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chemistry > 4.6 chemical analysis (tests) > Flashcards

4.6 chemical analysis (tests) Flashcards

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1
Q

4.6.1 what’s the test for hydrogen?

A

squeaky pop:
1. collect sample of gas in test tube
2. attempt to ignite the gas using a lit splint
3. squeaky pop sound

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2
Q

4.6.1 what is the chemical reaction taking place in the squeaky pop test?

A

the combustion of H2 to form H2O
2H2 (g) + O2 (g) —> 2H2O (g)

the pop sound caused by the hot gas rushing out of test tube

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3
Q

4.6.1 what’s the test for oxygen?

A

glowing splint test:
1. collect sample of gas in test tube
2. place glowing splint into tube
3. splint relights

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4
Q

4.6.1 what chemical reaction takes place in the glowing splint test?

A

the combustion of the wood in the splint

it relights bc pure O2 is more concentrated than the O2 in normal air. this means the wood in splint combusts faster

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5
Q

4.6.1 what’s the test for water?

A

presence of H2O: copper sulphate test
1. add anhydrous copper sulphate (CuSO4)
2. it changes colour from white to blue

purity of H2O: measure its bp & compare it to 100°C
1. measure its bp
2. closer to 100°C = more pure

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6
Q

4.6.1 what’s the test for carbon dioxide?

A

limewater test:
1. bubble a sample of gas through limewater
2. a white precipitate forms (i.e. it turns cloudy)

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7
Q

4.6.1 limewater is…

A

saturated calcium hydroxide solution, Ca(OH)2

it forms a white ppt of calcium carbonate (CaCO3) when it reacts with CO2:

Ca(OH)2 (aq) + CO2 (g) —> CaCO3 (s) + H2O (l)

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8
Q

4.6.1 what’s the test for chlorine?

A

damp blue litmus test:
1. place damp blue litmus paper in sample of gas
2. it turns white (bleaches)

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9
Q

4.6.1 why do we do the damp blue litmus paper test for Cl2?

A

Cl2 good bleaching agent, so easily removes colour from other substances. in this test, it dissolves into the dampness (H2O) & removes the colour from blue litmus.

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10
Q

4.6.1 what’s the test for ammonia?

A

damp red litmus paper:
1. place damp red litmus paper in sample of gas
2. it turns blue

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11
Q

4.6.1 why does ammonia (NH3) turn damp red litmus paper blue?

A

this happens bc the NH3 reacts w the dampness (H2O) to form OH- ions:
NH3 (g) + H2O (l) —> NH4 + (aq) + OH- (aq)

  • 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
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12
Q

4.6.2 what are cations

A

positive ions

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13
Q

4.6.2 how do you do the flame test?

A
  1. dip piece of nichrome/platinum wire into hydrochloric acid. these wires don’t colour the flame, and the acid cleans the wire
  2. dip wire into sample of solid or solution. this causes some of sample to stick onto wire
  3. place tip of wire into a non-luminous Bunsen flame (blue flame)
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14
Q

4.6.2 what are the results for the flame test?

A

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

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15
Q

4.6.2 what do we use to test for Cu2+, Fe2+ and Fe3+

A

sodium hydroxide (NaOH) - if one of those is present, a metal hydroxide ppt forms

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16
Q

4.6.2 how do you conduct a NaOH test?

A
  1. dissolve sample in water
  2. add few drops of NaOH (aq)
17
Q

4.6.2 what are the results for the NaOH test?

A

blue - ppt: Cu(OH)2 - Cu+
green - ppt: Fe(OH)2 - Fe2+
brown - ppt: Fe(OH)3 - Fe3+

18
Q

4.6.2 how do you test for ammonium?

A
  1. dissolve sample in water
  2. add some NaOH(aq) & warm it
  3. test the gas evolved with damp red litmus paper i.e. the ammonia test
  4. litmus paper goes red —> blue
19
Q

4.6.2 how does the ammonium test work?

A

it works bc the NH4 + (from sample) and OH- ions (from NaOH) react to produce NH3 (g):

NH4 + (aq) + OH- (aq) —> NH3 (g) + H2O (l)

20
Q

4.6.3 what are anions

A

negative ions

21
Q

4.6.3 how do you test for halide ions (Cl-, Br- and I-)?

A

silver nitrate test:
1. dissolve sample in water
2. add nitric acid (HNO3)
3. add silver nitrate (AgNO3) solution
4. if Cl-, Br- or I- ions are present, a ppt forms with the Ag+ ions from the AgNO3

22
Q

4.6.3 what are the results for the silver nitrate test?

A

ppt: white - ppt formula: AgCl - Cl-
ppt: cream - ppt formula: AgBr - Br-
ppt: yellow - ppt formula: AgI - I-

23
Q

4.6.3 why is it important to add the HNO3 in step 2 of the silver nitrate test?

A

bc Ag+ ions also form precipitates w other anions (e.g. OH- and CO3 2-). the HNO3 reacts w these anions, preventing false positives

24
Q

4.6.3 how do you test for carbonate?

A
  1. add hydrochloric acid (HCl), look for effervescence
  2. if there is effervescence, bubble gas through limewater
  3. if CO3 2- ions present, the gas evolved turns limewater cloudy
25
Q

4.6.3 what does the reaction equation look like for a carbonate test using calcium carbonate?

A

CaCO3 (s) + 2HCl (aq) —> CaCl2 (aq) + H2O (l) + CO2 (g)

26
Q

4.6.3 why is it important to confirm that the gas evolved from a carbonate test is CO2 using limewater?

A

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

27
Q

4.6.3 how do you test a sample for sulfate ions?

A
  1. add hydrochloric acid (HCl), look for effervescence
  2. if no effervescence, add barium chloride (BaCl2) solution
  3. if SO4 2- ions present, they’ll form a white ppt with the Ba2+ ions from the BaCl2
28
Q

4.6.3 what’s the equation for the sulphate test?

A

Ba2+ (aq) + SO4 2- (aq) —> BaSO4 (s)

29
Q

4.6.3 why is it important to add the HCl in step 1 of the sulfate test?

A

bc carbonate (CO3 2-) ions also form a white ppt with Ba2+ ions. by only continuing to step 2 if no effervescence, we can prevent a false positive

30
Q

4.6.5 how do you do paper chromatography?

A
  1. draw line near bottom of chromatography paper in pencil (pencil insoluble so won’t dissolve in solvent)
  2. add spots of diff dyes to the line at regular intervals
  3. place paper into beaker of solvent (make sure solvent below pencil line - otherwise inks will dissolve into solvent)
  4. place lid on beaker (to stop solvent evaporating)
  5. allow solvent to move up paper until it’s just below the top
  6. take paper out and mark on where solvent has reached (this called the solvent front)
  7. allow paper to dry
31
Q

4.6.5 how does chromatography work?

A

the dyes move up the paper at different rates:
- the more soluble the dye in the solvent - the further up the paper it’ll go
- if dye is insoluble it will not move from the baseline

32
Q

4.6.5 how do you calculate an Rf value? (chromatography)

A

Rf = distance moved by spot/distance moved by solvent

distance moved by spot = from baseline to middle of spot

Rf always < 1

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