SC25 - Qualitative Analysis: Test for Ions ✓

Question 1

SC25a - What colour do Lithium (Li+) ions turn in a flame test?

Answer 1

(Bright) Red

[Lith-le red riding hood]

Question 2

SC25a - What colour do Sodium (Na+) ions turn in a flame test?

Answer 2

Yellow

Question 3

SC25a - What colour do Potassium (K+) ions turn in a flame test?

Answer 3

Lilac

P̶u̶r̶p̶l̶e̶.̶ ̶A̶H̶H̶H̶H̶H̶ ̶I̶’̶M̶ ̶S̶O̶R̶R̶Y̶ ̶O̶K̶ ̶O̶K̶ ̶I̶T̶S̶ ̶N̶O̶T̶ ̶P̶U̶R̶P̶L̶E̶

Question 4

SC25a - What colour do Calcium (Ca2+) ions turn in a flame test?

Answer 4

Orange-red

Question 5

SC25a - What colour do Copper (Cu+) ions turn in a flame test?

Answer 5

Blue-Green

Question 6

SC25a - What are the 3 main steps in conducting a flame test?

Answer 6

• Light a Bunsen burner to a strong blue flame
• Pick up a sample of the substance with a small wire loop
• Hold the sample into the edge of the flame and observe the colour

Question 7

SC25a - Why may the flame test loop be a platinum one?

Answer 7

• It has a high melting point and so won’t melt
• It is inert and so won’t react It doesn’t produce/affect a flame colour

Question 8

SC25a - Why may a nichrome alloy be used as a wire loop for a flame test?

Answer 8

• It is cheaper
• However, it produces a light orange flame that could affect the results

Question 9

SC25a - What is flame photometry?

Answer 9

• When a solution containing the metal ions creates a flame and the light intensity is measured by a computer.
• This allows it to have quantitative and comparable data

Question 10

SC25a - What are the main three advantages of photometry over flame tests?

Answer 10

• Sensitivity: Much smaller sample sizes can be used
• Accuracy: They give results closer to the true value
• Speed: They occur with more quickness [look at me defining speed like a pro]

Question 11

SC25a - What is a callibration curve?

Answer 11

• It shows the light intensity produced by metal ions in different concentrations of solutions
• It is created by using standard solutions of known concentrations
• This allows data comparison to be accurate

Question 12

SC25a - What can flame photometers do to the light emitted by a flame?

Answer 12

They can separate the light produced into a spectrum of different colours (wavelengths) of light

Question 13

SC25a - What is an emission spectrum?

Answer 13

• Metal ions each produce their own unique combination of different wavelengths of light
• This different colours are the emission spectrum of an ion and can be used to identify ions
• Together, these different colours will produce the one colour seen in the flame test

Question 14

SC25b - How do you conduct a precipitate reaction to identify metal ions?

Answer 14

• A few drops of sodium hydroxide are added to a solution containing metal ions
• The metal and the hydroxide will react to form metal hydroxide
• This will be insoluble and form a precipitate of a colour that can be used to identify the metal

Question 15

SC25b - What colour precipitate do Iron(II) (Fe2+) ions form?

Answer 15

Green

Question 16

SC25b - What colour precipitate do Iron(III) (Fe3+) ions form?

Answer 16

Brown

Question 17

SC25b - What colour precipitate do Copper (Cu2+) ions form?

Answer 17

Blue

Question 18

SC25b - What colour precipitate do Calcium (Ca2+) ions form?

Answer 18

White

Question 19

SC25b - What colour precipitate do Aluminium (Al3+) ions form?

Answer 19

White

Question 20

SC25b - What is the problem with the precipitate test for calcium and aluminium and how is this overcome?

Answer 20

• Both calcium and aluminium ions will produce a whit precipitate when reacted with sodium hydroxide.
• Thus, a distinguishing test is required In this case, excess sodium hydroxide is added
• For aluminium ions, this causes the precipitate to eventually turn colourless

Question 21

SC25b - What is the test for ammonium ions?

Answer 21

• Dilute sodium hydroxide solution is added which produces ammonia gas
• Ammonia gas has a characteristic sharp smell but a confirmatory test is needed to be sure
• Ammonia gas turns damp red litmus paper blue

Question 22

SC25c - How do you test for carbonate ions?

Answer 22

• Add dilute hydrochloric acid to the sample If it does contain a carbonate, this will cause carbon dioxide to be produced
• This will cause effervesence
• To make sure this is carbon dioxide, bubble the gas produced through limewater
• If carbon dioxide is present, limewater will turn colourless to cloudy.milky

Question 23

SC25c - How do you test for sulfate ions?

Answer 23

• Add some dilute hydrochloric acid to the solution
• Then add barium chloride If sulfate ions are present, this will produce a white precipitate of (insoluble) barium sulfate

Question 24

SC25c - Why is hydrochloric acid added when testing for sulfate ions?

Answer 24

To remove any carbonate ions and therefore eliminate the chance of a false positive result

Question 25

SC25c - How do you test for halide ions?

Answer 25

• Add some dilute nitric acid to your sample
• Add some silver nitrate solution
• Observe the colour of the precipitate formed and compare to check what ion was present

Question 26

SC25c - What colour precipitate do Chloride (Cl-) ions form?

Answer 26

White

Question 27

SC25c - What colour precipitate do Bromide (Br-) ions form?

Answer 27

Cream

Question 28

SC25c - What colour precipitate do Iodide (I-) ions form?

Answer 28

Yellow