Identification of ions by chemical and spectroscopic means

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Apuntes de la lección

Flame Tests

Metal ions produce a characteristic colour when heated strongly in a flame, so flame tests identify metal cations.
Clean the wire by dipping it in dilute acid and holding it in a blue Bunsen flame until no colour is seen; this prevents contamination.
Dip the clean wire into the sample, then place it in the edge of the blue flame and note the colour.
Flame colours: lithium crimson, sodium yellow, potassium lilac, calcium orange-red, copper green.
If a mixture of ions is present, colours can mask each other, making identification unreliable.
Use a blue flame because the sample must be heated strongly; avoid overheating the wire until it glows red.

Diagram showing the technique for carrying out a flame test

Metal Hydroxides

Adding sodium hydroxide solution to metal cations forms coloured precipitates of metal hydroxides.
Add a few drops of NaOH first, then add excess, because some precipitates dissolve in excess NaOH.
Copper(II) gives a blue precipitate; iron(II) gives a green precipitate; iron(III) gives a brown precipitate.
Calcium, magnesium and aluminium ions form white precipitates with NaOH.
Aluminium hydroxide dissolves in excess NaOH to give a colourless solution, but calcium and magnesium hydroxides do not dissolve.
A flame test can distinguish between calcium and magnesium ions.

Testing for Ammonium Ions

Test for ammonium ions (NH₄⁺) by adding sodium hydroxide solution and gently heating.
If ammonium ions are present, ammonia gas is produced.
Ammonia gas turns damp red litmus paper blue; hold the paper near the mouth of the test tube without touching the sides.
Ammonia also reacts with hydrogen chloride gas to form a white smoke of ammonium chloride.
The ionic equation is: NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l).
Ammonium (NH₄⁺) is the aqueous cation; ammonia (NH₃) is the gas.

Testing for ammonium ions

Test for Carbonates

Carbonates contain the carbonate ion, CO₃²⁻.
Add dilute acid to the sample; if a carbonate is present, effervescence occurs and carbon dioxide gas is released.
Bubble the gas through limewater; carbon dioxide turns limewater cloudy/milky due to formation of insoluble calcium carbonate.
Equation: CO₃²⁻(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l).
Equation: CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l).
Connect the test tube to the limewater quickly so no carbon dioxide escapes.

Test for Halides

Halide ions are the negative ions formed by Group 7 elements: Cl⁻, Br⁻, I⁻.
Acidify the sample with dilute nitric acid, then add silver nitrate solution.
A silver halide precipitate forms: silver chloride white, silver bromide cream, silver iodide yellow.
Ionic equation: Ag⁺(aq) + X⁻(aq) → AgX(s).
Use nitric acid, not hydrochloric acid, because HCl contains chloride ions that would interfere.
Example: KCl(aq) + AgNO₃(aq) → KNO₃(aq) + AgCl(s).

Test for Sulfates

Acidify the sample with dilute hydrochloric acid, then add barium chloride solution.
A white precipitate of barium sulfate is a positive result for sulfate ions.
Ionic equation: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).
The test can also be carried out using barium nitrate solution.
HCl is added first to remove any carbonates, which would also produce a precipitate and interfere.
State symbols for the reaction are aq, aq, s.

Core Practical: Identifying Ions

The aim is to use chemical tests to identify the ions in unknown single ionic compounds.
Common tests include flame tests and tests for sulfate, carbonate and halide ions.
Tests can be carried out in any order; you may not need to do all tests on one sample.
Use only small amounts of sample and reagent; dissolve solid samples in distilled water if needed.
Record observations carefully in a results table and repeat any tests that give unclear results.
Identify the salt from the cation and anion present, balancing charges in the formula.
Example: a positive test for Fe²⁺ and sulfate ions means the salt is FeSO₄.
Hazards: limewater, dilute nitric acid and sodium hydroxide are harmful; ammonia gas is toxic if inhaled.

Instrumental Methods of Analysis

Instrumental methods include X-ray, infra-red and mass spectroscopy, gas chromatography and flame photometry.
They are more accurate, faster and easier to use, and more sensitive than traditional chemical tests.
They can be automated and can perform multiple simultaneous sampling and testing.
Modern instruments can work with very small sample sizes.
Advantages should be described in terms of sensitivity, speed and accuracy.

