CP7 - Identifying ions

Question 1

How do you carry out a flame test to identify metal ions?

Answer 1

• Clean a platinum wire loop by dipping it in HCl and
  then holding it in the blue flame until it burns without
  any colour
• Dip the loop into the sample you want to test and put
  it into the blue flame of the Bunsen burner
• Record the colour of the flame

Question 2

List precautions to take when using a

Bunsen burner

Answer 2

• Don’t leave unattended
• Turn off gas or leave on orange safety flame
  when not in use
• Tie back long hair
• Keep flammable chemicals away from the
  flame

Question 3

What is the result of the flame test on

lithium ions?

Answer 3

Lithium ions (Li+): Crimson flame

Question 4

What is the result of the flame test on

sodium ions?

Answer 4

Sodium ions (Na+): Yellow flame

Question 5

What is the result of the flame test on

potassium ions?

Answer 5

Potassium ions (K+): Lilac flame

Question 6

What is the result of the flame test on calcium ions?

Answer 6

Calcium ions (Ca2+): Orange-red flame

Question 7

What is the result of the flame test on

copper ions?

Answer 7

Copper ions (Cu2+): Green flame

Question 8

Why must the wire be cleaned before carrying out a flame test?

Answer 8

To remove any unwanted ions that might

obscure the colour of the flame

Question 9

Why can a flame test not be used when a compound contains a mixture of metal ions?

Answer 9

The flame colours of some ions may be

hidden by the colours of other metal ions

Question 10

How can you test for carbonate ions?

Answer 10

• Add a few drops of HCl to the sample in a test
  tube
• Connect this test tube to a test tube of limewater
• If carbonate ions are present, carbon dioxide will
  be produced. Limewater will turn cloudy when
  CO2 is bubbled through.

Question 11

Write the chemical equation for the

reaction between HCl and Na2CO3

Answer 11

Na2CO3 + 2HCl → CO2 + 2NaCl + H2O

Question 12

How can you test for sulfate ions?

Answer 12

- Add HCl to remove any CO3
2- ions as 
these will obscure the results
- Add a couple of drops of barium chloride
- If sulfate ions are present a white 
precipitate of barium sulfate will form

Question 13

Write the chemical equation for the

reaction between BaCl2 and MgSO4

Answer 13

BaCl2 + MgSO4 → BaSO4 + MgCl2

BaSO4 is a white precipitate

Question 14

How do you carry out a test for halide

ions?

Answer 14

• Add a couple of drops of nitric acid to react
  with any carbonate ions which might
  obscure the experiment
• Add a couple of drops of silver nitrate
• Observe the colour of the precipitate

Question 15

What colour precipitate is formed when silver nitrate is added to a chloride solution?

Answer 15

White precipitate of silver chloride

Question 16

What colour precipitate is formed when silver nitrate is added to a bromide solution?

Answer 16

Cream precipitate of silver bromide

Question 17

What colour precipitate is formed when
silver nitrate is added to an iodide
solution?

Answer 17

Yellow precipitate of silver iodide

Question 18

What colour precipitate forms when sodium hydroxide reacts with calcium ions?

Answer 18

White precipitate

Question 19

What colour precipitate forms when
sodium hydroxide reacts with copper(II)
ions?

Answer 19

Blue precipitate

Question 20

What colour precipitate forms when
sodium hydroxide reacts with iron(II)
ions?

Answer 20

Green precipitate

Question 21

What colour precipitate forms when sodium hydroxide reacts with iron(III) ions?

Answer 21

Brown precipitate

Question 22

What colour precipitate forms when
sodium hydroxide reacts with aluminium
ions?

Answer 22

White precipitate at first.

Redissolves with excess NaOH to form a
colourless solution.

Question 23

What colour precipitate forms when
sodium hydroxide reacts with
magnesium ions?

Answer 23

White precipitate

Question 24

Given two solutions, how can you identify which contains aluminium ions and which contains magnesium ions?

Answer 24

Add excess sodium hydroxide.

Both will form white precipitates but the one containing aluminium ions will redissolve to form a colourless solution.