|
|
 |
 |
||
 |
 |
|||
|
|
|
 |
 |
 |
 |
 |
 |
||
 |
 |
 |
||
 |
 |
 |
||
Custom Search
|
||||
Doc Brown's Chemistry Qualitative
Methods of Analysis Revision Notes
GCSE/IGCSE
Revision Qualitative Analysis Quiz on chemical test methods for
identifying ions, gases and compounds
Quantitative analysis:
acid-base, silver nitrate-chloride, EDTA titrations
|
3. INORGANIC Qualitative TESTS Cations and Acids |
|||
| TEST FOR | TEST METHOD | OBSERVATIONS | TEST CHEMISTRY-comments |
|
Test for the
Ammonium
ion NH4+ |
Add COLD sodium hydroxide solution to the suspected ammonium salt and test any gas above the solution with red litmus. |
 Smelly
ammonia released! and red litmus turns blue, gentle warming helps
BUT the ammonia should be released at room temperature. |
Ammonia
gas is evolved because alkali frees ammonia from its salts. NH4+(aq) + OH-(aq) ==> NH3(g) + H2O(l) The hydroxide ion removes a proton from the ammonium ion to release the ammonia. |
|
Test
for acids i.e. the aqueous hydrogen
ion i.e. H+ or H3O+ ion (note:
to completely identify acids you need to test for the anion e.g. chloride
for HCl hydrochloric or sulfate ion if sulfuric acid etc.) |
(i)
Litmus or universal indicator or pH meter.
(ii) Adding a little sodium hydrogen carbonate powder. |
(i)
Litmus turns red, and, a variety of colours with univ. ind. strong - red, weak
- yellow /orange, depending on strength of acid. (ii) Fizzing with any carbonate and test the gas to see if it is carbon dioxide - test for CO2. |
(i) A
pH meter reading gives a value of less than 7, the lower the pH number the
stronger the acid, the higher the H+ concentration,
(ii) HCO3-(aq) + H+(aq) ==> H2O(l) + CO2(g) However, some salts can give acid or alkaline solutions but advanced acid-base theory is needed to explain this. |
 Testing
for Positive
metal cations with a flame test(see also below for NaOH(aq) and NH3(aq) tests for metal ion) and heating carbonates too. This test can be done in a more precise and specific manner using an instrument called a spectroscope and the technique is called emission spectroscopy. Specific emission lines of a characteristic frequency are observed - a fingerprint pattern. |
The metal salt or other compound is mixed with concentrated hydrochloric acid and a sample of the mixture is heated strongly in a bunsen flame on the end of a cleaned nichrome wire (or platinum if you can afford it!) | Group 1: lithium Li/Li+ crimson | All
colours are due to electronic excitations to a higher electronic energy levels. You see the
light emitted as the electrons return to lower more stable electronic
energy levels.
This is the basis of atomic emission and absorption spectroscopy.
Aluminium, magnesium, iron and zinc do not produce a useful identifying
flame colours.
