Question

A shiny-brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’.
(A).Sodium
(B).Copper
(c).Calcium
(D).Aluminium

Answer 1

Hint: When we use the word ‘shiny’ we are usually describing a metal and not all metals are of brown colour. This metal when kept in the atmosphere, it slowly reacts with atmospheric oxygen to form a layer of this black coloured compound which protects the metal present underneath from further oxidation or corrosion . So now think for yourself.

Complete answer:
-The shiny elements are the metals.
 So, in the 4 options we can see that only copper and aluminium are metals. Sodium and calcium being non-metals can be ruled out.
-The question also says that this shiny compound is brown coloured. We know that copper is brown coloured and aluminium is silvery white in colour.
-Also when copper is heated in air it reacts with atmospheric oxygen to form black coloured compound known as copper oxide (CuO).
This reaction can be written as:
                                              $2C{u_{(s)}} + {O_2}_{(g)} \to 2Cu{O_{(S)}}$
This is an oxidation reaction of copper where oxygen acts as an oxidizer and a reactant.
-This layer of CuO that forms on the surface of Cu metal protects the underlying metal from any further corrosion.
-Copper is a soft, malleable and ductile metal. It also has electrical and thermal conductivity which makes it highly useful in making electrical wires. It is also used as a constituent for various alloys, for making jewellery and utensils.
-Copper is also an important dietary mineral since it is a key component of cytochrome c oxidase which is a respiratory enzyme complex.

So, the correct option is: (B) Copper.

Note: When we say heating in air we mean heating that element in the presence of oxygen (aerobic condition). Cu when kept in the atmosphere does not react with water due to its low reactivity but slowly reacts with the atmospheric oxygen. Also the best way to solve such a question is by ruling out the wrong options according to the facts given in the question itself.