RP7: Identifying Ions

Question 1

What is the aim of these experiments?

Answer 1

• Use of chemical tests to identify the ions in unknown single ionic compounds covering
  the ions from flame tests and sulphates.

Question 2

Describe how you would carry out a flame test to identify metal ions (method)

Answer 2

1. Pour 1 cm3 of each known chloride solution into 5 test tubes.
2. Clean the nichrome wire by dipping it in dilute hydrochloric acid.
3. Dip the nichrome wire into solution and hold the tip in a blue Bunsen flame.
4. Record the colour of the flame.
5. Repeat for the following solutions and make sure to clean the wire after each
   test.
6. Pour 1 cm3 of the unknown salt solution into test tube.
7. Dip the nichrome wire into solution and hold the tip in a blue Bunsen flame.
8. Record the colour of the flame, you should be able to compare results with the
   known chloride with the matching colour flame.

Question 3

List precautions to take when using a Bunsen burner

Answer 3

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

Question 4

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

Answer 4

• To remove any unwanted ions that might obscure the colour of the flame

Question 5

How can you test for carbonate ions?

Answer 5

1. Place a 2 cm3 of limewater in a clean test tube.
2. Add a little dilute hydrochloric acid to the unknown solution.
3. If you see bubbles, transfer the gas produced to the limewater using a delivery tube.
4. Repeat this process for the known sodium solutions to identify carbonates ions.
5. Lime water will turn cloudy when CO2 is bubbled

Question 6

Write the chemical equation for the reaction between HCl and Na₂CO₃

Answer 6

• Na₃CO₃+ 2HCl —-> CO₂ + 2NaCl + H₂O

Question 7

How can you test for sulfate ions?

Answer 7

1. Add a 10 drops of dilute hydrochloric acid to the unknown solution in a test tube.
2. Add a 2 cm3 barium chloride solution.
3. Pour 1 cm3 of the known sodium solutions into separate test tubes.
4. Add 5 drops of dilute hydrochloric acid and then 2 cm3 of barium chloride.
5. If sulfate ions are present a white precipitate of barium sulfate will form

Question 8

Write the chemical equation for the reaction between BaCl₂ and MgSO₄

Answer 8

• BaCl₂ + MgSO₄ —> BaSO₄ + MgCl₂
• BaSO₄ is a white precipitate

Question 9

How do you carry out a test for halide ions?

Answer 9

1. Add a 10 drops of dilute nitric acid to the unknown solution in a test tube.
2. Add a 1 cm3
   silver nitrate solution to the test tube with the unknown solution.
3. Pour 1cm3 of the know sodium solutions into separate test tubes.
4. Repeat steps 1 and 2 for which of the solutions.
5. Record colour of precipitate formed in each test tube.

Question 10

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

Answer 10

• Add excess sodium hydroxide
• Both will form white precipitates but the one containing aluminium ions will redissolve to form a colourless solution