Methods of Identifying Ions (AQA GCSE Chemistry)

Exam Questions

2 hours24 questions
1a2 marks

A student had three different solutions in beakers 1, 2 & 3 as shown in Figure 1.

Figure 1

beakers-1-to-3-unknown-chemicals

The beakers contained one of the following solutions but the student did not know which beaker contained each solution.

  • potassium bromide
  • calcium sulfate
  • copper nitrate

To identify the solutions, the student performed a flame test.

Describe how to carry out a flame test.

1b3 marks

The student recorded the flame colour that was observed with the solutions in beakers 1, 2 and 3.

Draw one line from each solution to the colour of the flame that it would produce.

flame-test-colours

1c1 mark

The student wanted to confirm the identity of the solutions in beakers 1, 2 and 3 so performed further tests on the solutions.

They added dilute nitric acid followed by barium nitrate solution to each of the solutions.

Which of the solutions would produce a white precipitate?

Tick (✓) one box.

Potassium bromide  
Calcium sulfate  
Copper nitrate  

1d1 mark

The student added dilute nitric acid and silver nitrate solution to each of the solutions.

Which solution would produce a cream precipitate?

Tick (✓) one box.

Potassium bromide  
Calcium sulfate  
Copper nitrate  

1e1 mark

Finally, the student added a few drops of sodium hydroxide solution to each of the solutions.

Which solution would form a white precipitate?

Tick (✓) one box.

Potassium bromide  
Calcium sulfate  
Copper nitrate  

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2a2 marks

Solutions containing iron(II) ions, Fe2+, and iron(III) ions, Fe3+, will produce precipitates when sodium hydroxide solution is added to them.

Draw one line from the iron ion to the colour of the precipitate formed.

metal-hydroxide-ppt

2b1 mark

Sodium hydroxide solution was added to another solution which contained metal ions.

A white precipitate was formed.

Which of the following ions could not be the metal ion.

Tick (✓) one box.

Copper  
Magnesium  
Aluminium  
Calcium  

2c1 mark

An excess of sodium hydroxide was added to the solution part b) and the white precipitate dissolved.

Which of the ions is the metal ion in solution?

Tick (✓) one box,

Copper  
Magnesium  
Aluminium  
Calcium  

2d1 mark

More than one metal ion produces a white precipitate that does not dissolve when sodium hydroxide is added in excess.

Suggest a method of distinguishing between these ions.

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3a4 marks

A student wanted to find the identity of different chemical compounds in solution.

Draw one line from each of the compounds they tested to the test and corresponding result.

jGvUYS-w_testing-for-ions

3b2 marks

The presence of carbon dioxide gas can be determined by a test.

Circle one word from each box to complete the sentences.

Bubble the gas through

barium chloride.

dilute hydrochloric acid.

limewater.

If carbon dioxide is present it will 

form a yellow precipitate.

turn cloudy.

form a blue solution.

3c1 mark

Another solution containing copper ions was tested using a flame test.

What colour flame would you expect to see?

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4a1 mark

A student had the three potassium halide solutions shown below:

  • potassium chloride
  • potassium bromide
  • potassium iodide

What should they add to the solutions to identify each of these solutions?

Tick (✓) one box.

Dilute hydrochloric acid followed by barium chloride  
Dilute nitric acid followed by barium nitrate  
Dilute hydrochloric acid followed by silver nitrate  
Dilute nitric acid followed by silver nitrate  

4b3 marks

When tested with the correct reagent, halides produce distinctive coloured precipitates which can be used to distinguish them from each other.

Draw one line from each of the potassium halide solutions to the colour of the precipitate it forms.

silver-nitrate-test

4c2 marks

The student also had a solution of potassium carbonate.

What could they add to this solution to identify the carbonate ion?

What would they observe? 

They should add _______________________________________________. 

They would observe ____________________________________________.

4d1 mark

The student could perform a flame test to confirm the presence of potassium ions in the solution in part c).

What colour flame would be seen?

 

Tick (✓) one box.

Orange-red  
Blue  
Lilac  
Yellow  

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5a2 marks

Instrumental analysis can be used to analyse elements and compounds.

Give two advantages of using instrumental analysis over chemical tests.

5b3 marks

Flame emission spectroscopy is an example of an instrumental method that can be used to analyse metal ions in solution.

Circle a word in each box to complete the sentences below.

  The sample to be tested is put into a 

flame.

solution.

beaker.

  The light given out is put through a  

mass spectrometer.

spectroscope.

coloured filter.

  The output is called a line

graph.

chromatogram.

spectrum.

5c1 mark

In addition to being able to identify the metal ions in solution, what other piece of information can flame emission spectroscopy tell us about the sample?

5d1 mark

The resulting output from a flame emission spectrometer can be compared against those of known metals.

Figure 1 shows the output for manganese, strontium, titanium and an unidentified solution.

Using the output in Figure 1, identify which metal ion is present in the unidentified solution.

Figure 1

XJfKyTIO_flame-emission-spectrum

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61 mark

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

  • Instrumental methods are more accurate

  • Instrumental methods are faster

  • Instrumental methods are cheaper

  • Instrumental methods are more sensitive

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1a1 mark

This question is about the chemicals that are used to make fireworks.

The metal ions present in the chemicals of fireworks are what produce coloured flames. 

What colour flame is produced by sodium ions?

1b1 mark

Name a metal ion that produces a green flame.

1c1 mark

A company wanted to produce fireworks with a bright red colour. 

Which metal would be most suitable?

1d1 mark

The same company also wanted to produce fireworks with a lilac colour to mimic a flower.

Which metal would be most suitable?

1e3 marks

Complete Table 1 below about four chemicals.

