Detecting the presence of chemical species

Transcription

1 Chapter 61 Detecting the presence of chemical species 61.1 Preliminary tests of substances 61.2 Detecting the presence of metallic ions in substances using the flame test 61.3 Detecting the presence of cations 61.4 Detecting the presence of anions 61.5 Detecting the presence of common gases and water P. 1 / 100

2 61.6 Deducing the chemical nature of a salt 61.7 Detecting the presence of various functional groups in carbon compounds 61.8 Possible risks associated with chemical tests Key terms Progress check Summary Concept map P. 2 / 100

3 61.1 Preliminary tests of substances Analytical chemistry Qualitative analysis Identification of a particular substance in a sample Quantitative analysis Determination of the amount of a particular substance in a sample P. 3 / 100

4 Preliminary tests on unknown substances includes observing: their colours their smells their solubilities in water action of heat on their samples the ph values of their aqueous solutions 61.1 Preliminary tests of substances P. 4 / 100

5 Colour Many metal oxides and transition metal compounds are coloured substances. Colour of solid ionic substance Black Yellow Blue Green Inference Copper(II) oxide Iron(II, III) oxide Lead(II) oxide Silver iodide Hydrated iron(iii) chloride Hydrated copper(ii) sulphate Hydrated iron(ii) chloride Hydrated copper(ii) chloride Table 61.1 Inference based on the colours of some solid ionic substances Preliminary tests of substances Think about P. 5 / 100

6 The colour of an aqueous ionic solution gives some ideas about the cation or anion it contains. Colour of aqueous ionic solution Inference of the cation/anion it contains Blue or green Copper(II) ion, Cu 2+ Pale green Iron(II) ion, Fe 2+ Yellow Iron(III) ion, Fe 3+ Purple Orange Permanganate ion, MnO 4 Dichromate ion, Cr 2 O 7 2 Table 61.2 Inference based on the colours of some aqueous ionic solutions Preliminary tests of substances P. 6 / 100

7 Smell Some substances give characteristic smells. Smell Characteristic pungent smell Pungent smell of bleaching solution Choking smell of burning sulphur Smell of bad egg Inference Ammonium salt or aqueous ammonia Chlorine gas Sulphur dioxide gas Hydrogen sulphide gas Table 61.3 Inference based on the characteristic smells of some substances Preliminary tests of substances P. 7 / 100

8 Many gases are harmful and some are even toxic. Never smell a gas by breathing in directly above the test tube containing the gas. Hold the tube about 15 cm away from the nose and then fan a little of the gas towards our nose. Figure 61.1 The proper way to smell a gas Preliminary tests of substances P. 8 / 100

9 Solubility in water The solubility of ionic compounds in water gives important hints on what ions are present. Ionic compound Solubility in water Remarks Potassium, sodium and ammonium salts Nitrates Hydrogencarbonates Halides (chlorides, bromides and iodides) Soluble Soluble Soluble Soluble Except: silver halides (AgCl, AgBr and AgI) lead(ii) halides (PbCl 2, PbBr 2 and PbI 2 ) Table 61.4 Some water-soluble and water-insoluble ionic compounds Preliminary tests of substances P. 9 / 100

10 Ionic compound Sulphates Carbonates Solubility in water Soluble Insoluble Remarks Except: silver sulphate (Ag 2 SO 4 ) lead(ii) sulphate (PbSO 4 ) barium sulphate (BaSO 4 ) calcium sulphate (CaSO 4 ; only sparingly soluble) Except: potassium carbonate (K 2 CO 3 ) sodium carbonate (Na 2 CO 3 ) ammonium carbonate ((NH 4 ) 2 CO 3 ) Table 61.4 Some water-soluble and water-insoluble ionic compounds Preliminary tests of substances P. 10 / 100

11 ph of aqueous solution The ph value of the aqueous solution of an unknown substance helps predict its identity. ph paper or universal indicator is used to find the ph value of a solution. Litmus paper is used to distinguish acidic and alkaline solutions Preliminary tests of substances P. 11 / 100

12 Acidic Neutral Basic Acids Some non-metallic oxides (e.g. SO 2, SO 3, NO 2, P 4 O 10 ) Salts formed from strong acids and weak bases (e.g. NH 4 Cl) Water Some non-metallic oxides (e.g. CO, N 2 O) Salts formed from strong acids and strong bases (e.g. NaCl) Alkalis Some metallic oxides (e.g. CaO, K 2 O, Na 2 O) Salts formed from weak acids and strong bases (e.g. CH 3 COO Na + ) Table 61.5 Examples of acidic, neutral and basic compounds Preliminary tests of substances P. 12 / 100

13 Action of heat on solid sample When the solid sample of an unknown substance is heated, the corresponding observations give some hints about its identity. Observations White solid sublimes Black solid sublimes and violet vapour evolves Yellow when hot, white when cold Orange when hot, yellow when cold Inference Ammonium chloride Iodine Zinc oxide Lead(II) oxide Table 61.6 Inference based on the observations when solid samples are heated Preliminary tests of substances P. 13 / 100

14 Figure 61.2 Action of heat on ammonium chloride. White solid deposits at the cooler part of the tube. Learning tip It seems that ammonium chloride sublimes on heating. In fact, ammonium chloride decomposes on heating. heat NH 4 Cl(s) NH 3 (g) + HCl(g) cool Ammonia and hydrogen chloride recombine to form ammonium chloride on cooling Preliminary tests of substances P. 14 / 100

