(c) Aluminium is a highly reactive metal, yet it is used to make utensils for cooking. Why?
Answer: Aluminium is a highly reactive metal and hence it easily reacts with the oxygen in air. This reaction results in the formation of a layer of aluminium oxide over the metal. The layer of aluminium oxide prevents further reaction of aluminium with oxygen. In other words, once a layer of aluminium oxide is formed, the aluminium utensil becomes corrosion resistant. This makes aluminium a corrosion resistant material, making it ideal for long term use. Moreover, aluminium is a good conductor of heat. Due to these reasons, aluminium is used to make cooking utensils. So, corrosion resistance and high conductivity make aluminium a good raw material for making cooking utensils.
(d) Carbonate and sulphide ores are usually converted into oxides during the process of extraction. Why?
Answer: Obtaining a metal from its oxide is much easier than doing that from a metal sulphide or carbonate. Due to this, sulphide and carbonate ores are first converted to metal oxides during metallurgy.
Question 13: You must have seen tarnished copper vessels being cleaned with lemon or tamarind juice. Explain why these sour substances are effective in cleaning the vessels.
Answer: When a copper article is left in open for a long time, it reacts with gases in air. This results in formation of copper carbonate or copper oxide over the copper article. These compounds of copper impart a dark greenish colour to the article. That is why a copper vessel becomes tarnished. Acids react with carbonates and oxides. When an acid reacts with carbonate then carbon dioxide and water are formed and metal salt if formed. When metal oxide reacts with an acid then neutralisation reaction takes place and salt is formed. These reactions helps in cleaning the crust formed on metal article. Thus, rubbing lemon juice or tamarind juice helps in cleaning the copper vessel.
Question 14: Differentiate between metal and non-metal on the basis of their chemical properties.
Answer: Difference between metals and non-metals on the basis of chemical properties is as follows:
|Metals form basic oxide when they react with oxygen.||Non-metals form acidic oxide when they react with oxygen.|
|Some metals displace hydrogen from water.||Non-metals do not react with water.|
|Some metals displace hydrogen from dilute acids.||Non-metals do not react with dilute acids.|
|Metals form ionic compounds.||Non-metals form covalent compounds.|
Question 15: A man went door to door posing as a goldsmith. He promised to bring back the glitter of old and dull gold ornaments. An unsuspecting lady gave a set of gold bangles to him which he dipped in a particular solution. The bangles sparkled like new but their weight was reduced drastically. The lady was upset but after a futile argument the man beat a hasty retreat. Can you play the detective to find out the nature of the solution he had used?
Answer: We know that goldsmiths use aqua-regia to clean gold ornaments. Gold does not react with acids at any temperature. But gold reacts with aqua-regia, which is a mixture of hydrochloric acid and nitric acid in three to one ratio. While cleaning a gold ornament with aqua-regia, some of the gold gets dissolved in it. This results in some weight loss of the gold ornament. That is what happened in the given case.
Question 16: Give reasons why copper is used to make hot water tanks and not steel (an alloy of iron).
Answer: Iron reacts with hot water but copper does not. If hot water tank is made of steel, there will be formation of rust and subsequent corrosion of steel tank in the long run. There is no such problem with copper because copper does not react with water at any temperature. Moreover, copper is a very good conductor of heat. Due to these reasons; copper (rather than steel) is used to make hot water tanks.
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