chap 6 qualitative analysis

• reagents used to identify cations

  aq sodium hydroxide and aq ammonia
• metal salt + sodium hydroxide -> metal hydroxide + sodium salt
• metal salt + aq ammonia -> metal hydroxide + ammonium salt
• zinc in aq NaOH

  white ppt, soluble in excess to form colourless solution
• aluminium in aq NaOH

  white ppt, soluble in excess to form a colourless solution
• lead (II) in aq NaOH

  white ppt, soluble in excess to form a colourless solution
• calcium in aq NaOH

  white ppt, insoluble in excess
• copper (II) in aq NaOH

  blue ppt, insoluble in excess
• iron (II) in aq NaOH

  green ppt, insoluble in excess
• iron (III) in aq NaOH

  red- brown ppt, insoluble in excess
• ammonium in aq NaOH

  ammonia produced on warming
• zinc in aq ammonia

  white ppt, soluble in excess to form colourless solution
• aluminium in aq ammonia

  white ppt, insoluble in excess
• lead (II) in aq ammonia

  white ppt, insoluble in excess
• calcium in aq ammonia

  no ppt
• copper (II) in aq ammonia

  light blue ppt, soluble in excess to form dark blue solution
• iron (II) in aq ammonia

  green ppt, insoluble in excess
• iron (III) in aq ammonia

  red- brown ppt, insoluble in excess
• how to distinguish between lead and aluminium

  add HCl, aluminium forms soluble salt while lead forms insoluble salt
• identify carbonate

  add dilute acid -> effervescence, carbon dioxide produced
• identify nitrate

  1)add sodium hydroxide 2)add aluminium foil and warm gently

  -> ammonia produced
• identify sulfate

  1)acidify with dilute nitric acid 2)add aq barium nitrate

  -> white ppt
• identify chloride

  1)acidify with dilute acid 2)add aq silver nitrate

  -> white ppt
• identify iodide

  1)acidify dilute nitric acid 2)aq silver nitrate

  -> yellow ppt
• why acidify anion before adding reagent ?

  it removes any possible carbonate anions present in the solution to prepare it for the test for sulfate/ chloride/ iodide. barium nitrate and silver nitrate reacts with carbonate ions to form white and yellow precipitate respectively
• identify ammonia

  turn moist red litmus paper blue
• identify carbon dioxide

  gives white ppt with limewater
• identify chlorine

  bleaches moist blue litmus paper
• identify hydrogen

  extinguishes lighted splint with a 'pop' sound
• identify oxygen

  relights glowing splint
• identify sulfuric dioxide

  turns aq acidifies potassium manganate (VII) from purple to colourless
• when can water be produced ?

  when a hydrated salt is heated, the colourless liquid that condenses near the top of the test tube is most likely to be water
• one way to confirm presence of water

  place cobalt (II) chloride paper at mouth of test tube

  -> turns anhydrous cobalt (II) chloride paper from blue to pink
• one way to confirm presence of water

  add few drops of sample to anhydrous copper (II) sulfate

  -> turns anhydrous copper (II) sulfate from white to blue
• carbonates generally decompose upon strong heating to produce metal oxide and carbon dioxide
• calcium carbonate, white, forms white calcium oxide when heated and remains white after cooling down
• copper (II) carbonate, green, forms black copper (II) oxide when heated and remains black after cooling down
• zinc carbonate, white, forms yellow zinc oxide when heated and turns white after cooling down
• lead (II) carbonate, white, forms white lead (II) oxide when heated and remains white when cooling down
• metal carbonate (s) -heated-> metal oxide (s) + carbon dioxide (g)