15 (a) (b) Figure 61.3 Action of heat on zinc oxide (a) yellow when hot (b) white when cold. (a) (b) Figure 61.4 Action of heat on lead(ii) oxide (a) orange when hot (b) yellow when cold Preliminary tests of substances P. 15 / 100

16 In some cases, when a solid sample is heated, it decomposes or burns and a gas evolves. The gas provides useful information on the identity of the sample. Example 61.1 Class practice Preliminary tests of substances P. 16 / 100

17 61.2 Detecting the presence of metallic ions in substances using the flame test The presence of certain metals or their compounds can be inferred by a flame test. Flame colour Inference Lilac Potassium ion, K + Golden yellow Sodium ion, Na + Brick-red Calcium ion, Ca 2+ Bluish green Copper(II) ion, Cu 2+ Table 61.7 Inferences based on the flame colours in a flame test. P. 17 / 100

18 Learning tip Potassium compounds give a lilac colour in the flame test but it appears crimson when viewed through cobalt glass. Simulation (Deducing the chemical nature of a sample) potassium ions (lilac) sodium ions (golden yellow) calcium ions (brick-red) Figure 61.5 Flame colours of some metal ions. copper(ii) ions (bluish green) 61.2 Detecting the presence of metallic ions in substances using the flame test P. 18 / 100

19 61.3 Detecting the presence of cations Test with sodium hydroxide solution To detect the presence of cations in a solution, a few drops of dilute sodium hydroxide solution (NaOH(aq)) is added to the solution. Certain metal ions form insoluble hydroxides with NaOH(aq). Learning tip The reactions between sodium hydroxide and solutions of different metal ions have been discussed in Book 2, Section 14.6, p P. 19 / 100

20 Some insoluble hydroxides are coloured precipitates. Cu 2+ (aq) + 2OH (aq) Cu(OH) 2 (s) pale blue precipitate Fe 2+ (aq) + 2OH (aq) Fe(OH) 2 (s) dirty green precipitate Fe 3+ (aq) + 3OH (aq) Fe(OH) 3 (s) reddish brown precipitate 61.3 Detecting the presence of cations P. 20 / 100

21 (a) (b) (c) Figure 61.6 The coloured precipitates formed by the addition of sodium hydroxide solution to solutions containing (a) Cu 2+ (aq), (b) Fe 2+ (aq) and (c) Fe 3+ (aq) ions respectively Detecting the presence of cations P. 21 / 100

22 Ca 2+ (aq) ions, Mg 2+ (aq) ions, Al 3+ (aq) ions, Zn 2+ (aq) ions and Pb 2+ (aq) ions all form white precipitate with NaOH(aq). Al(OH) 3, Zn(OH) 2 and Pb(OH) 2 dissolve in excess NaOH(aq) to give a colourless solution containing complex ions. Al(OH) 3 (s) + OH (aq) [Al(OH) 4 ] (aq) white precipitate from excess NaOH(aq) tetrahydroxoaluminate ion colourless solution Pb(OH) 2 (s) + 2OH (aq) [Pb(OH) 4 ] 2 (aq) white precipitate from excess NaOH(aq) 61.3 Detecting the presence of cations tetrahydroxoplumbate(ii) ion colourless solution P. 22 / 100

23 Zn(OH) 2 (s) + 2OH (aq) [Zn(OH) 4 ] 2 (aq) white precipitate from excess NaOH(aq) tetrahydroxozincate ion colourless solution add a few drops of NaOH(aq) add excess NaOH(aq) zinc nitrate solution white precipitate colourless solution Figure 61.7 The reaction of zinc nitrate solution with excess sodium hydroxide solution Detecting the presence of cations P. 23 / 100

24 Cation Addition of sodium hydroxide solution Small amount Excess Calcium Ca 2+ Magnesium Mg 2+ Aluminium Al 3+ White precipitate Ca 2+ (aq) + 2OH (aq) Ca(OH) 2 (s) white precipitate White precipitate Mg 2+ (aq) + 2OH (aq) Mg(OH) 2 (s) white precipitate White precipitate Al 3+ (aq) + 3OH (aq) Al(OH) 3 (s) white precipitate Insoluble precipitate Insoluble precipitate Precipitate dissolves to become colourless solution. Al(OH) 3 (s) + OH (aq) [Al(OH) 4 ] (aq) Table 61.8 Effects of adding a small amount and an excess of sodium hydroxide solution to the solutions containing certain cations Detecting the presence of cations P. 24 / 100

25 Cation Addition of sodium hydroxide solution Small amount Excess Zinc Zn 2+ Lead(II) Pb 2+ White precipitate Zn 2+ (aq) + 2OH (aq) Zn(OH) 2 (s) white precipitate White precipitate Pb 2+ (aq) + 2OH (aq) Pb(OH) 2 (s) white precipitate Precipitate dissolves to become colourless solution. Zn(OH) 2 (s) + 2OH (aq) [Zn(OH) 4 ] 2 (aq) Precipitate dissolves to become colourless solution. Pb(OH) 2 (s) + 2OH (aq) [Pb(OH) 4 ] 2 (aq) Table 61.8 Effects of adding a small amount and an excess of sodium hydroxide solution to the solutions containing certain cations Detecting the presence of cations P. 25 / 100

