Chemical tests - 4th

• using the squeaky pop test METHOD: 1. collect a sample of gas in a test tube 2. attempt to ignite the gas using a lit splint RESULT: • squeaky pop sound CHEMICAL REACTION: • combustion of H2 to form H2O • pop sound caused by hot gas rushing out of test tube CHEMICAL EQUATION: • 2H2 (g) -> O2 (g) -> 2H2O (g)

• using the glowing splint test METHOD: 1. collect a sample of gas in a test tube 2. place a glowing splint into the tube RESULT: • the splint relights CHEMICAL REACTION: • combustion of wood in the splint • it relights because pure O2 is more concentrated than the O2 in normal air • this means the wood in the splint combusts faster

• use the copper sulfate test METHOD: 1. add anhydrous copper sulfate (CuSO4) RESULT: • changes colour from white to blue

• measure its boiling point and compare it to 100 degrees celcius METHOD: • measure its boiling point RESULT: • closer to 100 degrees celcius = more pure

• using the limewater test METHOD: 1. bubble a sample of gas through limewater RESULT: • a white precipitate forms (i.e. it turns cloudy) CHEMICAL REACTION: • limewater is saturated calcium hydroxide solution (Ca(OH)2) • it forms a white precipitate of calcium carbonate (CaCO3) when it reacts with CO2 CHEMICAL EQUATION: • Ca(OH)2(aq) + CO2 (g) -> CaCO3(s) + H2O (l)

• using the damp blue litmus test METHOD: 1. place damp blue litmus paper in a sample of gas RESULT: • it turns white (bleaches) CHEMICAL REACTION: • Cl2 is a good bleaching agent -> easily removes colour from other substances • in this test, it dissolves into the dampness (H2O) and removes the colour from blue litmus

• using the damp red litmus test METHOD: 1. place damp red litmus paper in a sample of gas RESULT: • it turns blue CHEMICAL REACTION: • it turns blue because the NH3 reacts with the dampness (H2O) to form OH- ions • the NH3 acts as a base by accepting H+ ions from the H2O • the OH- ions are what’s left after H2O has lost H+, and they’re alkaline so the litmus turns blue CHEMICAL EQUATION: • NH3 (g) + H2O (l) -> NH4+ (aq) + OH- (aq)

• positive ions • e.g. Na+

• Li+ • Na+ • K+ • Ca2+ • Cu2+ • Fe2+ • Fe3+ • NH4 +

• many metal cations cause a non-luminous (blue) bunsen flame to change colour • by looking at that colour, we can tell which metal cation is in a substance

• dip a piece of nichrome/platinum wire into hydrochloric acid. these wires don’t colour the flame, and acid cleans the wire • dip the wire into a sample of solid or solution. this causes some of the sample to stick onto the wire. • place the tip of the wire into a non-luminous bunsen flame

• red = Li+ • yellow = Na+ • lilac = K+ • orange-red = Ca2+ • blue-green = Cu2+

• use sodium hydroxide to test for Cu2+, Fe2+ and Fe3+ • if one of these is present, a metal hydroxide precipitate forms

• dissolve the sample in water • add a few drops of NaOH (aq)

• blue = Cu(OH)2 = Cu2+ • green = Fe(OH)2 = Fe2+ • brown = Fe(OH)3 = Fe3+

• a fully balanced equation, e.g: CuCl2 (aq) + 2NaOH (aq) -> Cu(OH)2 (s) + 2NaCl(aq) • or you could write an ionic equation: Cu2+ (aq) + 2OH- (aq) -> Cu(OH)2 (s)

• negative ions, such as O2-

• Cl- • Br- • I- • CO3 2- • SO4 2-

• dissolve the sample in water • add nitric acid (HNO3) • add silver nitrate (AgNO3) solution RESULT: - if Cl-, Br- or I- ions are present, a precipitate forms with the Ag+ ions from the AgNO3

• white = AgCl = Cl - • cream = AgBr = Br- • yellow = AgI = I-

• Ag + (aq) + Cl- (aq) -> AgCl(s)

• because Ag+ ions also form precipitates with other anions, e.g. OH- and CO3 2- • the HNO3 reacts with those anions, preventing false positives

METHOD: 1. add hydrochloric acid (HCl), look for effervescence 2. if there is effervescence, bubble the gas through limewater RESULTS: - if CO3 2- ions were present in the sample, the gas evolved turns the limewater cloudy REACTION EQUATION: - CaCO3 (s) + 2HCl (aq) -> CaCl2 (aq) + H2O (l) + CO2 (g)

• because acids also evolve a gas when they react with some other things • checking that gas is CO2 prevents false positives