Applied Science BTEC Nationals/Chemical Laboratory Techniques/Qualitative analysis

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Qualitative analysis of inorganic compounds[edit]

The experimental details given here are given in good faith and are believed to be safe and workable methods. However, the authors cannot take responsibility for the consequences of performing these experiments.

The experiments are written for experienced science teaching staff to use as instructions for a supervised class of students. The experiments are not designed for students or inexperienced members of the public to perform without supervision. If you wish to attempt the experiments, ensure that you have completed a legally adequate risk assessment beforehand and that you work within the constraints of the risk assessment.

Test Observation Inference
X(s) + dilute acid solid dissolves - effervescence occurs - a colourless gas given off that turns limewater milky. CO2 given off. X contains carbonate ions CO32-.
solid dissolves - effervescence - a pungent colourless gas that turns acidified dichromate green. SO2 given off. X contains sulphite ions SO32-
solid dissolves - effervescence occurs - a colourless gas given off that gives characteristic pop with a lighted splint. H2(g) given off. X is a metal. X could be aluminium , zinc, iron, tin or magnesium
X(aq) + sodium hydroxide white ppt. - soluble in excess. aluminium hydroxide, Al(OH)3; lead (II) hydroxide, Pb(OH)2; or zinc hydroxide, Zn(OH)2, formed. X contains one of the following: aluminium ions, Al3+; lead (II) ions, Pb2+; zinc ions, Zn2+.
reddish brown ppt. - insoluble in excess. Iron (III) hydroxide, Fe(OH)3 formed. X contains iron (III) ions, Fe3+.
dirty green ppt. - insoluble in excess. Slowly turns reddish brown where exposed to air. iron (II) hydroxide, Fe(OH)2 formed. X contains iron (II) ions, Fe2+.
blue ppt. - insoluble in excess. copper (II) hydroxide Cu(OH)2 formed. X contains Copper (II) ions Cu2+.
light brown ppt. - darkens on standing. manganese (II) hydroxide formed. Mn(OH)2. X contains manganese (II) ions Mn2+
X(s) + sodium hydroxide .Warm Pungent colourless gas given off - turns moist red litmus blue. White fumes formed with conc. HCl The gas is ammonia NH3. X(s) contains ammonium ions NH4+
X(aq) + aqueous ammonia (ammonium hydroxide) white ppt. - insoluble in excess. aluminium hydroxide Al(OH)3 or Pb(OH)2. X contains Al3+ or Pb2+
white ppt. - soluble in excess. Zinc hydroxide Zn(OH)2 formed. X contains Zn2+.
reddish brown ppt. - insoluble in excess Iron (III) hydroxide formed; Fe(OH)3. X contains iron (III) ions, Fe3+
dirty green ppt. - insoluble in excess. Slowly turns reddish brown where exposed to air. Iron (II) hydroxide, Fe(OH)2 formed. X contains iron (II) ions, Fe2+
blue ppt. - dissolves in excess to form a deep blue solution. copper (II) hydroxide Cu(OH)2. complex ion formed. X contains Cu2+.
no ppt. formed but the solution becomes a different colour. transition metal complex ion formed. X contains a transition metal ion.
X(aq) + barium chloride (or nitrate). white ppt. - insoluble in excess. barium sulphate formed. BaSO4. X contains sulphate ions SO42-.
X(aq) + chlorine solution yellow or reddish brown colouration Bromine Br2 or iodine I2 formed
turns tetrachloromethane purple. Iodine I2 formed. X contains iodide ions I-
turns tetrachloromethane orange or yellow bromine Br2 formed. X contains Bromide ions Br-
X(aq) + lead nitrate or lead ethanoate bright yellow ppt. - insoluble in excess lead (II) iodide, PbI2; or lead (II) chromate, PbCrO4, formed. X contains iodide ions I- or chromate ions CrO42-
White ppt. - insoluble hot water. lead (II) carbonate PbCO3 formed. X contains carbonate ions, CO32-.