26 Cation Addition of sodium hydroxide solution Small amount Excess Iron(II) Fe 2+ Iron(III) Fe 3+ Copper(II) Cu 2+ Ammonium NH 4 + Dirty green precipitate Fe 2+ (aq) + 2OH (aq) Fe(OH) 2 (s) dirty green precipitate Reddish brown precipitate Fe 3+ (aq) + 3OH (aq) Fe(OH) 3 (s) reddish brown precipitate Pale blue precipitate Cu 2+ (aq) + 2OH (aq) Cu(OH) 2 (s) pale blue precipitate No observable change Insoluble precipitate Insoluble precipitate Insoluble precipitate No observable change Table 61.8 Effects of adding a small amount and an excess of sodium hydroxide solution to the solutions containing certain cations Detecting the presence of cations P. 26 / 100

27 If ammonium ion (NH 4+ ) is present, ammonia gas is produced upon warming with NaOH(aq). Ammonia has a characteristic pungent smell and turns moist red litmus paper blue. NH 4+ (aq) + OH (aq) NH 3 (g) + H 2 O(l) characteristic pungent smell moist red litmus paper turns blue ammonium chloride + sodium hydroxide solution Figure 61.8 Ammonium chloride solution reacts with sodium hydroxide solution to produce ammonia gas, which turns moist red litmus paper blue Detecting the presence of cations P. 27 / 100

28 Test with aqueous ammonia Aqueous ammonia precipitates insoluble hydroxides from solutions containing certain metal ions. Example Fe 2+ (aq) + 2OH (aq) Fe(OH) 2 (s) dirty green precipitate Think about 61.3 Detecting the presence of cations P. 28 / 100

29 A solution containing copper(ii) ions Cu 2+ (aq) + 2OH (aq) Cu(OH) 2 (s) blue pale blue precipitate Cu(OH) 2 (s) + 4NH 3 (aq) [Cu(NH 3 ) 4 ] 2+ (aq) + 2OH (aq) pale blue precipitate tetraamminecopper(ii) ion deep blue solution add a small amount of NH 3 (aq) add excess NH 3 (aq) CuSO 4 (aq) Cu(OH) 2 precipitate solution containing [Cu(NH 3 ) 4 ] 2+ (aq) ions Figure 61.9 The reaction of copper(ii) sulphate solution with a small amount and an excess aqueous ammonia Detecting the presence of cations P. 29 / 100

30 A solution containing zinc ions Zn 2+ (aq) + 2OH (aq) Zn(OH) 2 (s) colourless white precipitate Zn(OH) 2 (s) + 4NH 3 (aq) [Zn(NH 3 ) 4 ] 2+ (aq) + 2OH (aq) white precipitate tetraamminezinc ion colourless precipitate 61.3 Detecting the presence of cations P. 30 / 100

31 Cation Addition of aqueous ammonia Small amount Excess Calcium Ca 2+ Magnesium Mg 2+ Aluminium Al 3+ No observable change * Aqueous ammonia does not provide enough hydroxide ions to precipitate out calcium hydroxide. White precipitate Mg 2+ (aq) + 2OH (aq) Mg(OH) 2 (s) White precipitate white precipitate Al 3+ (aq) + 3OH (aq) Al(OH) 3 (s) white precipitate No observable change Insoluble precipitate Insoluble precipitate Table 61.9 Effects of adding a small amount and an excess of aqueous ammonia to the solutions containing certain cations Detecting the presence of cations P. 31 / 100

32 Cation Addition of aqueous ammonia Small amount Excess Zinc Zn 2+ Lead(II) Pb 2+ Iron(II) Fe 2+ White precipitate Zn 2+ (aq) + 2OH (aq) Zn(OH) 2 (s) white precipitate White precipitate Pb 2+ (aq) + 2OH (aq) Pb(OH) 2 (s) white precipitate Dirty green precipitate Fe 2+ (aq) + 2OH (aq) Fe(OH) 2 (s) dirty green precipitate Precipitate dissolves to become colourless solution. Zn(OH) 2 (s) + 4NH 3 (aq) [Zn(NH 3 ) 4 ] 2+ (aq) + 2OH (aq) Insoluble precipitate Insoluble precipitate Table 61.9 Effects of adding a small amount and an excess of aqueous ammonia to the solutions containing certain cations Detecting the presence of cations P. 32 / 100

33 Cation Iron(III) Fe 3+ Copper(II) Cu 2+ Ammonium NH 4 + Small amount Reddish brown precipitate Fe 3+ (aq) + 3OH (aq) Fe(OH) 3 (s) reddish brown precipitate Pale blue precipitate Cu 2+ (aq) + 2OH (aq) Cu(OH) 2 (s) pale blue precipitate No observable change Addition of aqueous ammonia P. 33 / 100 Excess Insoluble precipitate Precipitate dissolves to become deep blue solution. Cu(OH) 2 (s) + 4NH 3 (aq) [Cu(NH 3 ) 4 ] 2+ (aq) + 2OH (aq) No observable change Table 61.9 Effects of adding a small amount and an excess of aqueous ammonia to the solutions containing certain cations Detecting the presence of cations Class practice 61.2