Other metal flame colors in Group 1:rubidium - red and caesium/cesium - blue
|
| Group 1: sodium Na/Na+ yellow (can be slightly orangeish) | |||
| Group 1: potassium K/K+ violet/lilac (crimson through cobalt blue glass) | |||
| Group 2: calcium Ca/Ca2+ brick (yellowish) red (light green through cobalt blue glass) | |||
| Group 2: strontium Sr/Sr2+ crimson | |||
| Group 2: barium Ba/Ba2+ yellowish/apple green | |||
| Transition Metal: copper(II) Cu/Cu2+ blue (flashes of green too) | |||
|
Testing for positive
metal cations via sodium hydroxide (NaOH) or ammonia (NH3)
solutions. Note: (1) Both are alkalis, giving hydroxide ions, OH-, in their solutions. (2) Aluminium, magnesium, iron and zinc do not produce a useful identifying flame colour. (3) A more advanced test to distinguish iron(II) ions, Fe2+and iron(III) ions, Fe3+ (i) If potassium hexacyanoferrate(III) solution is added to the suspected iron solution, iron(II) ions give a deep blue precipitate of Turnbull's blue. (ii) If potassium hexacyanoferrate(II) solution is added to the suspected iron solution, iron(III) ions give a deep blue precipitate of Prussian blue. Note that Turnbull's blue is identical in composition to Prussian blue. For more chemical details see the transition metals page on iron. |
Dilute
sodium hydroxide (NaOH) solution is added to a solution containing the
suspected ion. Both the precipitate formed and the effect of excess alkali are important observations. All precipitates are white, unless otherwise stated, and all tend to be gelatinous in nature. The test should be repeated with aqueous ammonia solution (NH3, 'ammonium hydroxide'). The observations with ammonia solution are usually similar, but not always, the same and the differences can be important clues as to the identity of the metal ion. ppt. = precipitate. More on some of these hydroxide precipitates on the 3-d block Transition Metals Series pages. |
aluminium
ion: Al3+(aq) + 3OH-(aq)
==> Al(OH)3(s)
gives a white precipitate of aluminum hydroxide, which is not soluble in excess of the weak alkali ammonia, but dissolves in the stronger base/alkali sodium hydroxide (amphoteric) to give a clear colourless solution. Al(OH)3(s) + 3OH-(aq) ==> [Al(OH)6]3-(aq) (amphoteric behaviour because it dissolves in acids too) |
|
|
calcium
ion: Ca2+(aq) + 2OH-(aq)
==> Ca(OH)2(s)
gives a white precipitate of calcium hydroxide with sodium hydroxide IF the concentration of calcium ion is high. It is not soluble in excess of NaOH. No precipitate is formed with ammonia solution. |
|||
|
magnesium
ion: Mg2+(aq) + 2OH-(aq)
==> Mg(OH)2(s)
gives a white precipitate of magnesium hydroxide, which is not soluble in excess of either NH3 or NaOH. You could distinguish Mg from Ca with a flame test or ammonia test above. |
|||
|
copper(II)
ion: Cu2+(aq) + 2OH-(aq)
==> Cu(OH)2(s)
gives a blue/turquoise ppt. of copper(II) hydroxide, which dissolves in excess ammonia to give a deep blue solution of an ammine complex, but copper(II) hydroxide is NOT soluble in excess NaOH. Cu(OH)2(s) + 4NH3(aq) ==> [Cu(NH3)4]2+(aq) + 2OH-(aq) |
|||
|
iron(II)
ion: Fe2+(aq) + 2OH-(aq)
==> Fe(OH)2(s)
gives a dark green precipitate of iron(II) hydroxide, which is not soluble in excess of NH3 or NaOH. Darkens in air due to oxidation to Fe(OH)3. |
|||
|
iron(III)
ion: Fe3+(aq) + 3OH-(aq)
==> Fe(OH)3(s)
forms a brown precipitate of iron(III) hydroxide, which is not soluble in excess of NH3 or NaOH. Another test for iron(III) ions is to add a few drops of potassium/ammonium thiocyanate solution and a blood-red coloured compound is formed. |
|||
|
zinc
ion: Zn2+(aq) + 2OH-(aq)
==> Zn(OH)2(s)