Flame Emission Spectroscopy

Flame emission spectroscopy is an instrumental method used to analyse metal ions in solution.
The sample is put into a flame and the light emitted is passed through a spectroscope.
The output is a line spectrum; different elements produce lines in different parts of the spectrum.
The spectrum identifies the metal ions present and can measure their concentrations.
The intensity of light is proportional to the number of ions vaporised, so a calibration curve can determine concentration.
It works for mixtures of ions, unlike a flame test which can only analyse one ion at a time.
Unknown samples are identified by comparing their spectrum to reference spectra.

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Preguntas de práctica

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1.What colour flame is produced by potassium ions in a flame test?
Easy
- AYellow
- BLilac
- CCrimson
- DOrange-red
2.A student adds sodium hydroxide solution to a solution containing iron(II) ions. What is observed?
Easy
- AA blue precipitate
- BA green precipitate
- CA brown precipitate
- DA white precipitate that dissolves in excess
3.Which reagent is used to test for sulfate ions in solution?
Easy
- ASilver nitrate
- BBarium chloride
- CSodium hydroxide
- DLimewater
4.What is the correct test for ammonium ions?
Easy
- AAdd sodium hydroxide and warm gently; test the gas with damp red litmus
- BAdd silver nitrate; look for a white precipitate
- CAdd barium chloride; look for a white precipitate
- DAdd dilute acid; test the gas with limewater
5.Which instrumental method is used to analyse metal ions in solution by measuring the intensity and wavelength of light emitted?
Medium
- AGas chromatography
- BFlame emission spectroscopy
- CMass spectrometry
- DInfra-red spectroscopy
6.Which statements about testing for halide ions are correct? (select all that apply)
Medium
- ADilute nitric acid is added before silver nitrate solution
- BSilver chloride forms a white precipitate
- CSilver bromide forms a yellow precipitate
- DSilver iodide forms a yellow precipitate
- EDilute hydrochloric acid is used to acidify the solution
7.Sodium ions produce a yellow flame in a flame test.
Easy
True or false?
8.Instrumental methods are less accurate than traditional chemical tests.
Easy
True or false?