34 Identifying an unknown cation in a sample of solution K +, Na +, Ca 2+, Mg 2+, Al 3+, Zn 2+, Fe 2+, Fe 3+, Cu 2+ or NH 4 + Add NH 3 (aq) to the first portion Does a precipitate form? No Yes white precipitate dirty green precipitate reddish brown precipitate pale blue precipitate Mg 2+, Al 3+ or Zn 2+ Fe 2+ Fe 3+ Cu 2+ K +, Na +, Ca 2+ or NH + (soluble in 4 excess NH 3 (aq)) Figure A flow chart showing how to identify an unknown cation in a sample solution Detecting the presence of cations P. 34 / 100

35 K +, Na +, Ca 2+ or NH 4 + Does the sample give a characteristic flame colour? Perform flame test on the second portion No NH 4 +? Yes lilac flame golden yellow flame brick-red flame K + Na + Ca 2+ Add NaOH(aq) and warm the mixture Is any pungent colourless gas given off? Yes Test the gas with Is the gas alkaline? Yes moist red litmus paper Figure A flow chart showing how to identify an unknown cation in a sample solution Detecting the presence of cations P. 35 / 100 NH 4 +

36 Mg 2+, Al 3+ or Zn 2+ Add excess NH 3 (aq) Does the precipitate dissolve? Yes Zn 2+ No Mg 2+ or Al 3+ Add excess NaOH(aq) Does the precipitate dissolve? No Yes Al 3+ Example 61.2 Figure A flow chart showing how to identify an unknown cation in a sample solution. Mg 2+ Example 61.3 Class practice Detecting the presence of cations P. 36 / 100

37 61.4 Detecting the presence of anions Test for halide ions Using acidified silver nitrate solution Acidified silver nitrate (AgNO 3 (aq)/h + (aq)) solution is used to test for halide ions. It forms insoluble silver halide (AgX; X = Cl, Br, I) with halide ions. P. 37 / 100

38 Ag + (aq) + Cl (aq) AgCl(s) white precipitate Ag + (aq) + Br (aq) AgBr(s) pale yellow precipitate Ag + (aq) + I (aq) AgI(s) yellow precipitate AgCl AgBr AgI Figure Precipitates (from left to right: silver chloride, silver bromide and silver iodide) form when acidified silver nitrate solution is added to solutions containing halide ions Detecting the presence of anions P. 38 / 100

39 The silver nitrate solution should be acidified by adding dilute nitric acid. Nitric acid dissolves any precipitates formed from the reaction between silver nitrate and other anions like carbonate ions and sulphite ions. Ag 2 CO 3 (s) + 2HNO 3 (aq) 2AgNO 3 (aq) + CO 2 (g) + H 2 O(l) precipitate dissolved by nitric acid Ag 2 SO 3 (s) + 2HNO 3 (aq) 2AgNO 3 (aq) + SO 2 (g) + H 2 O(l) precipitate dissolved by nitric acid AgCl(s) + HNO 3 (aq) No reaction precipitate remains insoluble Concept check 61.4 Detecting the presence of anions P. 39 / 100

40 Test for carbonate ions Heating Most carbonates (except K 2 CO 3, Na 2 CO 3 ) decompose upon heating. They give carbon dioxide as a product. strong heating MgCO 3 (s) MgO(s) + CO 2 (g) colourless gas heating CaCO 3 (s) CaO(s) + CO 2 (g) colourless gas 61.4 Detecting the presence of anions P. 40 / 100

41 Learning tip Hydrogencarbonates (e.g. Mg(HCO 3 ) 2 ) will also decompose upon heating, giving carbon dioxide as a product. Ammonium carbonate decomposes to ammonia, carbon dioxide and water vapour when heating. heating (NH 4 ) 2 CO 3 (s) 2NH 3 (g) + CO 2 (g) + H 2 O(g) 61.4 Detecting the presence of anions P. 41 / 100

42 Using dilute hydrochloric acid Dilute hydrochloric acid reacts with carbonates to produce carbon dioxide. CaCO 3 (s) + 2HCl(aq) CaCl 2 (aq) + CO 2 (g) + H 2 O(l) Learning tip colourless gas Sulphuric acid is not used in the test for carbonate ions in calcium carbonate. This is because the formation of sparingly soluble CaSO 4 will prevent further reaction Detecting the presence of anions P. 42 / 100

43 Test for hypochlorite ions Using litmus paper The bleaching action of a bleach is due to the oxidizing power of hypochlorite ions (OCl ). OCl (aq) + dye Cl (aq) + (dye + O) coloured colourless a solution containing OCl (aq) ions blue litmus paper turns white Figure Action of hypochlorite ions on a piece of blue litmus paper Detecting the presence of anions P. 43 / 100

44 Using dilute hydrochloric acid If dilute HCl is added to a solution containing OCl (aq) ions, chlorine (with pungent smell of bleaching solution) will be produced. 2H + (aq) + Cl (aq) + OCl (aq) Cl 2 (g) + H 2 O(l) greenish yellow gas 61.4 Detecting the presence of anions P. 44 / 100

45 Test for sulphite ions Using oxidizing agents Sulphite ion is a strong reducing agent. A strong oxidizing agent is used to test for it. Oxidizing agent Colour change during test Ionic equation Br 2 (aq) Na 2 SO 3 (aq) Br 2 (aq) yellowish brown colourless Table Tests for sulphite ions by some oxidizing agents Detecting the presence of anions SO 3 2 (aq) + Br 2 (aq) + H 2 O(l) SO 4 2 (aq) + 2Br (aq) + 2H + (aq) P. 45 / 100

