Qualitative Analysis: Tests for Ions (Edexcel GCSE Chemistry)

Qualitative tests can be used to identify ions in substances.

Sodium hydroxide solution is warmed with a solution of ammonium ions.
Ammonia gas is given off.

Describe the test to show the gas is ammonia.

Two tests were carried out on copper sulfate solution.

(2)

(1)

A technician found some colourless crystals in an unlabelled beaker in a laboratory.

The technician knew that the substance was potassium chloride, potassium carbonate, sodium chloride or sodium iodide.

Plan a series of tests the technician could carry out to identify the colourless crystals.

Scientists test a solution from a bottle that is known to contain a copper (II) compound.

Table 1 shows their results.

Table 1

Which statement below is true?

Qualitative tests are used to identify ions.

The following tests were carried out on a substance containing two ions.

The test for chloride ions was carried out on a solution.
Dilute nitric acid was added to the solution, followed by a few drops of silver
nitrate solution.
A white precipitate formed.

Why is it necessary to add dilute nitric acid in this test?

Higher Only

Sodium hydroxide solution is used to identify some cations present in compounds.

(2)

(2)

(1)

   Fe³⁺ + 3OH^– → .............................................................................................

A compound is suspected of containing calcium ions, Ca²⁺.

Which test would not help to positively identify calcium?

Figure 3 shows two tests carried out on a white solid and the results of the tests.

Figure 3

(1)

(1)

A flame photometer can be used to measure the concentration of potassium ions in a solution.

Figure 4 shows the photometer readings for three different concentrations of potassium ions in solutions.

Figure 4

(2)

(1)

concentration =....................................... mg dm^-3

Which is not an advantage of using instrumental methods rather than small-scale lab methods for identifying ions?

Qualitative tests are used to identify ions in compounds.

Figure 10

Use the information in Figure 10 to name the cation and the anion in solid X.

name of cation ..................................

name of anion ...................................

Another solid, Y, also contains two ions.
A test was carried out on solid Y.
A few drops of sodium hydroxide solution were added to a solution of solid Y.
A white precipitate formed.

Instrumental methods are often used for analysis.

Give a reason why instrumental analysis may be better than other methods of analysis.

Higher Only

Iron(II) sulfate solution reacts with sodium hydroxide solution to form a pale green precipitate of iron(II) hydroxide, Fe(OH)₂.

Qualitative tests are carried out on ionic substances to identify the ions present in the substances.

The test for a given ion must be unique to that ion.

A white solid is known to be a chloride in which the metal ion is sodium, potassium, calcium or aluminium.

A chemist was told to carry out a test for each metal ion that could be present in this white solid.

Describe tests to show the presence of each of these metal ions.

Which element is present in this flame emission spectrum?

Tests are carried out to identify the ions in two solids, P and Q.

(2)

(2)

P and Q dissolve in water to form colourless solutions.

Figure 7 shows the results of tests on these solutions.

Figure 7

A few drops of sodium hydroxide solution are added to a solution of iron(II) sulfate.
Iron(II) hydroxide is formed.

A student adds a few drops of dilute nitric acid followed by a few drops of silver nitrate solution to a test tube containing an unknown colourless solution. A white precipitate is seen.

What ions could be present in the solution?

A chloride ion, a fluorine atom and a nanoparticle are all types of particle.

Which of the following shows the particles in order of size, starting from the smallest?

A solution, X, is thought to contain chloride, bromide or iodide ions.

solution 1  ...........................................................
solution 2 ............................................................

Explain why the anion in X cannot be known for certain.

(2)

Scientists test a solution of compound X. The table shows their results.

Which two ions are present in compound X?

A student carried out some tests on copper sulfate.

(1)

(1)

(1)

A solution of the copper sulfate was tested in a flame photometer.

(1)

(1)

(1)

concentration of copper ions = ................................. ppm

The student tested an unknown solution for sulfate ions by

1. 1. adding a few drops of dilute sulfuric acid to the unknown solution
   2. then adding a few drops of barium chloride solution.

A white precipitate was formed.

The student cannot conclude from this result that the unknown solution contains sulfate ions.

Explain what the student should change to obtain a valid result.

Calculate the percentage by mass of copper in copper sulfate, CuSO₄

(relative atomic masses: Cu = 63.5
relative formula mass of CuSO₄ = 159.5)

Give your answer to the nearest whole number.

percentage by mass of copper = ..........................

A sample of potassium carbonate is contaminated with a small amount of sodium carbonate.
When a flame test is carried out on the sample, a bright yellow flame is seen.

Describe how you could show that potassium and sodium ions are present in this sample.

Higher Only

Hydrochloric acid reacts with a solution of sodium carbonate. 

Write the ionic equation for this reaction.

A student tests solutions of three ionic substances, K, L and M.
The student carries out the same two tests on each of the three solutions.

Test 1 add dilute nitric acid and then silver nitrate solution.
Test 2 add a few drops of sodium hydroxide solution and warm the mixture.

Figure 7 shows the results of the tests and the student’s conclusions about the identity of each substance.

Figure 7

None of the student’s conclusions are fully justified.

Explain which part of each conclusion is justified and what further work can be carried out to fully justify each conclusion.

A technician was asked to find the concentration of potassium ions in a dilute solution using a flame photometer.

Figure 9

(3)

concentration = ...................................... mol dm^-3

Higher Only

In the test for chloride ions, silver nitrate solution is added to a solution containing chloride ions.
A white precipitate forms.

Write the ionic equation for this reaction.

Higher Only

A student was given a container of ammonium iron(II) sulfate, (NH₄)₂Fe(SO₄)₂ .
The student was also given a dilute solution of sodium hydroxide and access to other laboratory reagents.

Describe the tests the student should carry out to identify the ions in the ammonium iron(II) sulfate, including appropriate equations for the reactions involved.

A student was asked to identify the ions present in three different solutions of compounds, A, B and C.

The student's results from a series of tests on each solution are shown in Figure 1.

Figure 1.

Identify the two ions present in each of the compounds A, B, and C.

A: ____________________ and ____________________ 

B: ____________________ and ____________________ 

C: ____________________ and ____________________ 

What is the purpose of adding dilute acid in the tests with barium chloride and silver nitrate?

Suggest what could be seen if the acid was not added?

Explain why nitric acid is used before using silver nitrate instead of hydrochloric acid or sulfuric acid.

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 lithium.

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

Mineral C formed a white precipitate with sodium hydroxide solution.

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 photometer 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.

Describe other chemical tests that could be carried out to identify any unidentified elements.

You should not include any references to flame photometry.

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