Chemical tests - 4th Flashcards
how do you test for hydrogen (method, result, chemical reaction and chemical equation)?
- 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)
how do you test for oxygen (method, result and chemical reaction)?
- 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
how do you test for the presence of water (method and result)?
- use the copper sulfate test
METHOD:
1. add anhydrous copper sulfate (CuSO4)
RESULT: - changes colour from white to blue
how do you test for the purity of water (method and result)?
- measure its boiling point and compare it to 100 degrees celcius
METHOD: - measure its boiling point
RESULT: - closer to 100 degrees celcius = more pure
how do you test for carbon dioxide (method, result, chemical reaction and chemical equation)?
- 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)
how do you test for chlorine (method, result and chemical reaction)?
- 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
how do you test for ammonia (method, result, chemical reaction and chemical equation)?
- 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)
what are cations?
- positive ions
- e.g. Na+
which are the cations you need to know how to perform chemical tests on?
- Li+
- Na+
- K+
- Ca2+
- Cu2+
- Fe2+
- Fe3+
- NH4 +
how can we tell which metal cation is in a substance through flame tests?
- 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
how do you conduct a flame test?
- 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
what are the results of the flame test?
- red = Li+
- yellow = Na+
- lilac = K+
- orange-red = Ca2+
- blue-green = Cu2+
how can we use sodium hydroxide to test for metal cations?
- use sodium hydroxide to test for Cu2+, Fe2+ and Fe3+
- if one of these is present, a metal hydroxide precipitate forms
how is a NaOH test conducted?
- dissolve the sample in water
- add a few drops of NaOH (aq)
what are the results of the NaOH test (precipitate colour, formula and metal cation)
- blue = Cu(OH)2 = Cu2+
- green = Fe(OH)2 = Fe2+
- brown = Fe(OH)3 = Fe3+
what equation could you write for the NaOH test?
- 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)
what are anions?
- negative ions, such as O2-
what are the anions you need to know how to perform chemical tests on?
- Cl-
- Br-
- I-
- CO3 2-
- SO4 2-
how can we use the silver nitrate test to test for halide ions (Cl-, Br- and I-)?
- 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
what are the results of the silver nitrate test (precipitate colour, formula and halide ion)?
- white = AgCl = Cl -
- cream = AgBr = Br-
- yellow = AgI = I-
how do you write an equation for the formation of the precipitate in the silver nitrate test?
- Ag + (aq) + Cl- (aq) -> AgCl(s)
why is it important to add the HNO3 in step 2 of the silver nitrate test?
- because Ag+ ions also form precipitates with other anions, e.g. OH- and CO3 2-
- the HNO3 reacts with those anions, preventing false positives
how can we test for carbonates?
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)
why is it important to confirm the gas evolved is CO2 using limewater in the carbonate test?
- because acids also evolve a gas when they react with some other things
- checking that gas is CO2 prevents false positives
how can we test for sulfates?
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)
why is it important to add the HCl in step one of the test for sulfates?
- 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