46 Oxidizing agent Colour change during test Ionic equation Cr 2 O 7 2 (aq) /H + (aq) Na 2 SO 3 (aq) Cr 2 O 2 7 (aq) /H + (aq) orange green 3SO 3 2 (aq) + Cr 2 O 7 2 (aq) + 8H + (aq) 3SO 4 2 (aq) + 2Cr 3+ (aq) + 4H 2 O(l) MnO 4 (aq) /H + (aq) Na 2 SO 3 (aq) MnO 4 (aq) /H + (aq) purple colourless 5SO 3 2 (aq) + 2MnO 4 (aq) + 6H + (aq) 5SO 4 2 (aq) + 2Mn 2+ (aq) + 3H 2 O(l) Table Tests for sulphite ions by some oxidizing agents Detecting the presence of anions P. 46 / 100

47 Using dilute hydrochloric acid If dilute HCl is added to a solution containing SO 3 2 (aq) ions, sulphur dioxide (with choking smell of burning sulphur) will be produced. SO 3 2 (aq) + 2H + (aq) SO 2 (g) + H 2 O(l) Example 61.4 Experiment 61.1 Experiment Detecting the presence of anions P. 47 / 100

48 Identifying an unknown anion in a sample of solution Cl, Br, I, CO 3 2, OCl or SO 3 2 Add acidified AgNO 3 (aq) Does a precipitate form? No CO 3 2, OCl or SO 3 2 Yes white precipitate pale yellow precipitate yellow precipitate Cl Br I Figure A flow chart showing how to identify an unknown anion in a sample solution Detecting the presence of anions P. 48 / 100

49 CO 3 2, OCl or SO 3 2 Does acidified K 2 Cr 2 O 7 (aq) change from orange to green? No Add acidified K 2 Cr 2 O 7 (aq) Yes SO 3 2 CO 3 2 or OCl Does the litmus paper turn white? No CO 3 2? Test the bleaching power by adding it on a piece of litmus paper Add dilute HCl(aq) Yes 61.4 Detecting the presence of anions OCl Is any colourless gas given off? Yes Test the gas with limewater Class practice 61.4 CO 3 2 Yes Does the limewater turn milky? P. 49 / 100

50 61.5 Detecting the presence of common gases and water Test for hydrogen Hydrogen is a colourless and odourless gas. It is the lightest gas and almost insoluble in water. It can be tested with a burning splint. It burns with a pop sound to produce water. 2H 2 (g) + O 2 (g) 2H 2 O(l) P. 50 / 100

51 Test for oxygen Oxygen is a colourless and odourless gas. It is only slightly soluble in water. It relights a glowing splint. glowing splint gas to be tested glowing splint relighted Figure Testing for oxygen with a glowing splint. Oxygen relights the glowing splint Detecting the presence of common gases and water P. 51 / 100

52 Test for carbon dioxide Carbon dioxide is a colourless and odourless gas. It dissolves in water to give a slightly acidic solution. CO 2 (g) + H 2 O(l) H 2 CO 3 (aq) carbonic acid It can be identified by bubbling it through limewater. It turns limewater milky quickly. Ca(OH) 2 (aq) + CO 2 (g) CaCO 3 (s) + H 2 O(l) colourless white precipitate 61.5 Detecting the presence of common gases and water P. 52 / 100

53 carbon dioxide CO 2 (g) white precipitate of CaCO 3 (s) limewater Figure Carbon dioxide turns limewater from colourless to milky Detecting the presence of common gases and water P. 53 / 100

54 Test for chlorine Chlorine is a greenish yellow gas and has a pungent smell of bleaching solution. When chlorine dissolves in water, hydrochloric acid and hypochlorous acid are produced. Cl 2 (aq) + H 2 O(l) HCl(aq) + HOCl(aq) SBA note Chlorine is toxic. Perform experiments involving chlorine in a fume cupboard Detecting the presence of common gases and water P. 54 / 100

55 Coloured substances can be bleached by chlorine water. The bleaching action is due to the oxidizing power of the hypochlorite ions (OCl ) in hypochlorous acid. Chlorine turns moist blue litmus paper red (by the action of H + (aq) ions) and then white very quickly (by the bleaching action of OCl (aq) ions). moist blue litmus paper moist blue litmus paper turns red, and then white Think about 61.5 Detecting the presence of common gases and water Figure Chlorine turns moist blue litmus paper red, and then white. P. 55 / 100

56 Test for ammonia Ammonia is a colourless gas with a characteristic pungent smell. It is extremely soluble in water to give an alkaline solution. NH 3 (aq) + H 2 O(l) NH 4+ (aq) + OH (aq) SBA note Ammonia is toxic. Perform experiments involving ammonia in a fume cupboard Detecting the presence of common gases and water P. 56 / 100

57 Ammonia turns moist red litmus paper blue. Figure Ammonia turns moist red litmus paper blue Detecting the presence of common gases and water P. 57 / 100

58 The presence of ammonia can be further confirmed by using concentrated hydrochloric acid. NH 3 (g) + HCl(g) NH 4 Cl(s) from NH 3 (aq) from HCl(aq) white white fume containing NH 4 Cl(s) HCl(aq) NH 3 (aq) Figure When ammonia from aqueous ammonia reacts with hydrogen chloride from concentrated hydrochloric acid, a dense white fume forms. (Note: Aqueous ammonia is irritant and concentrated hydrochloric acid is corrosive.) 61.5 Detecting the presence of common gases and water P. 58 / 100

