Analysis - Unit 3 - Year 10 Chemistry Revision 2023

• Cation


  a positively charged ion
• Anion


  a negatively charged ion
• Testing for Cations


  1. Add Sodium hydroxide to the sample

  2. If no precipitates form, flame test or NH4+ test
• Testing for Fe2+


  1. Add NaOH to solution of Fe2+ compound

  2. Dark green gelatinous precipitate forms (turns brown on surface on standing due to aerial oxidation)

  
  

  Fe2+ (aq) + 2OH- (aq) --> Fe(OH)2 (s)
• Fe2+ Aerial Oxidation


  Fe2+ -(aerial oxidation)-> Fe3+
• Testing for Fe3+


  1. Add NaOH to solution of Fe3+ compound

  2. Brown gelatinous precipitate forms

  
  

  Fe3+ (aq) + 3OH- (aq) --> Fe(OH)3 (s)
• Testing for Cu2+ (Precipitation)


  1. Add NaOH to solution of Cu2+ compound

  2. Blue gelatinous precipitate forms

  
  

  Cu2+ (aq) + 2OH- (aq) --> Cu(OH)2 (s)
• Testing for Al3+


  1. Add NaOH to solution of Al3+ compound

  2. White precipitate forms and redissolves to give colourless solution

  
  

  Al3+ (aq) + 3OH- (aq) --> Al(OH)3 (s)
• Testing for Mg2+


  1. Add NaOH to solution of Mg2+ compound

  2. White precipitate forms but does not redissolve

  
  

  Mg2+ (aq) + 2OH- (aq) --> Mg(OH)2 (s)
• Testing for Ca2+ (Precipitation)


  1. Add NaOH to solution of Ca2+ compound

  2. White precipitate forms but does not redissolve

  
  

  Ca2+ (aq) + 2OH- (aq) --> Ca(OH)2 (s)
• Why can you not test for K+ or Na+ using NaOH?


  All potassium and sodium salts are soluble so no precipitate will form when NaOH is added (remains colourless)
• Testing for NH4+


  1. Add NaOH to solution of NH4+ compound (colourless solution forms as all NH4+ salts are soluble)

  2. When heated, a colourless, pungent alkaline gas is produced (NH3).

  3. Hold damp red litmus paper in gas.

  4. Litmus turns blue.

  
  

  NH4+ (aq) + OH- (aq) --> NH3 (g) + H2O (l)
• Chemical and Ionic Equations for Reaction Between Ammonium Chloride and Sodium Hydroxide

  NH4Cl (aq) + NaOH (aq) --> NaCl (aq) + NH3 (g) + H2O (l)

  NH4+ (aq) + OH- (aq) --> NH3 (g) + H2O (l)
• Testing for Li+


  1. Flame test

  2. Red flame
• Testing for Na+


  1. Flame test

  2. Yellow flame
• Testing for K+


  1. Flame test

  2. Lilac flame
• Testing for Ca2+


  1. Flame test

  2. Brick-red flame
• Testing for Ba2+


  1. Flame test

  2. Pale green flame
• Testing for Cu2+ (Flame)


  1. Flame test

  2. Green-blue flame
• Halides


  compound containing a halogen ion (F-, Cl-, Br-, I-) and one or more other elements
• Testing for Halides (Cl-, Br-, I-)


  Add dilute HNO3 and AgNO3 solution
• Why is HNO3 added when testing for halides?


  To remove any carbonate and hydroxide impurities that may be in the solution and that would form precipitates which could lead to false positive results for halide ions.

  
  

  Ag₂CO₃ is insoluble and yellow

  AgOH is insoluble and brown
• Testing for Cl-


  1. Add dilute HNO3 and AgNO3 solution to Cl- solution

  2. White precipitate forms (turns grey/purple on exposure to light)

  
  

  Ag+ (aq) + Cl- (aq) --> AgCl (s)

  
• Testing for Br-


  1. Add dilute HNO3 and AgNO3 solution to Br- solution

  2. Cream precipitate forms (turns grey/purple on exposure to light)

  
  

  Ag+ (aq) + Br- (aq) --> AgCl (s)

  
• Testing for I-


  1. Add dilute HNO3 and AgNO3 solution to I- solution

  2. Yellow precipitate forms (turns grey/purple on exposure to light)

  
  

  Ag+ (aq) + I- (aq) --> AgCl (s)
• Testing for SO4 2-


  1. Add dilute HCl and BaCl2 solution to SO4- solution

  2. White precipitate forms

  
  

  Br2+ (aq) + SO42- (aq) --> BaSO4 (s)
• Why is HCl added when testing for SO4 2-?


  To remove any carbonate ions as otherwise BaCO3 would be produced, giving a false positive result as BaCO3 is also a white precipitate. H+ ions react with carbonate ions to form carbon dioxide and water.
• Testing for CO3 2-


  1. Add dilute acid (e.g. HCl) to CO3 2- salt

  2. Gas (CO2) is released (effervescence)

  3. Bubble gas through limewater and it turns cloudy

  
  

  CO3 2- (s) + 2H+ (aq) --> CO2 (g) + H2O (l)
• When dilute HCl was added to a solid, fizzing was observed. This gas turned limewater cloudy. When heated, the substance turned from white to yellow, producing the same gas, and on cooling, the solid turned white again. Name the substance.

  Zinc carbonate ZnCO3
• Testing for H2O


  Chemical Test:

  1. Turns white anhydrous copper (II) sulphate crystals blue

  
  

  Physical Tests:

  1. Boils at exactly 100°C

  2. Pure water has a density of 1g/cm^3
• Methods of Gas Collection


  1. Downward Delivery - gases denser than air

  2. Upward Delivery - gases less dense than air

  3. Overwater - gases insoluble in water

  
  

  (Gas Syringe - for collecting and measuring volume of gas)
• Downward Delivery


  a method of collecting a gas which is denser than air by passing it downwards into a gas jar

  
• Upward Delivery


  a method of collecting a gas that is lighter than air by passing it upwards into an inverted gas jar

  
• Overwater (Displacement)


  a method of collecting a gas insoluble in water by filling and inverting a test tube over water and bubbling gas into the test tube using a delivery tube - as the gas fills the test tube it displaces the water
• Why can overwater not be used for soluble gases?

  the gas would dissolve in the water
• Testing for and Collecting H2


  1. Lit splint

  2. Squeaky pop

  
  

  Overwater/upward delivery (H2 less dense than air + insoluble in water)
• Testing for and Collecting O2


  1. Glowing splint

  2. Splint relights

  
  

  Overwater/upward delivery (O2 almost as dense than air + slightly soluble in water)
• Testing for and Collecting CO2


  1. Bubble through limewater ( Ca(OH)2 )

  2. Limewater goes cloudy (white precipitate forms)

  
  

  Overwater/downward collection (denser than air + slightly soluble in water)
• Testing for and Collecting NH3


  1. Damp red litmus turns blue

  OR

  2. Expose to hydrogen chloride gas (HCl) and white solid (smoke) forms

  
  

  Upward collection (less dense than air + highly soluble in water)
• Long Tube Diffusion of NH3 and HCl


  1. NH3 and HCl diffuse down the tube.

  2. A ring of white smoke forms closer to HCl (NH3 has a lower molecular mass)