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Doc Brown's Chemistry  Qualitative Methods of Analysis Revision Notes

Chemical identification TESTS Part 4 Qualitative tests to identify gases, water and some non-metals

Tests for the gases and non-metallic elements hydrogen gas H2, oxygen gas O2, hydrogen chloride HCl, hydrogen bromide HBr, hydrogen iodide HI, sulphur(IV) oxide/sulphur dioxide/sulfur(IV) oxide/sulfur dioxide SO2, ammonia NH3, chlorine Cl2, bromine Br2, iodine I2, hydrogen sulphide/hydrogen sulfide H2S, nitrogen(IV) oxide/nitrogen dioxide NO2 and water H2O

 Part 1 Introduction

 Part 2 Qualitative tests to identify organic molecule functional groups of homologous series

 Part 3 Metal cations (positive ions), metal carbonates, ammonium ion and hydrogen ions (acids)

 Part 4 Gases, water and non-metallic elements (this page)

 Part 5 Anions (negative ions) including hydroxide (alkalis)

some associated revising links

GCSE/IGCSE Revision Qualitative Analysis Quiz on chemical test methods for identifying ions, gases and compounds

(c) doc b Quantitative analysis: acid-base, silver nitrate-chloride, EDTA titrations * (c) doc b Quantitative analysis: Redox titrations


Full list of KEYWORDS for inorganic/organic identification methods in alphabetical order e.g. test/reagent for: * acid ==> H+ * acid/acyl chloride RCOCl * alcohols - general ROH/prim RCH2OH/sec R2CHOH/tert R3COH) * aldehyde RCHO * prim aliphatic amine R-NH2 * aliphatic/aromatic carboxylic acids * alkali ==> OH- * alkane/alkene >C=C</alkyne -CC- (saturated versus unsaturated) * aluminium/aluminum ion Al3+ * amide RCONH2 * prim aliphatic amines R-NH2 * ammonia gas NH3 * ammonium ion NH4+ * prim aromatic amine C6H5-NH2 etc. * barium ion Ba2+ * Benedict's solution * Brady's reagent * bromide ion Br- * bromine Br2 * caesium ion Cs+ * calcium ion Ca2+ by flame or hydroxide ppt. * carbonate CO32-/hydrogencarbonate HCO3- with acid or effect of heating metal carbonate e.g. MCO3 * carbon dioxide gas CO2 * carboxylic acid RCOOH * carboxylic acid (aliphatic) salts e.g. RCOO-Na+ * chloride ion Cl- * chlorine gas Cl2 * Chomate(VI) ion CrO42- * copper(II) ion Cu2+ by flame or hydroxide ppt. * 24DNPH (for aldehydes/ketones test) * esters RCOOR * Fehlings test/solution * flame test for metal ions * fluoride ion F- * haloalkanes/halogenoalkanes R-X * hydrogen gas H2 * hydrogen sulphide H2S * hydrogen ion, acids H+ * hydrogen bromide gas/hydrobromic acid HBr * hydrogen chloride gas/hydrochloric acid HCl * hydrogen iodide gas/hydriodic acid HI * hydroxide ion, alkali OH- * hydroxy/alcohol/phenol (organic) * iodide ion I- * iodine I2 * iodoform test - formation of CHI3 * iron(II) ion Fe2+ * iron(III) ion Fe3+ * ketone R2C=O * lead(II) ion Pb2+ * lithium ion Li+ * lime water Ca(OH)2(aq) * magnesium ion Mg2+ * metal carbonates-heating e.g. MCO3 * metal ions via hydroxide precipitate * nitrate or nitrate(V) NO3- * nitrite or nitrate(III) NO2- * nitrogen dioxide or nitrogen(IV) oxide NO2 * oxygen gas O2 * phenols C6H5OH etc. * potassium ion K+ * rubidium ion Rb+ * reducing sugars * saturated/unsaturated * silver nitrate AgNO3 (see chloride, bromide, iodide tests) * sugars (reducing) * sodium ion Na+ * strontium Sr+ * 'sulphate/sulfate' or sulphate(VI) SO42- * sulphide S2- * 'sulphite/sulfite' or sulphate(IV) SO32- * sulphur dioxide gas SO2 * Tollen's Reagent * unsaturated/saturated * water H2O * zinc ion Zn2+ *


Use the alphabetical test list above for identifying anions, cations, gases, molecules etc. to find what you require! for your KS3-KS4 Science-GCSE-IGCSE- Chemistry and GCE-AS-A2-IB-US grades 9-12 K12 advanced subsidiary chemistry course etc. and help you to identify unknown inorganic and organic compounds-molecules for qualitative analysis.

EMAIL query?comment?test missing? * Gas PreparationsHazard warning signs/symbols-examples of labelling

   Advanced Chemistry Page Index and Links

 4. INORGANIC Qualitative TESTS for gases and non-metallic elements

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

test for 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.

Advanced Chemistry Page Index and Links

Test for sulphur dioxide gas SO (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.Advanced Chemistry Page Index and Links

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+

Advanced Chemistry Page Index and Links

 

Tests for H2, O2, HCl, HBr, HI, SO2, NH3, Cl2, Br2, I2, H2S, NO2, H2O

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