59 Test for hydrogen chloride Hydrogen chloride is a colourless gas with a pungent smell. It is denser than air. It is highly soluble in water to give hydrochloric acid. It turns moist blue litmus paper red. moist blue litmus paper turns red hydrogen chloride 61.5 Detecting the presence of common gases and water Figure Hydrogen chloride turns moist blue litmus paper red. (Note: Hydrogen chloride gas is corrosive. Perform this experiment in fume cupboard.) P. 59 / 100

60 The presence of hydrogen chloride can be further confirmed by using ammonia. When it is placed near ammonia, a dense white fume forms. Test for sulphur dioxide Sulphur dioxide is a colourless gas with a choking smell of burning sulphur. It is denser than air. SBA note Sulphur dioxide is toxic. Perform experiments involving sulphur dioxide in a fume cupboard Detecting the presence of common gases and water P. 60 / 100

61 Sulphur dioxide turns a piece of filter paper soaked with acidified potassium dichromate solution from orange to green. Cr 2 O 7 2 (aq) + 3SO 2 (aq) + 2H + (aq) 2Cr 3+ (aq) + 3SO 4 2 (aq) + H 2 O(l) orange green filter paper soaked with acidified K 2 Cr 2 O 7 (aq) the paper turns from orange to green Figure Sulphur dioxide turns a piece of filter paper soaked with acidified potassium dichromate solution from orange to green Detecting the presence of common gases and water P. 61 / 100

62 Sulphur dioxide turns limewater milky. Ca(OH) 2 (aq) + SO 2 (g) CaSO 3 (s) + H 2 O(l) colourless white precipitate Carbon dioxide is odourless while sulphur dioxide has a strong smell of burning sulphur Detecting the presence of common gases and water P. 62 / 100

63 Test for water Using dry cobalt(ii) chloride paper Add a few drops of the liquid under test to blue dry cobalt(ii) chloride paper. The paper turns pink if the liquid contains water. This method can test for the presence of water vapour. dry cobalt(ii) chloride paper Figure Water turns dry cobalt(ii) chloride paper from blue to pink. water 61.5 Detecting the presence of common gases and water P. 63 / 100

64 Using anhydrous copper(ii) sulphate Add a few drops of the liquid under test to white anhydrous copper(ii) sulphate powder. The powder turns blue if water is present. CuSO 4 (s) + 5H 2 O(l) CuSO 4 5H 2 O(s) white blue anhydrous copper(ii) sulphate Example 61.5 Class practice Detecting the presence of common gases and water Figure Water turns anhydrous copper(ii) sulphate powder from white to blue. P. 64 / 100

65 61.6 Deducing the chemical nature of a salt If a salt is provided, various chemical tests are carried out to identify the cations and anions it contains. Example 61.6 Class practice 61.6 Experiment 61.2 P. 65 / 100

66 61.7 Detecting the presence of various functional groups in carbon compounds Test for carbon-carbon double bond (C=C) Reaction with bromine When an alkene is added to bromine (dissolved in an organic solvent), the colour of bromine changes from red-orange to colourless i.e. bromine solution is decolorized. Learning tip To distinguish alkanes and alkenes by using bromine, carry out the test in shade (not in direct sunlight). In case of alkenes, bromine is decolorized rapidly. For alkanes, the colour of bromine remains unchanged P. 66 / 100

67 Example hex-1-ene + bromine (dissolved in organic solvent) red-orange 1,2-dibromohexane colourless hex-1-ene Br 2 in organic solvent bromine solution decolorized Figure Bromine solution is decolorized when hex-1-ene is added to it Detecting the presence of various functional groups in carbon compounds P. 67 / 100

68 Reaction with acidified potassium permanganate solution When an alkene is added to acidified potassium permanganate solution, the colour of the solution changes from purple to colourless. Learning tip Alkenes can be tested by acidified potassium permanganate solution but not acidified potassium dichromate solution Detecting the presence of various functional groups in carbon compounds P. 68 / 100

69 Example + + hex-1-ene from acidified KMnO 4 (aq) (as an oxidizing agent) hexane-1,2,-diol (hexane-1,2-diol can be further oxidized) hex-1-ene acidified KMnO 4 (aq) KMnO 4 decolorized Figure Acidified potassium permanganate solution is decolorized when hex-1-ene is added to it Detecting the presence of various functional groups in carbon compounds P. 69 / 100

70 Test for hydroxyl group ( OH) Reaction with acidified potassium dichromate solution Acidified potassium dichromate solution changes from orange to green when a primary or secondary alcohol is added to it. Learning tip There is no reaction between acidified potassium dichromate solution and tertiary alcohol Detecting the presence of various functional groups in carbon compounds P. 70 / 100

71 The orange Cr 2 O 7 2 (aq) ions are reduced to green Cr 3+ (aq) ions as the alcohol is oxidized. ethanol acidified potassium dichromate solution solution containing Cr 3+ (aq) ions Figure Acidified potassium dichromate solution changes from orange to green when ethanol is added to it Detecting the presence of various functional groups in carbon compounds P. 71 / 100

72 For 1 alcohol from oxidizing agent [O] [O] 1 alcohol aldehyde carboxylic acid [O] [O] e.g. CH 3 CH 2 OH CH 3 CHO CH 3 COOH ethanol ethanal ethanoic acid 61.7 Detecting the presence of various functional groups in carbon compounds P. 72 / 100