Más preguntas

9.Match each metal ion to the colour of precipitate formed when sodium hydroxide solution is added.
Medium
- Cu2+
- Fe2+
- Fe3+
- Blue
- Green
- Brown
10.Put the steps of a flame test in the correct order.
Medium
- Dip the loop of the wire into dilute hydrochloric acid
- Hold the wire in a blue Bunsen flame until there is no colour change
- Dip the loop into the solid sample
- Place the loop in the edge of the blue Bunsen flame and observe the colour
11.Which metal ion is present in the unknown solution?
Medium
- ASodium
- BPotassium
- CCalcium
- DCopper
12.Which anion is present in the unknown solution?
Medium
- AChloride
- BBromide
- CIodide
- DSulfate
13.A student adds sodium hydroxide solution to a solution containing an unknown cation. A white precipitate forms. Which metal ions could be present? (select all that apply)
Medium
- AAluminium
- BCalcium
- CCopper(II)
- DIron(II)
- EMagnesium
14.Which statements about testing for ammonium ions are correct? (select all that apply)
Medium
- ASodium hydroxide solution is added and the mixture is gently heated.
- BAmmonia gas is produced if ammonium ions are present.
- CThe gas produced turns damp red litmus paper blue.
- DThe gas produced turns limewater cloudy.
- EThe gas produced gives a squeaky pop with a lit splint.
15.When testing for halide ions, dilute hydrochloric acid should be added before silver nitrate solution.
Easy
True or false?
16.The precipitate formed when sodium hydroxide solution is added to iron(III) ions is brown.
Easy
True or false?
17.Match each metal ion to the colour of flame it produces in a flame test.
Medium
- Lithium
- Sodium
- Potassium
- Calcium
- Copper
- Crimson
- Yellow
- Lilac
- Orange-red
- Green
18.Match each anion to the reagent used to test for it.
Medium
- Carbonate
- Sulfate
- Chloride
- Bromide
- Iodide
- Dilute acid, then bubble gas through limewater
- Dilute hydrochloric acid, then barium chloride solution
- Dilute nitric acid, then silver nitrate solution
19.Put the steps for carrying out a flame test in the correct order.
Medium
- Dip the loop of a clean platinum or nichrome wire into dilute hydrochloric acid.
- Hold the wire in the blue flame of a Bunsen burner until there is no colour change.
- Dip the loop into the solid sample.
- Place the loop in the edge of the blue Bunsen flame.
- Observe and record the colour of the flame.
20.Put the steps for testing an unknown solution for sulfate ions in the correct order.
Medium
- Add a few drops of dilute hydrochloric acid to the solution.
- Add a few drops of barium chloride solution.
- Observe whether a white precipitate forms.
21.A student adds sodium hydroxide solution drop by drop to a solution and a white precipitate forms. They then add excess sodium hydroxide solution and the precipitate dissolves to give a colourless solution. Which metal ion is present?
Medium
- ACalcium ion
- BAluminium ion
- CMagnesium ion
- DAmmonium ion
22.A student tests a solution for halide ions by adding dilute nitric acid and then silver nitrate solution. A cream precipitate forms. Which halide ion is present?
Medium
- AChloride
- BBromide
- CIodide
- DSulfate
23.Which reagent is added to a solution to test for sulfate ions after acidifying with dilute hydrochloric acid?
Medium
- ASilver nitrate solution
- BBarium chloride solution
- CSodium hydroxide solution
- DLimewater
24.A student adds dilute hydrochloric acid to a solution and bubbles the gas produced through limewater. The limewater turns cloudy. Which ion is present in the solution?
Medium
- ASulfate
- BCarbonate
- CChloride
- DAmmonium
25.A student carries out a flame test on a solution containing a mixture of sodium ions and potassium ions. What would they observe?
Medium
- AOnly a lilac flame
- BOnly a yellow flame
- CA yellow flame with a lilac tinge
- DNo flame colour
26.Which of the following is an advantage of using instrumental methods such as flame emission spectroscopy over traditional chemical tests?
Medium
- AThey are less accurate
- BThey are slower to use
- CThey can analyse mixtures of ions
- DThey require larger sample sizes
27.Which of the following statements about testing for ammonium ions are correct? (select all that apply)
Medium
- ASodium hydroxide solution is added and the mixture is gently heated.
- BAmmonia gas is produced.
- CThe gas turns damp red litmus paper blue.
- DThe gas turns limewater cloudy.
- EA white precipitate is formed.
28.Which of the following statements about flame emission spectroscopy are correct? (select all that apply)
Medium
- AIt measures the intensity and wavelength of light emitted.
- BIt can be used to determine the concentration of metal ions in solution.
- CIt produces a line spectrum.
- DIt can only analyse one ion at a time.
- EIt is less sensitive than chemical tests.
29.When carrying out a flame test, the wire loop should be cleaned by dipping it in dilute acid and heating it in a blue flame until no colour is seen.
Easy
True or false?
30.When testing for halide ions, dilute hydrochloric acid is added before silver nitrate solution.
Medium
True or false?
31.Match each metal ion to the colour of the precipitate formed when sodium hydroxide solution is added.
Medium
- Copper(II) ion
- Iron(II) ion
- Iron(III) ion
- Blue precipitate
- Green precipitate
- Brown precipitate
32.A student carries out a flame test on a solid salt. The wire loop is not cleaned before use. What is the most likely effect on the result?
Medium
- AThe flame colour will be more intense and easier to identify.
- BThe flame colour will be less intense and harder to identify.
- CThe flame colour may be a mixture of colours from different ions, making identification unreliable.
- DThe flame will not be produced at all.
33.A student adds a few drops of sodium hydroxide solution to a solution containing an unknown cation. A white precipitate forms. The student then adds excess sodium hydroxide solution and the precipitate dissolves, forming a colourless solution. Which cation is present?
Medium
- ACalcium ion
- BAluminium ion
- CMagnesium ion
- DAmmonium ion
34.A solution is tested for sulfate ions. The student adds dilute hydrochloric acid, then barium chloride solution, and a white precipitate forms. What is the identity of the precipitate?
