Revision notes on qualitative chemical tests for common gases and some non-metallic elements
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4. INORGANIC Qualitative TESTS for gases and non–metallic elements |
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CHEMICAL 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! |
Chemical 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! |
Chemical 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. |
Chemical
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. |
chemical 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. |
Chemical test for sulfur dioxide gas SO2 (sulfur dioxide, sulfur(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. |
Chemical
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) |
Chemical
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. |
Chemical
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. |
Chemical
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. |
Chemical test for hydrogen sulfide H2S (hydrogen sulfide) | Test gas with damp lead(II) ethanoate paper (old name lead acetate). | Rotten egg smell of hydrogen sulfide gas and the H2S gas turns lead(II) ethanoate paper black. |
Hydrogen sulfide gives sulfide ions in water, so
Pb2+(aq) + S2–(aq) => PbS(s) The gas is formed when acids react with sulfides. |
Chemical 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. |
Chemical tests for water
liquid H2O Easy to get these colour changes muddled! |
(i)
Add a few drops to white anhydrous copper(II)
sulfate. (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+ |
Key words & phrases: Gas Tests for hydrogen H2, oxygen O2, hydrogen chloride HCl, hydrogen bromide HBr, hydrogen iodide HI, sulfur dioxide SO2, ammonia NH3, chlorine Cl2, bromine Br2, iodine I2, hydrogen sulfide H2S, nitrogen dioxide NO2, water H2O What is the test for oxygen gas? How do you test for oxygen? What is the test for hydrogen gas? How do you test for hydrogen? What is the test for hydrogen chloride gas? How do you test for hydrogen chloride? What is the test for hydrogen bromide gas? How do you test for hydrogen bromide? What is the test for hydrogen iodide gas? How do you test for hydrogen iodide? What is the test for sulfur dioxide gas? (sulfur dioxide) How do you test for sulfur dioxide? What is the test for ammonia gas? How do you test for ammonia What is the test for chlorine gas? how do you test for chlorine? What is the test for bromine gas? How do you test for bromine? What is the test for iodine? how do you test for iodine? What is the test for hydrogen sulfide gas? (hydrogen sulfide) How do you test for hydrogen sulfide? What is the test for nitrogen dioxide gas? How do you test for nitrogen dioxide? What is the test for water? How do you test for water? analytical chemistry courses at Cambridge university analytical chemistry courses at Oxford university analytical chemistry courses at Durham university analytical chemistry courses at York university analytical chemistry courses at Edinburgh university analytical chemistry courses at St Andrews university analytical chemistry courses at Imperial College London university analytical chemistry courses at Warwick university analytical chemistry courses at Sussex university analytical chemistry courses at Bath university analytical chemistry courses at Nottingham university analytical chemistry courses at Surrey university analytical chemistry courses at Bristol university analytical chemistry courses at Cardiff university analytical chemistry courses at Birmingham university analytical chemistry courses at Manchester university analytical chemistry courses at University College London university analytical chemistry courses at Strathclyde university analytical chemistry courses at Loughborough university analytical chemistry courses at Southampton university analytical chemistry courses at Sheffield university analytical chemistry courses at Glasgow university analytical chemistry courses at Liverpool university analytical chemistry courses at Leeds university analytical chemistry courses at Queens, Belfast university analytical chemistry courses at Kings College, London university analytical chemistry courses at Heriot-Watt university analytical chemistry courses at Lancaster university analytical chemistry courses at East Anglia (UEA) university analytical chemistry courses at Newcastle university analytical chemistry courses at Keele university analytical chemistry courses at Leicester university analytical chemistry courses at Bangor university analytical chemistry courses at Nottingham Trent university analytical chemistry courses at Kent university analytical chemistry courses at Aberdeen university analytical chemistry courses at Coventry university analytical chemistry courses at Sheffield Hallam university analytical chemistry courses at Aston university analytical chemistry courses at Hull university analytical chemistry courses at Bradford university analytical chemistry courses at Huddersfield university analytical chemistry courses at Queen Mary, University of London university analytical chemistry courses at Reading university analytical chemistry courses at Glyndwr university analytical chemistry courses at Brighton university analytical chemistry courses at Manchester Metropoliten university analytical chemistry courses at De Montfort university analytical chemistry courses at Northumbria university analytical chemistry courses at South Wales university analytical chemistry courses at Liverpool John Moores university analytical chemistry courses at Central Lancashire university analytical chemistry courses at Kingston university analytical chemistry courses at West of Scotland university analytical chemistry courses at Lincoln university analytical chemistry courses at Plymouth university analytical chemistry courses at Greenwich university analytical chemistry courses at Liverpool Metropolitan university[ SEARCH BOX] |
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