8.3 Methods of Identifying Ions

A student had a solution of an unknown compound, Y.

They performed the following tests on fresh samples of the solution and recorded their observations:

The student concluded that the formula of compound Y was FeI₃,

Is the student's conclusion correct? 

Explain your answer. 

The student repeated Test 1 on a solution of another compound, Z.

A white precipitate formed.

The student concluded that magnesium ions were present.

Explain why the student cannot make that conclusion based on the test results.

Give details of how they could use other tests to identify the metal ion present in compound Z.

The student wanted to further identify compound Z.

They added dilute nitric acid followed by silver nitrate solution.

A cream precipitate formed confirming the presence of bromide ions.

Explain why a cream precipitate formed.

Minerals from a quarry were taken to a laboratory for analysis.

Four samples of minerals, A, B, C and D were submitted for identification of their ions. 

In the laboratory, water is added to each mineral to form solutions and further tests are carried out.

The results of these tests are shown in Table 1.

Table 1

It was concluded that mineral B contained two negative ions and the positive ion was unidentified.

How do the results show this?

It was believed that the positive ion present in mineral B was potassium.

Describe a further test, including the expected result, that could be used to confirm the presence of potassium ions.

Mineral C formed a white precipitate with sodium hydroxide solution.

One metal ion that gives a white precipitate with sodium hydroxide solution is aluminium.

Name two other metal ions that would also produce a white precipitate.

State an instrumental method that could be used to distinguish between the three metal ions from part (c).

Identify the ions in minerals A and D.

An unknown salt sample, X, has been sent off for analysis for identification.

A small sample of the unknown salt, X, is put into a flame emission spectroscope and its line spectrum is generated.

The line spectrum is compared to the line spectrum of Group 1 elements to see if the metal ion present in X can be identified.

The line spectra are shown in Figure 1.

Figure 1

State which elements are or are not present in the unknown salt sample, X.

Give reasons for your answers.

Other than using flame emission spectroscopy, give details of any further tests that could be carried out to identify any unidentified elements.

Explain how flame emission spectroscopy produces a line spectrum.

Give two advantages of using a flame emission spectroscopy machine, rather than chemical tests, to identify samples.

Various barium compounds are used as medicines by doctors. They can be used to help diagnose problems in the stomach and bowels or for treating irregularities with the heart beating.

A medical student has a sample that they believe to be a barium compound but wants to perform some tests on the solution to confirm its identity.

They perform a flame test and a green colour flame is observed.

Table 1 shows flame colours for some other elements.

Table 1

The student concluded that the solution did not contain calcium, potassium or sodium ions. 

Explain why this is correct and give any other conclusions that the student could make from this test, giving your reasons.

The student obtained an emission spectrum of barium.

How could this be used to show that the solution contains barium or any other elements?

The student noticed that the lines produced in the spectrum did not produce the same level of light intensity.

Explain what information this gives about the elements in the sample?

The student identified that iodide ions were present from the emission spectrum of the solution and confirmed their presence by adding dilute nitric acid and silver nitrate to the solution in question.

A yellow precipitate was formed.

Give the ionic equation, including state symbols, which shows the formation of the yellow precipitate.

A student investigated the composition of an egg shell as part of a wider study.

The student performed some tests on the egg shell and recorded the results as shown in Table 2 below.

Table 2

The student concluded that there were carbonate ions present in the egg shell.

Describe a test the student could use to identify the gas produced in test 1.

The student concluded that the egg shell also contains aluminium ions.

Is this conclusion correct? 

Use the results in the table to justify your answer.

The student concluded that the egg shell also contains chloride ions.

Is this conclusion correct?

Use the results in the table to justify your answer.

A group of scientists working on a similar project as the student wanted to investigate the amount of lead found in egg shells.

Instead of using older methods, they used a modern instrumental method which was more sensitive.

Name one modern instrument which is used to identify elements.

What does the term more sensitive mean?

State two other advantages of modern instrumental methods over older methods?

Low sodium salt is used on foods and is suitable for use by people following a low sodium diet. 

The label in Figure 1 is from a brand that produces low sodium salt.

  Figure 1

A student tests the low sodium salt for the substances written on the label.

The flame test can be used to identify sodium and potassium ions.

Describe the procedure of carrying out a flame test.

The student added a few drops of NaOH solution to a solution of the low sodium salt to test for magnesium ions. 

A white precipitate formed.

This test also gives a white precipitate with aluminium ions and calcium ions which makes it difficult to identify exactly which ion is responsible.

Describe how the student could confirm that the low sodium salt contains magnesium ions and not aluminium ions.

Describe how the student could confirm that the low sodium salt does not contain calcium ions.

Four different colourless solutions were given to a group of students in beakers 1, 2, 3 and 4, shown in Figure 3.

Figure 3

The students knew that the solutions were

• sodium carbonate
• sodium iodide
• sodium chloride
• potassium carbonate

but did not know which solution was in which beaker.

The teacher asked the class to plan a method that could be used to identify each solution. 

The teacher suggested using a flame test to identify the positive ions and provided them with the following reagents only: 

• silver nitrate solution
• dilute nitric acid

Outline a method the students could use to identify the four solutions.

You should include the results of the tests you describe.