Required practical 7 - 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 of bunsen burner 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 containing carbonate ions 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 → 2NaCl + CO₂ + H₂O

Question 12

How can you test for sulphate ions?

Answer 12

• Add HCl to remove any CO₃²⁻ as these will obscure the results.
• Add a couple of drops of barium chloride.
• If sulphate ions are present a white precipitate of barium sulphate 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 drops of nitric acid to react with any carbonate ions which might obscure the experiment.
• Add a couple drop of silver nitrate.
• Observe the colour of the precipitate.

Question 15

What colour precipitate is formed when silver nitrate is added to 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 forms 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 2 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.