White ppt. - soluble in hot water lead (II) chloride, PbCl2, or lead (II) bromide, PbBr2, formed. X contains chloride ions, Cl-, or bromide ions, Br-.
X(aq) + lead (II) nitrate or lead (II) ethanoate yellow ppt. - soluble in excess. lead (II) iodide, PbI2, formed. X contains iodide ions I-.
X(aq) + potassium iodide yellow ppt. - soluble in excess. lead (II) iodide, PbI2, formed. X contains lead (II) ions Pb2+.
Yellow or reddish brown colouration produced. Iodine I2 formed. X is an oxidising agent.
X(aq) + potassium manganate (VII) solution (acidified) purple colouration fades. permanganate has been reduced.Mn2+ formed. X is a reducing agent.
X(aq) + potassium dichromate (VI) solution (acidified) orange solution becomes green or blue/green. dichromate has been reduced. Cr3+ formed. X is a reducing agent.
X(aq) acidifed with nitric acid then silver nitrate added. white ppt. - darkens in bright daylight - soluble in excess dil. ammonia. silver chloride, AgCl, formed. X contains chloride ions Cl-.
off white creamy ppt. - soluble in excess conc. ammonia. silver bromide, AgBr, formed. X contains bromide ions Br-.
X(aq) + silver nitrate pale yellow ppt. - insoluble in dil. and conc. ammonia. silver iodide, AgI, formed. X contains iodide ions I-.
X(aq) + sodium carbonate solution. white ppt. - insoluble in excess. insoluble carbonate formed. X contains ONE of the following: magnesium ions, Mg2+; calcium ions, Ca2+; strontium ions, Sr2+; barium ions, Ba2+; zinc ions, Zn2+; lead (II) ions, Pb2+.
dirty green ppt. copper (II) carbonate, CuCO3 formed. X contains copper (II) ions, Cu2+.
effervescence - colourless gas which turns lime water milky. carbon dioxide formed. X is acidic (produces H+ ions in solution).
X(aq) + sodium hydroxide + aluminium and warm Pungent colourless gas given off - turns moist red litmus blue. White fumes formed with conc. HCl The gas is ammonia NH3. X(aq) contains nitrate ions NO3-
X(aq) + dil. sulphuric acid effervescence - colourless gas - turns lime water milky. carbon dioxide formed. X contains carbonate ions CO32-.
white ppt. formed - insoluble in excess insoluble sulphate formed. X contains ONE of the following : calcium ions Ca2+ barium ions Ba2+ strontium ions Sr2+ lead (II) ions Pb2+
X(s) + conc. sulphuric acid. effervescence - misty white fumes which turn moist blue litmus red. hydrogen chloride HCl given off. X contains chloride ions Cl-.
effervescence - misty white fumes + orange fumes - blue litmus bleached. hydrogen bromide HBr and bromine Br2 formed. X contains bromide ions Br-.
effervescence - purple black fumes formed. iodine I2 formed. X contains iodide ions I-
X(s) + conc. hydrochloric acid Pungent green-yellow gas given off, bleaches litmus paper white. Chlorine gas Cl2 given off. Solid is an oxidising agent. Red solid could be Pb3O4 Brown solid could be lead (IV) oxide. Black solid could be manganese (IV) oxide MnO2
effervescence, colourless gas given, gives a characteristic pop with a lighted splint hydrogen gas formed H2. X could be aluminium, magnesium , zinc, iron or tin.
X (aq) + conc. Hydrochloric acid White ppt. formed soluble in excess acid Lead (II) chloride formed. Forms a complex ion PbCl4- with excess conc. hydrochloric acid

Flame tests[edit]

A small amount of the unknown substance is added to a quiet blue Bunsen flame. A wire loop, cleaned and moistened with concentrated hydrochloric acid, is used for this purpose. Some metals will colour the flame:

FlammenfärbungLi.png (crimson) Lithium FlammenfärbungNa.png (yellow/orange) Sodium FlammenfärbungK.png (lilac) Potassium Flametest--Cu.swn.jpg (green/blue) Copper
FlammenfärbungCa.png (orange/red) Calcium FlammenfärbungSr.png (red) Strontium (apple green) Barium FlammenfärbungPb.png (white/blue) Lead

References[edit]

Davies, DG & Kelly, TVG Inorganic Reactions at Advanced Level published by Mills & Boon ISBN 0 263 05692 9