Q. 65.0( 1 Vote )

i. Why potato chips manufacturers fill the packet of chips with nitrogen gas?

ii. A student has been collecting silver coins and copper coins. One day she observed a black coating on silver coins and a green coating on copper coins. Which chemical phenomenon is responsible for these coatings? Write the chemical name of black and green coatings.

OR

i. Crystals of copper sulphate are heated in a test for some time.

(a) What is the colour of copper sulphate crystals before heating, and after heating?

(b) What is the source of liquid droplets seen on the inner upper side of the test tube during the heating process?

ii. A metal ‘X’ when dipped in an aqueous solution of aluminium sulphate no reaction is observed whereas when it is dipped in an aqueous solution of ferrous sulphate, the pale green solution turns colourless. Identify metal ‘X’ with reason.

Answer :

(i) Nitrogen being an inert gas prevents the oxidation of the fats and oils. Potato chips bags are filled with nitrogen gas to prevent the chips from oxidizing, and thus, from getting stale. Oxygen is very reactive, and easily combines with other molecules whenever it can, causing chemical changes to take place.


(ii) When silver and copper coins are left in the presence of air, it starts to develop a coating on them. Silver coins show black coating whereas copper coins show green colouration. The chemical reaction responsible for this color change is known as corrosion.


Corrosion is the gradual destruction of metals by chemical and electrochemical reaction with their environment.


1. Black coating on silver coins -


When silver reacts with hydrogen sulphide gas in the presence of air, then a black coating of silver sulphide is formed which gives black coloration to silver coins.


The reaction followed is shown below:


2Ag + H2S Ag2S + H2


The black coating is of Ag2S or silver sulphide.


2. The green coating on copper coins -


When copper reacts with air a mixture of copper carbonate and copper hydroxide is formed which turns the copper coins into green color after a long time.


The reaction followed is shown below:


2Cu + H2O + CO2 + O2 CuCO3.Cu(OH)2


The green coating is the result of a mixture of CuCO3 or copper carbonate and Cu(OH)2 or copper hydroxide in the ratio of 1:1.


OR


(a) The colour of copper sulphate crystals before heating is blue while the after after heating of the crystals is white. This is because of loss in water molecules from the copper sulphate molecule. Copper sulphate pentahydrate is converted to anhydrous copper sulphate thus turning the colour of the molecule from blue to white.


(b) The source of water droplets on the upper inner side of the test tube while heating of copper sulphate crystals is the water of crystallisation of hydrated copper sulphate crystals (CuS04.5H20). On heating, the water molecules come out leaving behind anhydrous copper sulphate.


(ii) When metal 'X' is dipped in aqueous solution of aluminium sulphate no reaction is observed, which means the metal 'X' is less reactive than aluminium. But when it is dipped in ferrous sulphate solution, the solution turns from pale green to colourless, so 'X' is more reactive than iron and thus displaces it from its solution.


Therefore, according to the metal reactivity series, metal ‘X' must be zinc. It reacts with ferrous sulphate to form colourless zinc sulphate solution by displacing iron.


Zn + FeSO4 ZnSO4 + Fe


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