This on treatment with cobalt nitrate and on subsequent heating forms a double salt of the formula MgO.CoO, which is pink in colour. This combines with the metallic-oxides present as white residue in the charcoal cavity forming coloured compounds.įor example, when a magnesium salt undergoes charcoal cavity test, a while residue of MgO is left behind. The test is based on the fact that cobalt nitrate decomposes on heating to give cobalt oxide, CoO. This test is based on the fact that metallic carbonates when heated in a charcoal cavity decomposes to give corresponding oxides. This test is applied to those salts that leave white residue in the charcoal cavity test. In certain cases, the oxides formed partially undergo reduction to the metallic state producing metallic beads or scales. The oxides appear as coloured incrustation or residue in the cavity. Characteristic smell helps to identify some ions like ammonium ion. The test is not much reliable, but certainly helpful in identifying some coloured cations. The physical examination of the unknown salt involves the study of colour, smell and density. (a) Physical Appearance: Colour and Smell Some preliminary tests needs to be done before doing the analysis of cations. Qualitative Analysis of Cations Preliminary Tests Let us discuss about the Qualitative Analysis of Cations. There are separate procedures for detecting cations and anions, called the Cation Analysis and Anion Analysis. Testing with various reagents gives characteristic reaction of certain ions, which may be a colour change, a solid formation or any other visible changes. The common procedure for testing any unknown sample is to make its solution and test this solution with various reagents for the ions present in it. It is mainly concerned with the detection of ions in an aqueous solution of the salt. Qualitative analysis is a method of Analytical chemistry that deals with the determination of elemental composition of inorganic salts. For example, when you obtain silver chloride precipitate by treating the unknown with silver nitrate, you can say that the unknown contains chloride ions.Our objective is to determine the cation present in a given salt. In qualitative analysis of inorganic ions, the formation of a particular precipitate usually indicates the presence of a particular ion. Note: A precipitate is obtained when the ionic product exceeds the solubility product. When you add silver nitrate reagent to a solution containing sodium nitrate, you will NOT obtain any precipitate of silver chloride. When you add silver nitrate reagent to a solution containing sodium chloride, you will obtain a white precipitate of silver chloride. (ii) To distinguish between sodium chloride solution and Sodium nitrate solution, you can use a silver nitrate test. This white precipitate does not dissolve in excess ammonium hydroxide. Lead nitrate reacts with ammonium hydroxide solution to form white precipitate. This white precipitate dissolves in excess ammonium hydroxide. Zinc nitrate reacts with ammonium hydroxide solution to form white precipitate. If the reagent forms precipitate with both compounds, then one of the precipitate should be soluble in the excess of the reagent, whereas other precipitate should not dissolve in the excess of the reagent.Ĭomplete Step by step answer: (i) To distinguish between Lead nitrate solution and Zinc nitrate solution, you can add excess ammonium hydroxide solution. Hint: To distinguish between a pair of compounds, find out a suitable reagent that forms a precipitate with one of the compounds, but the same reagent should not form a precipitate with another compound.
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