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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 | |||||||||||||||||||||||||
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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
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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! |
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Apply a glowing splint or spill. |
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C(in
wood) + O2(g) ==> CO2(g) The relighted splint is mainly combustible carbon. |
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![]() 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. |
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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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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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![]() 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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![]() 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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![]() ![]() 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. |
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![]() 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. |
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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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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) 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+ |
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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? (sulphur 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 sulphide) 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?
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