73 For 2 alcohol [O] [O] no reaction 2 alcohol ketone [O] [O] e.g. CH 3 CH(OH)CH 3 CH 3 COCH 3 no reaction propan-2-ol propanone Class practice Detecting the presence of various functional groups in carbon compounds P. 73 / 100

74 Reaction with carboxylic acids When an alcohol is heated with a carboxylic acid in the presence of concentrated sulphuric acid, esterification takes place. During the reaction, an ester forms. The ester gives a characteristic pleasant fruity smell. conc. H 2 SO heat carboxylic acid + alcohol ester + water 61.7 Detecting the presence of various functional groups in carbon compounds P. 74 / 100

75 Test for carbonyl group ( C=O) in aldehydes and ketones Reaction with 2,4-dinitrophenylhydrazine 2,4-dinitrophenylhydrazine reacts with aldehyde or ketone to give yellow, orange or red precipitate of 2,4-dinitrophenylhydrazone. This reaction is used to test for the presence of carbonyl group ( C=O). ketone or aldehyde 2,4-dinitrophenylhydrazine 2,4-dinitrophenylhydrazone 61.7 Detecting the presence of various functional groups in carbon compounds P. 75 / 100

76 Example 1 ethanal 2,4-dinitrophenylhydrazine ethanal 2,4-dinitrophenylhydrazone yellow precipitate yellow precipitate of ethanal 2,4-dinitrophenylhydrazone Figure Ethanal 2,4-dinitrophenylhydrazone forms when ethanal reacts with 2,4-dinitrophenylhydrazine Detecting the presence of various functional groups in carbon compounds P. 76 / 100

77 Example 2 propanone 2,4-dinitrophenylhydrazine propanone 2,4-dinitrophenylhydrazone orange-yellow precipitate orange-yellow precipitate of propanone 2,4-dinitrophenylhydrazone Figure Propanone 2,4-dinitrophenylhydrazone forms when propanone reacts with 2,4-dinitrophenylhydrazine Detecting the presence of various functional groups in carbon compounds P. 77 / 100

78 Reaction with Tollens reagent Tollens reagent is used to distinguish between an aldehyde and a ketone. It is a specific test for aldehydes as Tollens reagent has no reaction with ketones. Tollens reagent is an aqueous solution of silver nitrate in excess ammonia. It contains the diamminesilver(i) ion, [Ag(NH 3 ) 2 ] +, in an alkaline solution Detecting the presence of various functional groups in carbon compounds P. 78 / 100

79 When an aldehyde reacts with Tollens reagent, a silver mirror forms inside the test tube. silver mirror Learning tip Figure Ethanal reacts with Tollens reagent to form a silver mirror. Practically, it is possible to coat silver metal (like a mirror) on the inner wall of the test tube. However, grey or black precipitate forms if the inner wall of the test tube is not clean enough Detecting the presence of various functional groups in carbon compounds P. 79 / 100

80 During the reaction, diamminesilver(i) ion is reduced to silver by the aldehyde. RCHO + 2[Ag(NH 3 ) 2 ] + + 3OH RCOO + 2Ag + 4NH 3 + 2H 2 O diamminesilver(i) ion silver mirror Example CH 3 CHO + 2[Ag(NH 3 ) 2 ] + + 3OH CH 3 COO + 2Ag + 4NH 3 + 2H 2 O ethanal diamminesilver(i) ion ethanoate ion silver mirror 61.7 Detecting the presence of various functional groups in carbon compounds P. 80 / 100

81 Test for carboxyl group ( COOH) Reaction with sodium carbonate/sodium hydrogencarbonate solution When a carboxylic acid reacts with sodium hydrogencarbonate or sodium carbonate solution to form carbon dioxide which turns limewater milky. 2RCOOH + Na 2 CO 3 2RCOO Na + + H 2 O + CO 2 sodium carbonate RCOOH + NaHCO 3 RCOO Na + + H 2 O + CO 2 sodium hydrogencarbonate Think about 61.7 Detecting the presence of various functional groups in carbon compounds P. 81 / 100

82 Reaction with alcohol A carboxylic acid reacts with an alcohol in the presence of concentrated sulphuric acid. After heating, an ester with a characteristic pleasant fruity smell will form. Example 61.7 Example 61.8 Class practice 61.8 Experiment 61.3 Experiment Detecting the presence of various functional groups in carbon compounds P. 82 / 100

83 61.8 Possible risks associated with chemical tests Risk assessment Hazardous nature Hazard warning label Hazardous nature Hazard warning label Explosive Toxic or Oxidizing or Harmful or Flammable or Corrosive or Carcinogenic Irritant or P. 83 / 100

84 Procedure for carrying out risk assessment 1. Identify the hazards (hazardous chemicals used or made, hazardous equipment or procedure). 2. Assess how likely the hazard will cause harm and how serious that harm could be. 3. Decide what safety precautions should be taken to reduce the risk to an acceptable level. For example, (a) use a smaller amount of chemicals or a more dilute solution, (b) use a less hazardous chemical, (c) perform the experiment in the fume cupboard, (d) use personal protective equipment Possible risks associated with chemical tests P. 84 / 100

85 safety spectacles laboratory coat glove Figure Personal protective equipment for performing experiments. 4. Find out how to dispose of the hazardous residues after performing the experiment Possible risks associated with chemical tests P. 85 / 100

