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.

i) Sodium hydroxide solution was added to a small amount of copper sulfate solution. A blue precipitate of copper hydroxide formed. Complete the word equation for the reaction. Include state symbols.

(2)

copper sulfate (aq) + sodium hydroxide (aq) → .......... (..........) + ............ (..........)

ii) Dilute hydrochloric acid was added to a different sample of copper sulfate solution. Barium chloride solution was then added. State what would be seen.

(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.

A student carries out a flame test on an unknown solid.
A red flame is seen.
The student concludes that the solid is lithium carbonate.

Explain why this conclusion is not justified.

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

1. A flame test on the solid substance produced a yellow colour.

2. Dilute hydrochloric acid was added to a solution of the substance followed by a few drops of barium chloride solution. A white precipitate formed.

Give the name and formula of the substance.

 Name of substance ...........................................................................................

 Formula of substance........................................................................................

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.

i) 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.

(2)

ii) Sodium hydroxide solution is also used to distinguish between iron(II) ions, Fe²⁺ and iron(III) ions, Fe³⁺, in solution.

You are given a solution containing iron(II) ions and another solution containing iron(III) ions.

Describe what is seen when sodium hydroxide solution is added to each of these solutions.

(2)

iii) Iron(III) ions, Fe³⁺, react with hydroxide ions in solution to form iron(III) hydroxide. Complete the ionic equation for this reaction.

(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

Which ion is shown to be present by the result of test 1?

i) State the name of the gas given off in test 2.

(1)

ii) State the name of the ion shown to be present in the white solid by the result of test 2.

(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

i) A solution containing a concentration of potassium ions of 35 mg dm^-3 is placed in the photometer. The photometer reading is 7.0. Plot this point on the graph and then draw the straight line of best fit.

(2)

ii) Another solution of potassium ions gives a photometer reading of 9.0. Use the graph to find the concentration of potassium ions in this solution.

(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.

Solid X contains two ions.

The tests for these ions and their results are shown in Figure 10.

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.

i) Give the reason why this test does not identify the cation in solid Y.

(1)

ii) Give the reason why this test does not identify the anion in solid Y.

(1)

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.

i) Write the ionic equation for this reaction.

(3)

ii) The green iron(II) hydroxide precipitate gradually turns brown when exposed to air. Explain this observation.

(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.

i) Explain why the test for a given ion must be unique to that ion. 

(2)

ii) In the test for the carbonate ion, CO₃^2–, dilute hydrochloric acid is added to the solid being tested.

 State the name of the gas produced in the test if carbonate ions are present. 

(1)

iii) Tests for three ions are described.

 Draw one straight line from the test for each ion to the observation that shows that ion to be present.

Each observation may be correct for one test, more than one test, or for none of the tests.

(3)

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.

A flame test is used to identify the metal ions in each of these solids.

i) Describe how to do a flame test.

(2)

ii) Different metal ions produce different coloured flames. Draw one straight line from each metal ion to its flame colour.

(2)

P and Q dissolve in water to form colourless solutions. Figure 7 shows the results of tests on these solutions.

Figure 7

i) The anions in solutions of P and Q can be identified from the results of the tests shown in Figure 7.

Draw one straight line from each solution to the anion present.

(2)

ii) The formula of barium chloride is BaCl₂.
Give the total number of ions in the formula BaCl₂.

(1)

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

i) State what would be seen.

(2)

ii) One other product is formed in this reaction. What is the name of this other product?

(1)

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.

i) The solution is tested to see whether it contains one of these ions. In the test, a few drops of two different solutions are added to X. Name the two solutions that are added in the test.

(2)

solution 1  ...........................................................

solution 2 ............................................................

ii) The student carrying out the test records the following result.

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

(2)

iii) The metal ions in X could be identified using a flame test. There is a more sensitive and accurate instrumental method that can be used.

Give the name of an instrument that can be used to identify the metal ions in X.

(1)

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.

A flame test was carried out on some copper sulfate crystals.

The student used the following method.

i) State why in step 1 the wire is dipped in hydrochloric acid and held in a roaring Bunsen burner flame.

(1)

ii) State why in step 2 the wire is dipped in hydrochloric acid again before dipping it in the copper sulfate crystals.

(1)

iii) What colour should be seen when the flame test is carried out on copper sulfate?

(1)

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

i) Give an advantage of using a flame photometer, rather than a flame test, to test for copper ions.

(1)

ii) Figure 7 shows some values obtained when producing a calibration graph for copper ions in a flame photometer.

Draw a line of best fit through the points.

(1)

iii) A solution containing copper ions gave a display reading of 0.030.

Use the calibration graph in Figure 7 to find the concentration of copper ions in this solution.

(1)

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

The student tested an unknown solution for sulfate ions by

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. 

2HCl (aq) + Na₂CO₃ (aq) → 2NaCl (aq) + CO₂ (g) + H₂O (l)

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.

i) The technician first produced a calibration curve using solutions with known concentrations of potassium ions. Figure 9 contains the data for the calibration curve.

Figure 9

Use the information in Figure 9 to plot the calibration curve on the grid below.

(3)

ii) The technician then obtained a reading of 360 for a dilute solution containing potassium ions.

Use the calibration curve to find the concentration of the potassium ions in this solution.

(1)

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.

i) Identify which two metal ions this could be.

(1)

ii) Describe a further test that would confirm the identify of the ion and give the result.

(2)

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.