to remove the dust particles and its oxides that may be present on its upper surface, so that it comes directly in contact with air.

Question: Name the oxidizing and reducing agent in the following reaction:
CuO + H2 → Cu + H2O

Answer: Oxidizing agent is CuO
Reducing agent is H2

Question: On adding dilute hydrochloric acid to copper oxide powder, the solution formed is blue-green. Predict the new compound formed which imparts a blue – green colour to the solution.

Answer: The new compound formed is copper (II) chloride (CuCl2) which imparts a blue-green colour to the solution.

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