Tests for ions
- Created by: [email protected]
- Created on: 22-02-19 09:16
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- Tests for ions
- Negative ions
- Halides
- Dissolve the
halide in water
if not already
aqueous, add
some dilute
nitric acid
and aqueous
silver nitrate
- Coloured precipitate will form, if hard to distinguish add dilute NH3 then concentrated NH3, note the solubility
- Cl-
- With AgNO3(aq) Cl-
= white
precipitate
- Solubility of precipitate = soluble in dilute NH3
- Ag+(aq) + Cl- (aq) --> AgCl(s)
- With AgNO3(aq) Cl-
= white
precipitate
- Br-
- With AgNO3(aq)
= cream
precipitate
- Ag+(aq) + Br- (aq) --> AgBr(s)
- Solubility of precipitate = soluble in concentrated NH3(aq) only
- With AgNO3(aq)
= cream
precipitate
- I-
- With AgNO3(aq)
= yellow
precipitate
- Ag+(aq) + I- (aq) --> AgI(s)
- Solubility of precipitate = insoluble in both dilute and concentrated NH3(aq)
- With AgNO3(aq)
= yellow
precipitate
- Dissolve the
halide in water
if not already
aqueous, add
some dilute
nitric acid
and aqueous
silver nitrate
- Sulfate ions,
(SO4)2-
- Half fill a test
tube with
the sample,
add dilute HCl(aq) followed by BaCl2(aq)
- White precipitate
if (SO4)2-
is present
- Ba2+(aq) + (SO4)2-(aq) - > BaSO4(s)
- White precipitate
if (SO4)2-
is present
- Half fill a test
tube with
the sample,
add dilute HCl(aq) followed by BaCl2(aq)
- Carbonate ions, (CO3)2-
- Half fill a test
tube with
sample, add dilute HCl(aq)
- collect bubbles
and pass
it through
limewater
- If gas turns limewater cloudy, (CO3)2- is present
- (CO3)2-(s) + 2H+(aq) -- > H2O(l) + CO2(g)
- collect bubbles
and pass
it through
limewater
- Half fill a test
tube with
sample, add dilute HCl(aq)
- Hydroxides, OH-
- Will turn damp red litmus paper blue
- Nitrate(V) ions, NO3-(aq)
- Warm solution with NaOH(aq) and either aluminium foil or Devardas's alloy
- Aluminium reduces the NO3-(aq) to produce ammonia gas, NH3(g)
- Ammonia turns damp red litmus paper blue
- 3NO3-(aq) + 8Al(s) + 5OH-(aq) +18H2O(l) --> 3NH3(g) + 8[Al(OH)4]-(aq)
- Aluminium reduces the NO3-(aq) to produce ammonia gas, NH3(g)
- Warm solution with NaOH(aq) and either aluminium foil or Devardas's alloy
- Halides
- Positive ions
- Ammonium, NH4+(aq)
- Fill half of a
test tube, add
NaOH(aq) and
warm gently
- If ammonia gas, NH3(g), given out then ammonium ions are present
- NH3(g) turns damp red litmus paper blue
- NH4+(aq) + OH-(aq) -- > NH3(g) + H2O(l)
- If ammonia gas, NH3(g), given out then ammonium ions are present
- Fill half of a
test tube, add
NaOH(aq) and
warm gently
- Metal ions
- Flame tests
- Dip dichrome wire loop in conc HCl(aq). Dip wire loop in sample. Hold loop in clear blue part of flame and observe colour change.
- Li+ = crimson
- Na+ = yellow
- K+ = lilac
- Ca2+ = Brick red
- Ba2+ = green
- Cu2+ = blue
- Metal hydroxides
- Add NaOH(aq) drop by drop. An insoluble metal hydroxide will form
- Ag+ =brown
- Ag+(aq) +2OH-(aq) --> Ag2O(s) +H2O(l)
- Ca2+ = white
- Ca2+(aq) + 2OH-(aq) --> Ca(OH)2(s)
- Cu2+ = blue
- Cu2+(aq) + 2OH-(aq) --> Cu(OH)2(s)
- Pb2+ =white
- Pb2+(aq) + 2OH-(aq) --> Pb(OH)2(s)
- Fe2+ = green
- Fe2+(aq) + 2OH-(aq) --> Fe(OH)2(s)
- Fe3+ = brown
- Fe3+(aq) + 3OH-(aq) --> Fe(OH)3(s)
- Zn2+ = white then redissolves in excess NaOH(aq) to form colourless solution
- Zn2+(aq) + 2OH-(aq) --> Zn(OH)2(s) then Zn(OH)2(s) +OH-(aq) -->Zn(OH)3-(aq)
- Al3+ = white then redissolves in excess NaOH(aq) to form colourless solution
- Al3+(aq) + 3OH-(aq) --> Al(OH)3(s) then Al(OH)3(s) +OH-(aq) -->Al(OH)4-(aq)
- Flame tests
- Ammonium, NH4+(aq)
- Negative ions
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