Testing for ions: RP

Question 1

How do we test for group 2 ions (cations) in a compound using flame tests?

Answer 1

1.) Dip nichrome wire in concentrated HCl
2.) Dip into the sample.
3.) Place the loop into the BLUE bunsen flame/ observe the colour.

Question 2

What type of samples are flame tests used for?

Answer 2

• Solid samples.
• The samples can be made into solutions and sprayed on bunsen however, this may be difficult if sample tested = insoluble.

Question 3

What different coloured flames would be produced by flame test of these 3 ions:
Ca²⁺ (caclium ions.)
Sr²⁺ (Strontium ions.)
Ba²⁺ (barium ions)

Answer 3

• Ca²⁺ (caclium ions.) –> dark red
  Sr²⁺ (Strontium ions.) —> red
  Ba²⁺ (barium ions) –> green.

Question 4

What is used to test for ammonium compounds and hydroxides?

Answer 4

• Litmus paper.

Question 5

How would you test for ammonium compounds?

Answer 5

• Add sodium hydroxide (NaOH), gently heat and if ammonium compound = present, ammonia gas will be produced.
• The ammonia gas will dissolve in water (on the litmus paper.) Use damp red litmus at MOUTH of the test tube, the ammonia will turn the litmus blue.

Question 6

Give the ionic equation for the test of ammonium compounds.

Answer 6

NH₄⁺ (aq) + OH⁻ (aq) –> NH₃ (g) + H₂O (l)

Question 7

How do you test for hydroxides? What is the issue with this test- how can this be solved?

Answer 7

• Hydroxides = alkaline, will turn red litmus paper blue.
• Issue: doesn’t DEFINETELY prove you have hydroxide, just proves you have alkali.
• Further tests needed to confirm you have alkali

Question 8

How do you test for carbonate ions? Give the ionic equation.

Answer 8

• HCl will react with carbonates to make CO2 gas. When CO2 gas bubbled through limewater, limewater will turn cloudy.
• CO₃²⁻ + 2H⁺ –> CO₂ + H₂O

Question 9

How do you test for sulphate ions? Give ionic equation for formation of precipitate.

Answer 9

• Add HCl
• Then add BaCl2
• White precipitate observed if sulphates are present. White precipitate = barium sulphate (insoluble.)
• Ba²⁺ (aq) + SO₄²⁻ (aq) –> BaSO₄ (s)

Question 10

Why do we add HCl FIRST when testing for sulphate ions?

Answer 10

• HCl will remove any carbonates, these would precipitate out after adding BaCl2, giving false result.

Question 11

We are testing a sample to see whether it contains: carbonate, sulphate or halide ions. What order should we test for each of these and why?

Answer 11

• Test for carbonates first
• Then test for sulphates
• Then test for halides
• WHY: testing in this specific order prevents any false positives.