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GCSE Chemistry > Chemical tests - 4th > Flashcards

Chemical tests - 4th Flashcards

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1
Q

how do you test for hydrogen (method, result, chemical reaction and chemical equation)?

A
  • 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)
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2
Q

how do you test for oxygen (method, result and chemical reaction)?

A
  • 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
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3
Q

how do you test for the presence of water (method and result)?

A
  • use the copper sulfate test
    METHOD:
    1. add anhydrous copper sulfate (CuSO4)
    RESULT:
  • changes colour from white to blue
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4
Q

how do you test for the purity of water (method and result)?

A
  • measure its boiling point and compare it to 100 degrees celcius
    METHOD:
  • measure its boiling point
    RESULT:
  • closer to 100 degrees celcius = more pure
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5
Q

how do you test for carbon dioxide (method, result, chemical reaction and chemical equation)?

A
  • 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)
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6
Q

how do you test for chlorine (method, result and chemical reaction)?

A
  • 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
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7
Q

how do you test for ammonia (method, result, chemical reaction and chemical equation)?

A
  • 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)
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8
Q

what are cations?

A
  • positive ions
  • e.g. Na+
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9
Q

which are the cations you need to know how to perform chemical tests on?

A
  • Li+
  • Na+
  • K+
  • Ca2+
  • Cu2+
  • Fe2+
  • Fe3+
  • NH4 +
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10
Q

how can we tell which metal cation is in a substance through flame tests?

A
  • 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
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11
Q

how do you conduct a flame test?

A
  1. dip a piece of nichrome/platinum wire into hydrochloric acid. these wires don’t colour the flame, and acid cleans the wire
  2. dip the wire into a sample of solid or solution. this causes some of the sample to stick onto the wire.
  3. place the tip of the wire into a non-luminous bunsen flame
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12
Q

what are the results of the flame test?

A
  • red = Li+
  • yellow = Na+
  • lilac = K+
  • orange-red = Ca2+
  • blue-green = Cu2+
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13
Q

how can we use sodium hydroxide to test for metal cations?

A
  • use sodium hydroxide to test for Cu2+, Fe2+ and Fe3+
  • if one of these is present, a metal hydroxide precipitate forms
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14
Q

how is a NaOH test conducted?

A
  1. dissolve the sample in water
  2. add a few drops of NaOH (aq)
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15
Q

what are the results of the NaOH test (precipitate colour, formula and metal cation)

A
  • blue = Cu(OH)2 = Cu2+
  • green = Fe(OH)2 = Fe2+
  • brown = Fe(OH)3 = Fe3+
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16
Q

what equation could you write for the NaOH test?

A
  • 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)
17
Q

what are anions?

A
  • negative ions, such as O2-
18
Q

what are the anions you need to know how to perform chemical tests on?

A
  • Cl-
  • Br-
  • I-
  • CO3 2-
  • SO4 2-
19
Q

how can we use the silver nitrate test to test for halide ions (Cl-, Br- and I-)?

A
  1. dissolve the sample in water
  2. add nitric acid (HNO3)
  3. add silver nitrate (AgNO3) solution
    RESULT:
    - if Cl-, Br- or I- ions are present, a precipitate forms with the Ag+ ions from the AgNO3
20
Q

what are the results of the silver nitrate test (precipitate colour, formula and halide ion)?

A
  • white = AgCl = Cl -
  • cream = AgBr = Br-
  • yellow = AgI = I-
21
Q

how do you write an equation for the formation of the precipitate in the silver nitrate test?

A
  • Ag + (aq) + Cl- (aq) -> AgCl(s)
22
Q

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

A
  • because Ag+ ions also form precipitates with other anions, e.g. OH- and CO3 2-
  • the HNO3 reacts with those anions, preventing false positives
23
Q

how can we test for carbonates?

A

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)

24
Q

why is it important to confirm the gas evolved is CO2 using limewater in the carbonate test?

A
  • because acids also evolve a gas when they react with some other things
  • checking that gas is CO2 prevents false positives
25
Q

how can we test for sulfates?

A

METHOD:
1. add hydrochloric acid (HCl) and look for effervescence
2. If there’s no effervescence, add barium chloride (BaCl2) solution
RESULTS:
- if SO4 2- ions are present, they will form a white precipitate with the Ba 2+ ions from the BaCl2
REACTION EQUATION:
- Ba 2+ (aq) + SO4 2- (aq) -> BaSO4 (s)

26
Q

why is it important to add the HCl in step one of the test for sulfates?

A
  • because carbonate (CO3 2-) ions also form a white precipitate with Ba 2+ ions
  • by continuing to step 2 if there’s no effervescence, we can prevent a false positive

GCSE Chemistry (15 decks)

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