Question

State reason for the following
i. Silver articles turn black in colour when exposed to atmosphere.
ii. Magnesium floats on water.
Hydrogen gas is not evolved when copper metal reacts with
hydrochloric acid.
Aluminium is a highly reactive metal, yet it is used to make utensils
for cooking
Electric wires are coated with polyvinyl chloride or a rubber like material.​

Answer 1

Answer:

1. Silver reacts with the sulphur in the air. This results in the formation of Silver Sulphide (Ag2S) and it turns black.
2. Magnesium does not react with cold water. It reacts with hot water to form Magnesium Oxide (MgO). Magnesium metal starts floating because the bubbles of hydrogen stick to its surface. The heat evolved is not sufficient.
3. Copper lies below Hydrogen in the Reactivity series. It implies that Copper is less reactive than Hydrogen. Hence it does not displace Hydrogen from HCl and Hydrogen gas is not evolved.
4. At normal temperatures, aluminium is coated with a layer of oxide. This Aluminium oxide makes it resistant to corrosion. The resistance can be improved by making the oxide layer thicker which is known as anodising. This layer can be dyed, giving it an attractive finish. Besides it is a good conductor of heat. Hence it is used in making utensils.
5. Electric wires are made of a good conductor such as copper or aluminium. This conducting nature can cause a shock to the person handling them. Hence they are coated with rubber of polyvinyl chloride (PVC) as they are good insulators.

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Answer 2

i. Silver article turns black when kept in the air because silver reacts with sulphur compounds such as hydrogen sulphide (H2S) present in air and forms silver sulphide ( Ag2S) . The phenomenon is called corrosion. For silver particularly, it is called tarnishing of silver.

ii. Magnesium reacts with hot water. The hydrogen gas evolved doesn't catch fire because the heat evolved is very low . But it forms bubbles and stick to the surface of magnesium and thus as a result magnesium starts floating on water due to the hydrogen gas.

iii.Copper doesn't normally react with hydrochloric acid to produce hydrogen gas because as a metal, copper isn't active enough to displace hydrogen. So hydrogen is not evolved.

iv. This is because aluminium reacts with oxygen present in air to form a thin layer of aluminium oxide. This oxide layer is very stable and prevents further reaction of aluminium with oxygen. Also, it is light in weight and a good conductor of heat. Hence, it is used to make cooking utensils.

v. Electric wires and cables are coated with substance like PVC to prevent from electric shock since PVC is insulator( bad conductor of electricity). and it does not conduct electricity it is therefore on excellent material to use for electrical applications such as insulation sheathing for cables.

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