* CHEMICAL TESTS - QUALITATIVE ANALYSIS 4 To identify/test for gases/water/non-metals at Doc Brown's Chemistry
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* CHEMICAL TESTS - QUALITATIVE ANALYSIS 4 To identify/test for gases/water/non-metals at Doc Brown's Chemistry
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4. INORGANIC Qualitative TESTS for gases and non-metallic elements |
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| TEST FOR | TEST METHOD | OBSERVATIONS | TEST CHEMISTRY and comments | |
  hydrogen
gas H2 |
Apply a lit splint or spill. | A squeaky pop! (might see condensation on test tube) | 2H2(g) + O2(g) ==> 2H2O(l) + energy! | |
| carbon dioxide gas CO2 | Bubble the gas into limewater (aqueous calcium hydroxide solution). | It turns cloudy - fine milky white precipitate of calcium carbonate. BEWARE - the calcium carbonate precipitate dissolves in excess carbon dioxide! |
Ca(OH)2(aq)
+ CO2(g) ==> CaCO3(s) + H2O(l) For diagram of a possible procedure see test for carbonate If excess carbon dioxide bubbled through you form a clear colourless solution of calcium hydrogencarbonate. CaCO3(s) + H2O(l) + CO2(aq) ==> Ca(HCO3)2(aq) |
|
 oxygen
gas O2 |
Apply a glowing splint or spill. |
It
re-ignites to a flame. |
C(in
wood) + O2(g) ==> CO2(g) The relighted splint is mainly combustible carbon. |
|
 HCl
Hydrogen
chloride gas,
in water
forms hydrochloric acid. |
(i) Damp blue
litmus. (ii) Apply a drop of silver nitrate on the end of a glass rod |
(i)
Litmus
turns red (ii) A white precipitate. |
(i)
Strongly
acid gas. (ii) In water forms chloride ions - hence precipitate with silver nitrate, see chloride test. |
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Hydrogen
bromide HBr
and Hydrogen iodide HI |
As above. In water they are hydrobromic acid and hydriodic acid. | as above but cream precipitate with HBr or yellow precipitate with HI. |
As
above - combination of acid and halide ion tests.
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 Sulphur
dioxide gas SO2
(sulfur dioxide, sulphur(IV) oxide,
sulfur(IV) oxide) |
Freshly made potassium dichromate(VI) paper. | paper changes from orange to green. |
The
orange dichromate(VI) ion, Cr2O72-(aq) is reduced to the green Cr3+(aq) ion. |
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Ammonia
gas NH3 Strong pungent odour.* |
(i) Damp red litmus. (ii) Near fumes of conc. hydrochloric acid. |
(i)
Litmus
turns blue. (ii) Gives white clouds with HCl fumes. |
(i)
Ammonia is the only
common alkaline gas. (ii) It forms fine ammonium chloride crystals with HCl. (*volatile organic aliphatic amines give the same result, and smell more fishy) |
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Bromine
Br2 (l or aq) A dark red liquid - orange-brown fumes, yellow-orange aqueous solution. The other common orange-brown gas is nitrogen dioxide |
(i) Shake with
a liquid alkene.
(ii) Mix with silver nitrate solution. |
(ii)
Decolourised. See alkene test.
(ii) Cream ppt. of silver bromide. See bromide test. |
(i)
Forms a colourless organic dibromo-compound
>C=C< + Br2 ==> >CBr-CBr< (ii) Ag+(aq) + Br-(aq) ==> AgBr(s) Any soluble bromide gives a silver bromide precipitate. |
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Chlorine
gas Cl2 A pungent green gas. Test (ii) on its own is no good, could be HCl. |
(i)
Apply damp blue litmus. (Can use red litmus and just see
bleaching effect.) (ii) A drop silver nitrate on the end of a glass rod into the gas. |
(i) litmus
turns red and then is bleached white. (ii) White precipitate. |
(i)
Non-metal, is acid in aqueous solution and a powerful oxidising agent
(ii) It forms a small amount of chloride ion in water,
so gives a positive result for the chloride test. |
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Iodine
(i)
solid or (ii) solution A dark coloured solid. |
(i)
Gently heat the solid. (ii) Test aqueous solution or solid with starch solution. |
(i)
Gives brilliant purple vapour. (ii) A blue black colour. |
(i) Iodine forms a distinctive coloured vapour. (ii) Forms a blue-black complex with starch and in biology the test is used to detect starch with iodine solution. |
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Hydrogen
sulphide H2S
(hydrogen sulfide) |
Test gas with damp lead(II) ethanoate paper (old name lead acetate). | Rotten egg smell of hydrogen sulphide gas and the H2S gas turns lead(II) ethanoate paper black. |
Hydrogen sulphide gives sulphide ions in water, so
Pb2+(aq) + S2-(aq) => PbS(s) The gas is formed when acids react with sulphides. |
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Nitrogen(IV)
oxide nitrogen dioxide NO2 |
There is no simple relatively unambiguous test. The other common orange-brown gas is bromine. | Its a nasty orange-brown gas | Its a strong oxidising agent. Dissolved in water it gives a solution of nitrite and nitrate ions. The other common brown gas is bromine and the solution of nitrogen dioxide shouldn't give a cream ppt. with silver nitrate solution. | |
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Water
liquid H2O Easy to get these colour changes muddled! |
(i)
Add a few drops to white anhydrous copper(II) sulphate. (ii) Dip in dry blue cobalt chloride paper. |
(i)
Turns
from white to blue. (ii) Turns from blue to pink. |
(i)
Blue
hydrated copper(II) crystals or solution formed (ii) Pink hydrated cobalt ion formed [Co(H2O)6]2+
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