Question

Give reasons for the following:
(i) Metal conduct electricity.
ii) Reaction of nitric acid with metals generally does not evolve hydrogen gas.
(iii) For making gold ornaments, 22 carat gold is preferred to 24 carat gold.
(class 10 CBSE SAMPLE PAPER 2017-18 SCIENCE)

Answer 1

i) Metals conduct electricity because they have free  electrons to move. The free electrons in metals are responsible for the high electrical conductivity .The free electrons  offer little resistance to the flow of current. Silver and copper are the best conductor of electricity.

ii) Reaction of metals with nitric acid does not evolve hydrogen gas. It is because HNO3 is a strong oxidizing agent. It oxidises H2 to produce water and itself gets reduced to any of its oxides such as N2O(dinitrogen monoxide),NO(nitrogen monoxide) or NO2(nitrogen dioxide).

iii) Pure gold is 24 carat and is very soft. Therefore it is not suitable for making jewellery. It is alloyed with Cu or Ag to make it hard. Generally 22 carat gold is used for making ornaments which contains 22 parts of pure gold and 2 parts of either Cu or Ag.

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Answer 2

hey mate

here's your answer

(i) Metals conduct electricity because they have electrons which are free to move. They offer little resistance to the flow of current. Silver and copper are the best conductors of electricity.

(ii) It depends on the metal. If the metal is below hydrogen in the reactivity series, then the oxidising agent is the nitrate ion, and the hydrogen is not released, but combines to form water. An example is the reaction with copper.

3Cu + 8HNO3 → 3Cu(NO3)2 + 2NO + 4H2O

Otherwise, the soluble metal nitrate forms, and hydrogen is released.

2Na + 2HNO3 → 2NaNO3 + H2

(iii) Pure gold is 24 carat and is very soft. Therefore, it is not suitable for making jewellery. It is alloyed with Cu or Ag to make it hard. Generally, 22 carat gold is used for making ornaments, which contains 22 parts pure gold and 2 parts of either Cu or Ag.

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