chpt 10 QA

Question 1

Observations of aluminium.

Answer 1

1. aq NAOH
   white ppt, soluble in excess to form colourless solution
2. aq NH3
   white ppt, insoluble in excess

Question 2

Observations of zinc.

Answer 2

1. in NAOH
   white ppt soluble in excess forming colourless solution
2. in aq NH3
   white ppt, soluble in excess forming colourless solution

Question 3

Observations of calcium.

Answer 3

1. aq NAOH
   white ppt, insoluble in excess
2. in NH3
   no visible change no ppt

Question 4

Observations of copper.

Answer 4

1. aq NAOH
   blue ppt insoluble in excess
2. aq NH3
   blue ppt, soluble in excess forming dark blue solution

Question 5

Observation of iron

Answer 5

FE2+
in both
green ppt insoluble in excess
FE3+
in both
red brown ppt insoluble in excess

Question 6

Observations of ammonium.

Answer 6

1. aq NAOH
   no ppt
   upon warming, effervescence of colourless, pungent gas that turns red litmus paper blue
   ammonia gas produced

Question 7

Observations of ammonium.

Answer 7

1. aq NAOH
   no ppt
   upon warming, effervescence of colourless, pungent gas that turns red litmus paper blue
   ammonia gas produced

Question 8

How to test for nitrate ion ?

Answer 8

1. add aq NAOH
2. add small piece of aluminium foil
3. warm mixture
4. effervescence of ammonia gas (desc what it does)
   Reasoning
5. aluminium reduces nitrate ion into ammonium ion
6. ammonium ion rxt with hydroxide ions to produce ammonia gas and water

Question 9

Why do you add dilute nitric acid to tests for sulfate, chloride and iodide ions ?

Answer 9

nitrate reacts with any carbonate present and removes any carbonate ions
ensure that carbonate ions do not rxt with metal compound to from metal carbonate -> false result
wrong ppt may occur so wrong anion might be inferred to be present
eg
mixture + dilute nitric acid + aq barium nitrate -> baso4 (insoluble ppt)
vs
mixture + aq barium nitrate -> white ppt could be either barium carbonate or barium sulfate (inconclusive test）