Required Practical 4

Question 1

How do we test for group 2 metals?

Outline the method and the results to distinguish the group 2 metals.

Answer 1

METHOD 1

1. Place about 10 drops of 0.1 mol dm⁻³ metal ion solution in a test tube.
2. Add about 10 drops of 0.6 mol dm⁻³ sodium hydroxide solution, mixing well.
3. Continue to add sodium hydroxide, dropwise with gentle shaking, until in excess.

METHOD 1 RESULTS

• The results in this test are an application of the trend that group II hydroxides become more soluble down the group.
• Magnesium hydroxide is classed as insoluble in water and will appear as a white precipitate.
  
  • A suspension of magnesium hydroxide in water will appear slightly alkaline (pH 9) so some hydroxide ions must have been produced by very slight dissolving.
• Calcium hydroxide is classed as partially soluble in water and will appear as a white precipitate.
  
  • A suspension of calcium hydroxide in water will appear more alkaline (pH 11) than magnesium hydroxide as it is more soluble so there will be more hydroxide ions present in solution.
• Strontium and barium salts will not form a hydroxide precipitate on addition of sodium hydroxide due to their high solubility - the solutions will be highly alkaline.

METHOD 2

1. Place about 10 drops of 0.1 mol dm⁻³ metal ion solution in a test tube.
2. Add about 10 drops of 0.6 mol dm⁻³ sulfuric acid or other soluble sulfate solutions.
3. Continue to add sulfuric acid solution, dropwise with gentle shaking, until in excess.

METHOD 2 RESULTS

• The results in this test are an application of the trend that group II sulfates become less soluble down the group.
• Magnesium and calcium salts will not form a sulfate precipitate on addition of sulfate ions due to their high solubility.
• Strontium and barium solutions will form white precipitates with addition of sulfate ions.

Question 2

How do we test for the presence of ammonium ions, NH4⁺?

Outline the method and the results to distinguish ammonium ions.

Answer 2

METHOD

1. Place about 10 drops of 0.1 mol dm⁻³ ammonium chloride in a test tube.
2. Add about 10 drops of 0.4 mol dm⁻³ sodium hydroxide solution. Shake the mixture.
3. Warm the mixture in the test tube gently using a water bath.
4. Test the fumes released from the mixture by holding a piece of damp red litmus paper in the mouth of the test tube.

RESULTS

• Alkaline ammonia gas is released which turns the red litmus paper blue.

Question 3

How do we test for the presence of sulfate ions, SO₄²⁻?

Outline the method and the results to distinguish sulfate ions.

Answer 3

METHOD

1. BaCl₂ solution acidified with hydrochloric acid is used as a reagent to test for sulfate ions.
   
   • The reagent is acidified to react with carbonate impurities that are often found in salts which would form a white barium carbonate precipitate and so give a false result.
   • You could not use sulfuric acid because it contains sulfate ions and so would give a false positive result.

RESULTS
- If acidified barium chloride is added to a solution that contains sulfate ions, a white precipitate forms.

Question 4

How do we test for the presence of halide ions?

Outline the method and the results to distinguish between halide ions.

Answer 4

METHOD

1. Solution is made acidic with nitric acid.
   
   • The role of nitric acid is to react with any carbonates present to prevent the formation of the precipitate Ag₂CO₃. This would mask the desired observations.
2. Then silver nitrate solution is added dropwise.

RESULTS

• Fluorides produce no precipitate.
• Chlorides produce a white precipitate.
• Bromides produce a cream precipitate.
• Iodides produce a pale yellow precipitate.

(The silver halide precipitates can be treated with ammonia solution to help differentiate between them if the colours look similar)

Question 5

How do we test for the presence of carbonate ions?

Outline the method and the results to distinguish carbonate ions.

Answer 5

METHOD

• Add any dilute acid and observe effervescence.

OR

• Bubble gas through limewater to test for CO₂.

RESULTS

• Fizzing due to CO₂ would be observed if a carbonate is present.

OR

• Limewater will turn cloudy.

Question 6

How do we test for the presence of hydroxide ions?

Outline the method and the results to distinguish hydroxide ions.

Answer 6

Alkaline hydroxide ions will turn red litmus paper blue.