Q97 of 400 Page 195

When an object made of metal A is kept in air for a considerable time, it loses its shine and becomes almost black due to the formation of a layer of substance B. When an object made of another metal C is kept in damp air for a considerable time, it gets covered with a green layer of substance D. Metal A is the best conductor of electricity whereas metal C is the next best conductor of electricity.

(a) What is metal A?


(b) What is metal C?


(c) Name the substance B.


(d) Name the substance D.


(e) What type of chemical can be used to remove the green layer from metal C and clean it? Why?

(a) Metal A is silver. Because when silver is kept in air for a considerable time, it loses its shine and becomes almost black due to the formation of a layer of silver sulphide.

(b) The name of the metal C is copper.


(c) The name of the substance is silver sulphide.


(d) The name of the substance D is basic copper carbonate.


(e) Dilute acidic solution can be used to remove the green layer from metal C and clean it because the acid solution dissolves green coloured basic copper carbonate present on the corroded copper object makes it look shiny, red brown again.



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95 In an electrolytic tank, aluminium metal is being extracted by the electrolysis of molten aluminium oxide using carbon electrodes. It is observed that one of the carbon electrodes is gradually burnt away and has to be replaced.

(a) Which carbon electrode (cathode or anode) is burnt away?


(b) Why is this carbon electrode burnt away?

 96 A metal X which is resistant to corrosion is produced by the electrolysis of its molten oxide whereas another metal Y which is also resistant to corrosion is produced by the reduction of its oxide with carbon. Metal X can be used in powder form in thermite welding whereas metal Y is used in making cathodes of ordinary dry cells.

(a) Name the metals X and Y.


(b) Which of the two metals is more reactive: X or Y?


(c) Name one ore or metal X. Also write its chemical formula.


(d) Name one ore of metal Y. Also write its chemical formula.


(e) Name one alloy of metal X and one alloy of metal Y.

 98 Four metals P, Q R and S are all obtained by the reduction of their oxides with carbon. Metal P is used to form a thin layer over the sheets of metal S to prevent its corrosion. Metal Q is used for electroplating tiffin boxes made of metal S whereas metal R is used in making car batteries. Metals Q and R form an alloy called solder. What are metals P, Q R and S? How have you arrived at this conclusion? 99 A black metal oxide X02 is used as a catalyst in the preparation of oxygen gas from potassium chlorate. The oxide XO2 is also used in ordinary dry cells. The metal oxide XO2 cannot be reduced satisfactorily with carbon to form metal X.

(a) Name the metal X.


(b) Name the metal oxide XO2


(c) Which reducing agent can be used to reduce XO2 to obtain metal X?


(d) Name another metal which can also be extracted by the reduction of its oxide with the above reducing agent.