86 Risk assessment form Risk assessments are made on a standard form. Example 61.9 Class practice 61.9 Activity Possible risks associated with chemical tests P. 86 / 100

87 Key terms 1. 2,4-dinitrophenylhydrazine 2,4- 二硝基苯肼 2. 2,4-dinitrophenylhydrazone 2,4- 二硝基苯腙 3. qualitative analysis 定性分析 4. quantitative analysis 定量分析 5. risk assessment 風險評估 6. risk assessment form 風險評估表 7. Tollens reagent 托倫斯試劑 P. 87 / 100

88 Progress check 1. How do we deduce the ions present in a sample by observing the colour? 2. How do we make a guess of the identity of a substance by smelling? 3. What are the examples of water-soluble and water-insoluble ionic compounds? 4. What are the examples of acidic, neutral and basic compounds? 5. What are the effects of heat on some ionic compounds? 6. What are the flame colours of compounds of potassium, sodium, calcium and copper? P. 88 / 100

89 7. How do we test for the presence of cations in a solution? 8. How do we test for the presence of anions in a solution? 9. How do we detect the presence of common gases and water by chemical methods? 10.How do we detect the presence of various functional groups in carbon compounds? 11.How do we assess the risks associated with chemical tests? Progress check P. 89 / 100

90 Summary 61.1 Preliminary tests of substances 1. The colour of an ionic compound is usually determined by the ions it contains. Refer to Table 61.2 on p.4 for the colours of some ions in aqueous solutions. 2. Some ionic compounds are soluble in water but others are not. Refer to Table 61.4 on p The ph values of some compounds can be found by using ph paper. Refer to Table 61.5 on p.5 for examples of acidic, neutral and basic compounds. P. 90 / 100

91 4. The action of heat on a solid sample can provide useful information on the identity of the solid. Refer to Table 61.6 on p Detecting the presence of metallic ions in substances using the flame test 5. We can infer the presence of certain metals or their compounds by the flame test. Refer to Table 61.7 on p Detecting the presence of cations 6. Refer to Table S61.1 on p.46 for the summary of some tests for certain cations. Summary P. 91 / 100

92 61.4 Detecting the presence of anions 7. Refer to Table S61.2 on p.47 for the summary of some tests for certain anions Detecting the presence of common gases and water 8. Refer to Table S61.3 on p.48 for the summary of some tests for common gases and water Deducing the chemical nature of a salt 9. If we are provided with a salt, we can carry out various chemical tests to identify the cations and anions it contains. Summary P. 92 / 100

93 61.7 Detecting the presence of various functional groups in carbon compounds 10. Refer to Table S61.4 on p.49 for the summary of some tests for various functional groups in carbon compounds Possible risks associated with chemical tests 11. Risk assessment is a systematic way of identifying the potential hazards in an experiment and deciding the preventive measures to be taken to control these risks. Its procedure can be simplified by completing a risk assessment form. Summary P. 93 / 100

94 Concept map Colour Smell Solubility In water ph of aqueous solution Action of heat on solid sample Preliminary test IDENTIFY AN UNKNOWN SUBSTANCE Test for cations Test for anions P. 94 / 100

95 Test for cations Flame test Test with NaOH(aq) Test with NH 3 (aq) K + Na + Ca 2+ Cu 2+ Lilac Golden yellow Brick-red Bluish green Concept map P. 95 / 100

96 Test for anions Halide Carbonate Hypochlorite Sulphite acidified AgNO 3 Forms precipitate strong heating* Decomposes to give CO 2 dilute acids Gives CO 2 litmus paper Paper turns white HCl(aq) Gives Cl 2 K 2 Cr 2 O 7 (aq)/ H + (aq) Solution changes from orange to green HCl(aq) Gives SO 2 Concept map *except K 2 CO 3 and Na 2 CO 3 P. 96 / 100

97 Burns with a pop sound Relights Turns red and then white Turns milky burning splint glowing splint moist blue litmus paper limewater Hydrogen Oxygen Chlorine Carbon dioxide IDENTIFY AN UNKNOWN SUBSTANCE Concept map P. 97 / 100

98 IDENTIFY AN UNKNOWN SUBSTANCE Ammonia Hydrogen chloride Sulphur dioxide Water moist red litmus paper Turns blue conc. HCl Forms a dense white fume moist blue litmus paper Turns red Solution changes from orange to green NH 3 (aq) K 2 Cr 2 O 7 (aq)/ H + (aq) Forms a dense white fume limewater anhydrous copper(ii) sulphate powder Changes from white to blue Turns milky dry cobalt(ii) chloride paper Changes from blue to pink Concept map P. 98 / 100

99 Turns from red-orange to colourless Turns from purple to colourless Solution changes from orange to green Forms ester Br 2 (dissolved in organic solvent) KMnO 4 (aq)/ H + (aq) K 2 Cr 2 O 7 (aq)/ H + (aq) heat with carboxylic acid Carbon-carbon double bond Hydroxyl group IDENTIFY AN UNKNOWN SUBSTANCE P. 99 / 100

100 IDENTIFY AN UNKNOWN SUBSTANCE Carbonyl group Carboxyl group 2,4-dinitrophenylhydrazine Tollens reagent Na 2 CO 3 (aq)/ NaHCO 3 (aq) heat with alcohol Forms orange/ yellow/red precipitate Forms a silver mirror inside the test tube Gives CO 2 Forms ester P. 100 / 100