Medium
- ABarium sulfate
- BBarium chloride
- CSilver chloride
- DCalcium carbonate
35.Which reagent is added to a solution to test for halide ions, after acidifying with dilute nitric acid?
Medium
- ABarium chloride solution
- BSilver nitrate solution
- CSodium hydroxide solution
- DLimewater
36.In a flame test, the wire loop should be cleaned by dipping it in dilute acid and heating it in a blue Bunsen flame until no colour is seen.
Easy
True or false?
37.A student tests an unknown solution. They add dilute nitric acid and silver nitrate solution, and a yellow precipitate forms. They then add sodium hydroxide solution to a fresh sample and a blue precipitate forms. Which ions are present in the solution?
Medium
- ACopper(II) and iodide
- BCopper(II) and chloride
- CIron(II) and iodide
- DIron(III) and bromide
38.Which of the following are advantages of using instrumental methods such as flame emission spectroscopy compared with traditional chemical tests? (select all that apply)
Medium
- AThey are more sensitive and can detect very small amounts.
- BThey are faster and easier to use.
- CThey can analyse mixtures of ions.
- DThey do not require any sample preparation.
- EThey are always less accurate than chemical tests.
39.A solution contains a mixture of sodium ions and potassium ions. A flame test is carried out. What is observed and why?
Hard
- AA lilac flame is seen because potassium ions produce a more intense colour.
- BA yellow flame is seen because sodium ions mask the lilac colour of potassium ions.
- CA crimson flame is seen because the colours mix to give crimson.
- DNo flame colour is seen because the colours cancel out.
40.Flame emission spectroscopy can be used to determine the concentration of metal ions in a solution.
Medium
True or false?
41.Name the gas that turns limewater cloudy when testing for carbonate ions.
Medium
42.A flame emission spectrum shows a line at a wavelength of 589 nm. Using reference spectra, which metal ion is present?
Medium
43.Complete the sentence about flame tests.
Easy
In a flame test, a clean wire is dipped in dilute ____ and then held in a blue Bunsen flame to prevent ____.
44.What is the name of the compound in the unknown solution?
Medium
45.A flame emission spectrum of an unknown solution is compared with reference spectra. The unknown shows a line at 589 nm, which matches the reference spectrum for sodium. If the calibration graph shows that an intensity of 30 units corresponds to a concentration of 0.05 mol/dm³, what is the concentration of a solution with an intensity of 60 units? Give your answer in mol/dm³.
Hard
46.Complete the sentences about testing for halide ions.
Medium
When testing for halide ions, the solution is first acidified with dilute ____. Then ____ solution is added. A white precipitate indicates ____ ions, a cream precipitate indicates ____ ions, and a yellow precipitate indicates ____ ions.
47.What is the colour of the flame produced by potassium ions in a flame test?
Easy
48.A flame emission spectrum shows a line at a wavelength of 589 nm. Using reference spectra, which metal ion is present? Give the name of the metal ion.
Medium
49.Complete the sentence about testing for carbonate ions.
Easy
When dilute acid is added to a carbonate, ____ gas is produced, which turns limewater ____.
50.Name the reagent used to test for sulfate ions after acidifying with dilute hydrochloric acid.
Medium
51.A flame emission spectrum shows a line at a wavelength of 589 nm. Using reference spectra, this line matches the reference spectrum for sodium ions. Give the wavelength of this sodium line in metres. Give your answer in standard form.
Medium
52.A calibration curve for calcium ions in flame emission spectroscopy is a straight line through the origin. A standard solution of concentration 0.10 mol/dm³ gives a light intensity of 40 units. An unknown calcium solution gives an intensity of 100 units. Calculate the concentration of calcium ions in the unknown solution in mol/dm³.
Medium
53.Describe how to test a solution for sulfate ions, including the reagent used and the expected observation.
Medium
54.Describe how you would test an unknown solution to identify the metal ion present using a flame test. Include the expected observations for two different metal ions.
Hard
55.A student is given a solution that may contain sulfate ions or carbonate ions. Describe a test that would distinguish between these two ions, including the reagents used and the observations expected.
Hard
56.Explain why a flame test cannot be used to identify a mixture of sodium ions and potassium ions in the same solution.
Medium
57.Describe how you would test a solution for carbonate ions, including the reagent used and the observation that confirms the ion is present.
Medium
58.A flame emission spectrum shows a line at a wavelength of 589 nm. Using reference spectra, which metal ion is present?
Medium
- A648 nm
- B530 nm
- C589 nm
- D707 nm
59.A flame emission spectrum of an unknown solution is compared with reference spectra. The unknown shows a line at 589 nm, which matches the reference spectrum for sodium. If the calibration graph shows that an intensity of 30 units corresponds to a concentration of 0.05 mol/dm³, what is the concentration of a solution with an intensity of 60 units? Give your answer in mol/dm³.
Hard
- A-0.9 mol/dm³
- B0.1 mol/dm³
- C1.1 mol/dm³
- D2.1 mol/dm³
60.A flame emission spectrum shows a line at a wavelength of 589 nm. Using reference spectra, which metal ion is present? Give the name of the metal ion.
Medium
- A2
- B0
- C1
- D-1
61.A flame emission spectrum shows a line at a wavelength of 589 nm. Using reference spectra, this line matches the reference spectrum for sodium ions. Give the wavelength of this sodium line in metres. Give your answer in standard form.
Medium
- A5.89e-7 m
- B-1 m
- C2 m
- D1 m
62.A calibration curve for calcium ions in flame emission spectroscopy is a straight line through the origin. A standard solution of concentration 0.10 mol/dm³ gives a light intensity of 40 units. An unknown calcium solution gives an intensity of 100 units. Calculate the concentration of calcium ions in the unknown solution in mol/dm³.
Medium
- A0.25 mol/dm³
- B2.25 mol/dm³
- C-0.75 mol/dm³
- D1.25 mol/dm³

