Question

Aluminium, deposited as vapour on a glass, forms good mirrors, essentially because:
A. Coating is much smoother
B. Does not scratch
C. It has better shine than silver
D. It does not tarnish in air

Answer 1

Hint: Aluminium on contact with air undergoes oxidation. This forms an oxide layer on the surface of aluminium which helps protect the rest of the layers of aluminium. This is a characteristic feature of aluminium that can be utilised to make mirrors.

Complete step by step answer:
Aluminium belongs to the p block. It is part of group $13$ . It does not react with air or water. It can be exposed to air and it will not tarnish. This happens because on exposure to air it forms a layer of oxide that protects it from the attack from air. This property also prevents aluminium from getting rusted. The superficial layer of aluminium reacts with oxygen to form aluminium oxide. This oxide layer is made of two atoms of aluminium and three atoms of oxygen and has the molecular formula of: $A{l_2}{O_3}$ .
Therefore the answer to the above question is option D. this means that aluminium which is deposited on the surface of a glass can form a good mirror as it does not tarnish in air by undergoing oxidation.

So, the correct answer is Option D.

Additional information:
Aluminium also does not react with water. This happens because the aluminium oxide is formed. Therefore, any water molecule that may come in contact with the metal causes the oxide to get hydrated and causes it to change its geometry such that the distance between the bonds will now be almost double the initial bond length of the atoms. Thus, this change in the geometry of the atoms causes the lower layers of the metal to become chemically inert to water. In this way, aluminium does not undergo any chemical change on contact with water.

Note: aluminium is chemically inert to both water and air as the outermost layer of aluminium forms an oxide. Due to the presence of an oxide layer the metal does not undergo corrosion because it creates a layer of protection from the water molecules present in the air that could contribute to the rust. The tarnish of a mirror can reduce the visibility of the mirror as it will lead to black or darker spots forming in different regions of the mirror.