a white precipitate formed of zinc hydroxide, which dissolves in both excess (i) sodium hydroxide or (ii) ammonia to give a clear colourless solution: (i) Zn(OH)2(s) + 2OH-(aq) ==> [Zn(OH)4]2-(aq) (amphoteric behaviour because zinc hydroxide dissolves in acids too). (ii) Zn(OH)2(s) + 4NH3(aq) ==> [Zn(NH3)4]2+(aq) + 2OH-(aq) (soluble complex ion formation) |
|||
|
chromium (III)
ion: Cr3+(aq) + 3OH-(aq)
==> Cr(OH)3(s)
a grey-green precipitate forms of chromium(III) hydroxide, which is soluble in excess of NaOH (amphoteric, dissolves in acids too) but not soluble in excess ammonia NH3. With sodium hydroxide a dark green soluble hexahydroxo-complex ion is formed. Cr(OH)3(s) + 3NaOH(aq) ==> [Cr(OH)6]3-(aq) |
|||
|
manganese(II)
ion: Mn2+(aq) + 2OH-(aq)
==> Mn(OH)2(s)
produces an off-white precipitate of manganese(II) hydroxide, which is NOT soluble in excess of NH3 or NaOH and rapidly turns brown=>black in air due to oxidation to manganese(III) oxide Mn2O3 and then manganese(IV) oxide, MnO2. |
|||
|
lead(II)
ion: Pb2+(aq) + 2OH-(aq)
==> Pb(OH)2(s)
a white precipitate forms of lead(II) hydroxide, which dissolves in excess sodium hydroxide (amphoteric) to give a clear colourless solution but does not dissolve in excess ammonia solution. Pb(OH)2(s) + 2OH-(aq) ==> [Pb(OH)4]2-(aq) (amphoteric behaviour with NaOH as with zinc hydroxide which also dissolves in acids too) |
|||
| The barium ion, Ba2+(aq) does not give a hydroxide precipitate because barium hydroxide, Ba(OH)2, is too soluble. | |||
|
|
|||
|
MISCELLANEOUS
CATION TESTS:
(i) Lead(II) ion (ii) - |
(i) add potassium iodide solution ==> yellow precipitate | (i) Pb2+(aq) +2I-(aq) ==>PbI2(s) a yellow precipitate of lead(II) iodide is formed | |
|
Metal
Carbonates
|
Sometimes
heating a metal carbonate strongly to decompose it provides some clues
to its identity. Adding acid to a carbonate ==> CO2 and the colour of the resulting solution e.g. blue copper(II) ion Cu2+(aq), may also provide clues, but no good in most cases because most carbonates you come across are white giving colourless solutions except for some transition metals like copper, nickel and cobalt. The metal ion solution might also give a flame colour or a hydroxide precipitate with sodium hydroxide e.g. copper. |
(i) copper(II)
carbonate==> copper(II) oxide + carbon dioxide CuCO3(s) ==> CuO(s) + CO2(g) observations [green solid] ==> [black solid residue] + [colourless gas, test with limewater, white precipitate] (ii) zinc carbonate==> zinc oxide + carbon dioxide ZnCO3(s) ==> ZnO(s) + CO2(g) observations [white] ==> [solid
residue, pale yellow hot,
white cold] + [colourless
gas, test with limewater ==>
white precipitate] |
|
|
Tests for NH4+, H+/H3O+, Li+, Na+, K+, Ca2+, Sr2+, Sr2+, Ba2+, Cu2+, Al3+, Mg2+, Fe2+, Fe3+, Zn2+, Cr3+, Mn2+, Pb2+, CuCO3, ZnCO3 private tuition IB AS A2 advanced level residential student tuition courses pupil resources teacher resources publishing company revision guides published by Amazon Harper Collins Education Letts Lonsdale Revision Guides Heinemann Educational Books publishers catalogue CGP Books Educational Hodder Education Philip Adam publications Nelson Thornes Philip Allan Textbooks examination boards AQA Edexcel OCR sciences higher education secondary school education college science education institutions Science GCE courses Advanced Level A AS A2 level tuition help tutors colleges semesters books revision guides college textbooks university education courses GCE biology GCSE chemistry courses GCE physics university courses medicine biochemistry university medical sciences university chemistry university biology university physics environmental science biomedical physics courses revision books worksheets workbooks practice examination paper questions science seminars university entrance examinations exam tuition science teacher training
Alphabetical Index for Science Pages Content A B C D E F G H I J K L M N O P Q R S T U V W X Y Z |
|||
