Chemistry Qualitative Analysis What is qualitative analysis Qualitative analysis is used to determine the chemical composition of an unknown substance. Below is a list of some simple tests used to determine the presence of numerous anions and cations using the bench reagents present in the laboratory. Test for carbonates and hydrogen carbonates Carbonates react with dilute acid with effervesce, liberating carbon dioxide. Test for carbon dioxide- carbon dioxide outs a lit wooden splint. Carbonates are present when a dilute acid is added to an unknown substance and carbon dioxide is liberated. Test for the chloride and bromide ions chloride and bromide ions react with aqueous lead nitrate to form a white precipitate (ppt.) This ppt. dissolves when heated and solidifies when cooled. Conformation of chloride ions (done after test for halides) Chlorides react with silver nitrate solution to form a white ppt. This ppt. then dissolves in a solution of aqueous ammonium Test for the metal ions lead, zinc and aluminum Lead, zinc and aluminum ions react with aqueous sodium hydroxide to form a white ppt. White ppt. is soluble in excess sodium hydroxide. Conformation of zinc ions (done after test for lead, zinc and aluminum) Zinc ions react with aqueous ammonia to form a white ppt. Precipitate soluble in excess aqueous ammonia. Conformation of aluminum ions (done after test for lead, zinc and aluminum) Aluminum ions react with aqueous ammonia to form an white gelatinous ppt. Precipitate insoluble in excess aqueous ammonia. Test for calcium ions Calcium ions form a white ppt. in aqueous sodium hydroxide Precipitate insoluble in excess aqueous sodium hydroxide. Conformation of calcium ions (done after test for calcium) Add aqueous ammonia solution to an unknown solution until in excess- No ppt. formed. Test for absence of lead and copper ions Add aqueous potassium iodide to an unknown solution No visible reaction. Test for copper ions Copper ions react with aqueous sodium hydroxide to form a pale blue ppt. Insoluble in excess aqueous sodium hydroxide Conformation of copper ions (done after test for copper) Copper ions react with aqueous ammonia to form a pale blue ppt. Precipitate dissolves in excess ammonia to form a dark blue solution Or add aqueous potassium iodide Dark brown ppt. formed (formation of the brown copper iodide). Test for sulphate Sulphate ions react with barium chloride solution to form a white ppt. This ppt. is soluble in dilute HCl without effervescence Test for nitrate Heat solid in test tube- colorless gas evolved followed by a pungent brown gas. Colorless gas relights a glowing wooden splint and brown gas turns damp blue litmus red. Colorless gas is oxygen Brown gas is NO2 Example of Qualitative Analysis Identify the ions in the unknown Z using the bench reagents.
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