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.