Tarjetas

What is a flame test used to identify?

A flame test is used to identify metal ions (cations) by the colour of the flame they produce when heated strongly.

Why is a platinum or nichrome wire dipped in dilute acid and heated before a flame test?

To clean the wire and remove any contamination, so that only the sample's ions are present and the flame colour is not affected by impurities.

What colour flame do lithium ions produce in a flame test?

Lithium ions produce a crimson flame.

What colour flame do sodium ions produce in a flame test?

Sodium ions produce a yellow flame.

What colour flame do potassium ions produce in a flame test?

Potassium ions produce a lilac flame.

What colour flame do calcium ions produce in a flame test?

Calcium ions produce an orange-red flame.

What colour flame do copper(II) ions produce in a flame test?

Copper(II) ions produce a green flame.

Why might a flame test fail to identify a metal ion in a mixture?

If a sample contains a mixture of ions, the flame colours can mix or one colour can mask another, making identification difficult or impossible.

What is the test for ammonium ions (NH4+)?

Add sodium hydroxide solution and gently heat. If ammonium ions are present, ammonia gas is produced.

How do you test for ammonia gas?

Hold damp red litmus paper near the mouth of the test tube; ammonia turns it blue. Alternatively, ammonia reacts with hydrogen chloride to form a white smoke of ammonium chloride.

What is the difference between ammonium and ammonia?

Ammonium is the aqueous cation, NH4+, while ammonia is the gas, NH3.

What is the test for carbonate ions (CO3 2–)?

Add dilute acid; if carbonate is present, effervescence occurs and carbon dioxide gas is released. Bubble the gas through limewater; it turns milky/cloudy.

What is the test for sulfate ions (SO4 2–)?

Acidify with dilute hydrochloric acid, then add barium chloride solution. A white precipitate of barium sulfate forms.

What is the test for halide ions (Cl–, Br–, I–)?

Acidify with dilute nitric acid, then add silver nitrate solution. A silver halide precipitate forms: white for chloride, cream for bromide, yellow for iodide.

Why is dilute nitric acid used before silver nitrate in the halide test, rather than hydrochloric acid?

Hydrochloric acid contains chloride ions, which would react with silver nitrate and give a false positive result for chloride.

Why is dilute hydrochloric acid added before barium chloride in the sulfate test?

It removes any carbonate ions present, which would also form a white precipitate with barium chloride and interfere with the test.

What is the effect of adding sodium hydroxide solution to copper(II) ions?

A light blue precipitate of copper hydroxide forms, which is insoluble in excess sodium hydroxide.

What is the effect of adding sodium hydroxide solution to iron(II) ions?

A green precipitate of iron(II) hydroxide forms, which is insoluble in excess sodium hydroxide.

What is the effect of adding sodium hydroxide solution to iron(III) ions?

A brown precipitate of iron(III) hydroxide forms, which is insoluble in excess sodium hydroxide.

How can aluminium ions be distinguished from calcium ions using sodium hydroxide solution?

Both form a white precipitate with sodium hydroxide, but aluminium hydroxide dissolves in excess sodium hydroxide to give a colourless solution, while calcium hydroxide does not dissolve.

What are two advantages of instrumental methods over chemical tests?

Instrumental methods are more accurate and more sensitive; they are also faster and easier to use.

What is flame emission spectroscopy used for?

It is an instrumental method used to analyse metal ions in solution, identifying the metal ions present and measuring their concentrations.

How does flame emission spectroscopy work?

The sample is put into a flame, and the light emitted is passed through a spectroscope. The intensity and wavelength of the light are measured, producing a line spectrum.

How can flame emission spectroscopy identify metal ions in a mixture?

The line spectrum of the sample is compared with reference spectra; each metal ion produces a unique pattern of lines, so mixtures can be analysed.

What is the advantage of flame emission spectroscopy over a flame test?

Flame emission spectroscopy can analyse mixtures of ions, whereas a flame test can only analyse one ion at a time.

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