8.3 Identification of ions by chemical and spectroscopic means

Question 1

What can flame tests be used for? (1)

Answer 1

To identify some metal ions (cations).

Question 2

What do lithium compounds produce when put through a flame? (1)

Answer 2

Crimson flame test

Question 3

What do sodium compounds produce when put through a flame? (1)

Answer 3

Yellow flame test

Question 4

What do potassium compounds produce when put through a flame? (1)

Answer 4

Lilac flame test

Question 5

What do calcium compounds produce when put through a flame? (1)

Answer 5

Orange-red flame tests

Question 6

What do copper compounds produce when put through a flame? (1)

Answer 6

Green flame test

Question 7

What are the drawbacks of using flame tests to identify compounds? (2)

Answer 7

• Colours of flames can be hard to distinguish- certainly when low concentration of metal compound used.
• Sample may contain mixture of metal ions which may mask the colour of the flame.

Question 8

What is an alternative to using flame tests? (1)

Answer 8

Flame emission spectroscopy

Question 9

How does flame emission spectroscopy work? (3)

Answer 9

1. Metal ion sample placed into a flame
2. the light given out is passed through a spectroscope
3. The light is converted into a line spectrum.

Question 10

What can line spectrums produced by flame emission spectroscopy be used to do? (2)

Answer 10

• Identify metal ions in the solution (lines in line spectrum specific to given metal ions)
• Measure their concentrations through analysis (lines become more intense at higher concentrations)

Question 11

What are the advantages of using instrumental methods?

Answer 11

• Rapid: analyse samples faster than flame tests.
• Sensitive: even works on tiny sample of metal compounds.
• Accurate: more likely to identify metal ion correctly than flame test.

Question 12

What can sodium hydroxide solutions be used for? (1)

Answer 12

To identify some metal ions (cations).

Question 13

How do you test for calcium using a sodium hydroxide solution? (3)

Answer 13

1. White precipitate formed when sodium hydroxide solution added.
2. When excess is added, there is still a white precipitate.
3. Green flame produced in flame test.

Question 14

How do you test for aluminium using a sodium hydroxide solution? (2)

Answer 14

1. White precipitate formed when sodium hydroxide solution added.
2. When excess is added, the precipitate dissolves.

Question 15

How do you test for magnesium using a sodium hydroxide solution? (3)

Answer 15

1. White precipitate formed when sodium hydroxide solution added.
2. When excess is added, there is still a white precipitate.
3. No colour produced in flame test.

Question 16

How do you test for copper (II) using a sodium hydroxide solution?

Answer 16

Blue (copper hydroxide) precipitate formed when sodium hydroxide is added.

Question 17

How do you test for iron (II) using a sodium hydroxide solution? (1)

Answer 17

Green (iron hydroxide) precipitate firmed when sodium hydroxide is added.

Question 18

How do you test for iron (III) using a sodium hydroxide solution? (1)

Answer 18

Brown (iron hydroxide) precipitate formed when sodium hydroxide added.

Question 19

What is the test for carbonate ions? (3)

Answer 19

1. Add a dilute acid to the sample
2. Effervescence is seen when acid reacts with carbonate ions - carbon dioxide produced.
3. Bubble the gas through limewater - if limewater is cloudy, carbon dioxide present - started with a carbonate ion.

Question 20

What is the test for halide ions? (5)

Answer 20

1. Add nitric acid to sample.
2. Add silver nitrate solution to sample.
3. If halide present:
   - (a) Silver chloride produces white precipitate.
   - (b) Silver bromide ion produces cream precipitate.
   - (c) Silver iodide ion produces yellow precipitate.

Question 21

How do you test for sulfate ions? (3)

Answer 21

1. Add dilute hydrochloric acid to sample.
2. Add barium chloride solution to sample.
3. If sulphate ions present, white precipitate formed.