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Explain the terms:
(a) Anodising
(b) Aqua regia.


(a) Anodising is a process of forming a thick oxide layer of aluminium. During anodising, a clean aluminium article is made the anode and is electrolysed with dilute sulphuric acid. The oxygen liberated at the anode reacts with aluminium to produce a thick protective oxide layer on its surface. Also the oxide layer can be dyed easily to give an attractive finish to the aluminium articles.

(b) Aqua regia is a freshly prepared mixture of concentrated hydrochloric acid and concentrated nitric acid in the ratio 3: 1. It is also called royal water. It is highly corrosive and faming liquid. It can dissolve noble metals like gold and platinum.

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A solution of copper sulphate was kept in an iron pot. After a few days, the iron pot was found to have a number of holes in it. Write the equation of the reaction that took place. Explain this reaction in terms of reactivity.

Iron is more reactive than copper and so iron displaces copper from copper sulphate solution. Since iron is being consumed, holes appear in the pot. On the other hand, Cu2+ ions in solution change into copper metal.


Iron is more reactive than copper and so iron displaces copper from c
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Why should we not throw small sodium pieces into a sink in the laboratory?

Sodium is a very reactive metal. Hence it reacts with water to form hydrogen gas and large aount of heat. This makes hydrogen gas to catch fire. Hence explosion could result. Thus to prevent explosion sodium piece should not be thrown in sink.

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In nature aluminium is found in the form of its compounds while gold is found in free state. Give reasons.

Aluminium finds place higher up in the activity series and is thus quite reactive and is found as compounds in nature while gold is just at the bottom in the activity series and is thus least reactive. It does not react with other metal thus found in free state in nature.


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How would you show that silver is chemically less reactive than copper?

Take copper sulphate solution in a test tube and place a silver few granule in it. After some time you observe that there is no change in the test tube, i.e., blue colour of copper sulphate does not fade away. This shows silver is less reactive than copper.

Ag(s) + CuSO4(ag) → No reaction

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