Table 1

Chemical Colour produced in firework
barium chloride green
                                    carbonate crimson
__________________ sulfate orange-red
sodium nitrate ________________

1f3 marks

Some fireworks contain a mixture of two or more metal ions.

Why is it difficult to identify the metal ions using a flame test?

1g2 marks

Flame emission spectroscopy can be used instead of flame tests to identify metal ions in a firework.

Figure 1 shows the results of flame emission spectra of five individual metal ions.

It also shows a flame emission spectrum for a mixture of two different metal ions.

Figure 1

8-3-methods-of-identifying-ions-medium-q1g

Identify two metal ions that are in the mixture?

Tick two boxes.

ion-options

1h1 mark

Typically, the compounds used in fireworks also contain non-metal ions.

A scientist tests a sample from a batch of fireworks.

Silver nitrate and dilute nitric acid are added to a solution of the sample and a cream precipitate is formed.

Identify the ion present.

1i3 marks

Describe a test to detect the presence of sulfate ions in the solution and the result you would expect to see if they were present. 

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21 mark

Scientists test a solution of compound X.

Table 1 shows their results.

Table 1

Test Result
Add dilute nitric acid followed by silver nitrate solution White precipitate
Add sodium hydroxide solution Green precipitate

Which two ions are present in compound X?

  • Fe3+ and Cl-

  • Fe3+ and SO42-

  • Fe2+ and Cl-

  • Fe2+ and SO42-

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31 mark

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

Table 1 shows their results.

Table 1

Test Result
Flame test Yellow flame
Add sodium hydroxide solution Blue precipitate formed
Add dilute nitric acid followed by silver nitrate solution Yellow precipitate formed

Which statement below is true?

  • The flame test was performed incorrectly

  • Bromide ions are present, so the compound is CuBr2

  • The test tube was contaminated for the sodium hydroxide test

  • The copper compound in the bottle was contaminated with another metal compound

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4a2 marks

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

  Test Observation
1 Dilute HCl was added to the shell A gas was produced.
 
The egg shell dissolved and formed a colourless solution.
2 A flame test was done on the colourless solution from test 1. The flame turned red.
3 NaOH solution was added to the colourless  solution from test 1. A white precipitate formed that did not dissolve in excess NaOH solution.
4 AgNO3 solution was added to the colourless solution from test 1. A white precipitate formed.


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.

4b4 marks

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.

4c3 marks

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.

4d1 mark

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.

4e1 mark

What does the term more sensitive mean?

4f2 marks

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

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5a5 marks

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

8-3-methods-of-identifying-ions-medium-q3a

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.

5b2 marks

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.

5c2 marks

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

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66 marks

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

Figure 3

aqa-gcse-8-3m-4-unknowns

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.

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71 mark

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

Which test would not help to positively identify calcium?

  Test Result
A Flame test Orange-red flame
B Add sodium hydroxide solution White precipitate forms
C Add nitric acid and silver nitrate White precipitate forms
D Flame emission spectroscopy Characteristic lines match the reference spectrum

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    81 mark

    Calcium carbonate can react with hydrochloric acid by the following equation:

    CaCO3 (s) + 2HCl (aq) rightwards arrowCaCl2 (aq) + H2O (l) + CO2 (g)

    Which observation does not tell you that this reaction has occurred?

      Test Result
    A Add dilute nitric acid then by silver nitrate solution White precipitate forms
    B Bubble any gases present through limewater Limewater turns cloudy
    C Observe the solutions when they are mixed Solutions bubble and fizz upon mixing
    D Test pH of the solution at the start and after 30 s pH increases

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      1a3 marks

      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 Table 1.

      Table 1

      Test
      Compound Flame Test Add dilute hydrochloric acid Add sodium hydroxide solution Add dilute hydrochloric acid and barium chloride Add dilute nitric acid and silver nitrate
      A green flame fizzing blue precipitate fizzing, no precipitate no reaction
      B red flame no reaction white precipitate no reaction cream precipitate
      C yellow flame no reaction no reaction white precipitate no reaction

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

      A
      : ____________________ and ____________________ 

      B: ____________________ and ____________________ 

      C: ____________________ and ____________________ 

      1b1 mark

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

      1c1 mark

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

      1d3 marks

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

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      2a4 marks

      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:

      Test 1: Add sodium hydroxide solution
      Observation: Green precipitate formed
         
      Test 2: Add dilute nitric acid followed by silver nitrate solution
      Observation: Yellow precipitate formed

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

      Is the student's conclusion correct? 

      Explain your answer. 

      2b4 marks

      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.

      2c2 marks

      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.

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      3a3 marks

      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

      Test A B C D
      Add sodium hydroxide solution blue precipitate  no change white precipitate green precipitate
      Add dilute hydrochloric acid fizzed fizzed no change no change
      Add dilute hydrochloric acid and barium chloride no change no change white precipitate no change
      Add dilute nitric acid and silver nitrate no change yellow precipitate no change cream precipitate

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

      How do the results show this?

       

      3b2 marks

      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.

      3c1 mark

      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.

      3d1 mark

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

      3e2 marks

      Identify the ions in minerals A and D.

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      4a5 marks

      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

      line-spectrum

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

      Give reasons for your answers.

      4b2 marks

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

      4c2 marks

      Explain how flame emission spectroscopy produces a line spectrum.

      4d2 marks

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

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      5a3 marks

      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

      Element Flame colour
      Barium Green
      Calcium Orange-red
      Copper Green
      Phosphorus Blue-green
      Potassium Lilac
      Sodium Yellow

      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.

      5b2 marks

      The student obtained an emission spectrum of barium.

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

      5c2 marks

      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?

      5d2 marks

      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.

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