Question

estions Based on High Order Thinking Skills (HOTS)
44. When a piece of limestone reacts with dilute HCI, a gas X is produced
water then a white precipitate Y is formed. On passing excess of
forming a soluble compound Z.​

Answer 1

TOPIC :- MILKY TEST FOR CO2

CONCEPT :- Co2 gives a white milky precipitate on reaction with calcium hydroxide.On passing excess Co2 ,milkiness disappears due to the formation of a soluble compounder calcium hydrogen carbonate.

------>SOLUTION TO THE QUESTION<-------

• Limestone is CaCO3.

The reaction which occurs on passing dil. HCl is

CaCO2 + dil. HCL = Cacl2 + Co2 + H20 (Not balanced)

So, the gas produced is CO2.

Hence, X = CO2

Ca(OH)2 + CO2 = CaCO3 (Y) (white precipitate) + H2O

Hence, y is CaCO3

Ca(OH)2 + CO2 (excess)= CaHCO3 (Y) () + H2O

Hence, Z is CaHCO3

ADDITIONAL IMPORTANT DETAILS FOR IIT-JEE

SO2 also gives lime water test but the distinction between CO2 and SO2 can be made by KMn04/H+ .

1. CO2 doesn't react with KMn04/H+

2.So2 decolorizes the pink colour of the solution.

Now, the question arises why this happens ?

• The first reaction is not possible because CO2 can't act as reducing agent as carbon is in it's maximum oxidation state.And Since, both are acidic, further acid bas e reaction is not possible.

• The second reaction is possible as sulphur is oxidised to SO4^2-.

Hope helped :)