Carbon dioxide gas and sulphur dioxide gas both turn lime water milky. how will you distinguish between the two
Answers
How to distinguish between Carbon dioxide and Sulphur dioxide?
While performing Salt Analysis, the action of heat on a salt with or without an acid will release gases that can be used to identify the anion of the salt. For example, carbonate salts will release CO2CO2 and bisulphite salts release SO2SO2. These gases are then identified with appropriate tests.
CO2CO2 when passed through lime water makes it turbid (milky) and if the passage of CO2CO2 is prolonged the turbidity disappears and the solution becomes clear again.
CO2+Ca(OH)2→CaCO3↓+H2OCO2+Ca(OH)2→CaCO3↓+H2O
CaCO3↓+CO2→Ca(HCO3)2CaCO3↓+CO2→Ca(HCO3)2
CaCO3CaCO3 is a precipitate and is responsible for the turbidity of the solution.
Ca(HCO3)2Ca(HCO3)2 is soluble in water and is responsible in making the solution clear again.
SO2SO2 when passed through lime water makes it turbid (milky) and if the passage of SO2SO2 is prolonged the turbidity disappears and the solution becomes clear again.
SO2+Ca(OH)2→CaSO3↓+H2OSO2+Ca(OH)2→CaSO3↓+H2O
CaSO3↓+SO2→Ca(HSO3)2CaSO3↓+SO2→Ca(HSO3)2
CaSO3CaSO3 is a precipitate and is responsible for the turbidity of the solution.
Ca(HSO3)2Ca(HSO3)2 is soluble in water and is responsible in making the solution clear again.
Visually, the same visual thing is observed when CO2CO2 and SO2SO2 is passed through lime water. What is the test to distinguish between the two, then?
Here is the reaction to distinguish between them. The gas is passed into to an aqueous solution of acidified Potassium Dichromate. If the colour of the solution (which is originally orange) turns to green, then the gas is SO2SO2.
Chromium in its +6 oxidation state and in the dichromate (Cr2O2−7Cr2O72−) form is orange in colour and in its +3 oxidation state is green in colour. The change in colour in the above reaction shows that Cr2O2−7Cr2O72− ion is reduced to the Cr3+Cr3+ionic state. This also means that the compound that reacted with Cr2O2−7Cr2O72− ion should have been oxidised.
Out of CO2CO2 and SO2SO2, only SO2SO2 can be oxidized as in CO2CO2, Carbon is already in its highest oxidation state (+4). The reaction between SO2SO2 and acidified Cr2O2−7Cr2O72− ion is given below.
Cr2O2−7+3SO2+2H+→2Cr3++3SO2−4+H2OCr2O72−+3SO2+2H+→2Cr3++3SO42−+H2O
What do you think will the colour change be if acidified KMnO4KMnO4 is used instead of acidified K2Cr2O2−7K2Cr2O72−?
Hint: Mn7+Mn7+ is Purple in colour, Mn4+Mn4+ is buff coloured, Mn2+Mn2+ is pale pink coloued and Mn6+Mn6+ is green coloured.