Suppose the given mixture contains calcium carbonate and sand. Both are insoluble in water. But calcium carbonate is, however, soluble in dil acids. Hence the mixture is first treated with dil HCI, when calcium carbonate gradually dissolves to form calcium chloride. Subsequently
calcium carbonate is recovered back by treating the filtrate with Na₂CO3 or (NH4)2 CO3 solution.
a) CaCO3 + 2 HCl———> CaCl₂ + CO₂ + H₂O
b) CaCl₂ + Na₂CO3———-> CaCO3 + 2 NaCl
3) Filter stand
4) Filter paper
5) Wash bottle
6) Air oven.
Mixture of sand and CaCO3
i) Take about 2 gms of the given mixture of sand and calcium carbonate in a beaker, and add dilute HCl little by little at a time till the effervescence ceases.
ii) Warm the solution, and filter off the liquid.
iii) Wash the sand in the beaker two or three times with water, and filter off the clear liquid.
iv) Transfer the sand on to the filter paper, and wash it with hot water a few times till it is free from chloride [test with AgNO3 Solution]
v) When the sand is completely free from chloride, remove the filter paper along with the residue sand and dry it in the air oven or over the asbestos.
vi) Now, warm the filtrate in the beaker, and add sodium carbonate solution slowly when CaCO3 will come down as white precipitate. Test for the completeness of precipitation as usual.
vii) Filter off the clear supernatant liquid by the method of washing by decantation, and finally transfer the CaCO3 precipitate on to the filter paper.
viii) Wash the ppt. a number of times with cold water, till it is free from soluble carbonate (Test with BaCl2 solution).
ix) Dry the CaCO3 ppt. along with the filter paper in the air oven or over the asbestos.
Air as A Mechanical Mixture
The air around us is a mixture of several gasses consisting mainly of nitrogen, oxygen, varying amounts of water vapour, carbon dioxide and little amounts of inert gasses like helium, neon, argon etc. The major constituents of air can be separated by some easy mechanical means showing thereby that air is a mechanical mixture of gasses not a chemical compound.