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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! |
|
Test for
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, so don't overdo the test! |
Ca(OH)2(aq)
+ CO2(g) ==> CaCO3(s) + H2O(l) 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) so beware of overdoing this test! |
 Test
for 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. |
 Test
for HCl
hydrogen
chloride gas,
in water
forms hydrochloric acid.Note that HCl does NOT bleach litmus paper like chlorine does. |
(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. |
|
Test
for 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.
|
 Test for 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. |
|
Test
for 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) |
|
Test
for 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. |
|
Test
for chlorine
gas Cl2 A pungent green gas. Test (ii) on its own is no good, could be HCl, but hydrogen chloride does not bleach litmus paper. |
(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-metallic chlorine is acid in aqueous solution and a powerful oxidising agent
which is why you get the double colour change.
(ii) It forms a small amount of chloride ion in water,
so gives a positive result for the chloride test. |
|
Test
for 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
and note it sublimes - changes directly from a solid to a gas-vapor. (ii) Forms a blue-black complex with starch and in biology the test is used to detect starch with iodine solution. |
|
Test
for 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. |
|
Test
for 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. |
|
Tests for water
liquid H2O Easy to get these colour changes muddled! |
(i)
Add a few drops to white anhydrous copper(II) sulphate. (ii) Dip in a piece of 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+
|
|
Tests for H2, O2, HCl, HBr, HI, SO2, NH3, Cl2, Br2, I2, H2S, NO2, H2O 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 |
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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
