Identification Of Ions And Gases

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Identification of Anions

Carbonate (CO₃²⁻): Add dilute acid; bubble gas through limewater – turns cloudy (white precipitate of CaCO₃).
Halide ions (Cl⁻, Br⁻, I⁻): Acidify with nitric acid, then add AgNO₃ solution. Precipitate colour: Cl⁻ = white (AgCl), Br⁻ = cream (AgBr), I⁻ = yellow (AgI).
Nitrate (NO₃⁻): Add NaOH(aq) and aluminium foil; warm gently. Releases ammonia gas – turns damp red litmus blue.
Sulfate (SO₄²⁻): Acidify with nitric acid, then add barium nitrate solution. White precipitate of BaSO₄ forms.

Testing for carbonate ions

Identification of Cations – NaOH & Ammonia Tests

Add few drops NaOH(aq) then excess; observe precipitate colour and solubility.
Ca²⁺: white precipitate, insoluble in excess NaOH and excess ammonia.
Cu²⁺: light blue precipitate, insoluble in excess NaOH, dissolves in excess ammonia to form dark blue solution.
Fe²⁺: green precipitate, insoluble in excess NaOH and ammonia.
Fe³⁺: brown precipitate, insoluble in excess NaOH and ammonia.
Zn²⁺: white precipitate, dissolves in excess NaOH and excess ammonia to form colourless solution.
NH₄⁺: warm with NaOH – releases ammonia gas (damp red litmus turns blue).

Metal ion tests summary

Flame Tests for Metal Cations

Clean nichrome/platinum wire loop by dipping in concentrated acid and heating in blue Bunsen flame.
Dip loop into sample and hold in edge of blue flame; observe colour.
Li⁺ → crimson; Na⁺ → yellow; K⁺ → lilac; Cu²⁺ → blue-green.
Avoid wire glowing red (confuses colour); use blue flame for strong heating.

Flame test colours

Identification of Gases

Ammonia (NH₃): damp red litmus turns blue; sharp choking smell.
Carbon dioxide (CO₂): bubble through limewater – turns cloudy/milky (white CaCO₃ precipitate).
Chlorine (Cl₂): damp blue litmus turns red then bleached white; toxic – use fume cupboard.
Hydrogen (H₂): lighted splint at mouth of tube gives ‘squeaky pop’ (rapid combustion).
Oxygen (O₂): glowing splint relights (or glows brightly).

Testing for ammonia gas

Key Tips & Common Mistakes

For carbonate test, say ‘bubble gas through limewater’ – not just ‘limewater turns cloudy’.
Halide test: use nitric acid not HCl (Cl⁻ would interfere).
Sulfate test: add nitric acid first to remove carbonates that also give white precipitate.
Add NaOH/ammonia slowly – some precipitates dissolve in excess and may be missed.
Distinguish colourless (no colour) from clear (transparent, may have colour).

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Câu hỏi luyện tập

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1.Which row correctly describes the test for each gas?
Easy
- AA: oxygen - lighted splint pops; hydrogen - glowing splint relights; chlorine - damp blue litmus turns red then white
- BB: oxygen - glowing splint relights; hydrogen - lighted splint pops; chlorine - damp blue litmus turns red then white
- CC: oxygen - glowing splint relights; hydrogen - lighted splint pops; chlorine - damp red litmus turns blue then white
- DD: oxygen - lighted splint pops; hydrogen - glowing splint relights; chlorine - damp red litmus turns blue then white
2.Halide ions in solution are detected using silver nitrate solution. Which row correctly describes the colour of the precipitate for each halide?
Easy
- AA: chloride - cream; bromide - white; iodide - silver
- BB: chloride - yellow; bromide - cream; iodide - white
- CC: chloride - white; bromide - cream; iodide - yellow
- DD: chloride - white; bromide - cream; iodide - silver
3.A solution containing an unknown substance was analysed. On the addition of acidified silver nitrate solution a yellow precipitate formed. A flame test produced a lilac coloured flame. What is the identity of the unknown solid?
Medium
- AA: Lithium iodide
- BB: Lithium bromide
- CC: Potassium iodide
- DD: Potassium bromide
4.Metal cations can be detected using flame tests. Which row correctly describes the colour of the flame for each metal cation?
Easy
- AA: Li+ - lilac; K+ - green; Cu2+ - yellow
- BB: Li+ - red; K+ - lilac; Cu2+ - green
- CC: Li+ - green; K+ - lilac; Cu2+ - red
- DD: Li+ - red; K+ - orange; Cu2+ - blue
5.A student tested copper sulfate solution and potassium iodide solution using flame tests. The student's method did not produce the distinct lilac flame colour using potassium iodide solution. Which statement would not cause the lack of lilac flame?
Medium
- AA: The metal wire loop being overheated
- BB: Using the safety flame of the Bunsen burner
- CC: The same metal wire loop being used for each experiment
- DD: The metal wire loop being dipped in concentrated acid and burned between flame tests
6.The results of two tests on a solid are shown below. Test 1: Acidified silver nitrate added - cream precipitate formed. Test 2: Aqueous sodium hydroxide added - red-brown precipitate formed. What is the solid?
Medium
- AA: Iron(II) bromide
- BB: Iron(III) bromide
- CC: Iron(II) chloride
- DD: Iron(III) chloride
7.Which reaction would result in the production of a gas that would produce a squeaky pop with a lit splint?
Medium
- AA: Iron(II) oxide + sulfuric acid
- BB: Methane + oxygen during complete combustion
- CC: Magnesium + hydrochloric acid
- DD: Carbon dioxide and water in the presence of light and chlorophyll
8.A solution of an unknown salt is tested by adding aqueous sodium hydroxide. A blue precipitate is formed which is insoluble in excess. The mixture is then warmed and aluminium foil is added. A gas is released which turns damp red litmus paper blue. What is the salt?
Hard
- AA: Copper(II) nitrate
- BB: Copper(II) sulfate
- CC: Ammonium nitrate
- DD: Ammonium sulfate

Thêm câu hỏi

9.A student is given a white insoluble solid to analyse. He first added hydrochloric acid and observed bubbles of colourless gas. He then added a small amount of aqueous ammonia to the solid-acid mixture and observed a white precipitate being formed. The precipitate dissolved in excess aqueous ammonia. What is the identity of the white solid?
Hard
- AA: Ammonium nitrate
- BB: Aluminium nitrate
- CC: Zinc carbonate
- DD: Calcium carbonate
10.Aqueous sodium hydroxide is used in chemical analysis. Which statement about aqueous NaOH is correct?
Medium
- AA: A white precipitate is formed when added to a solution containing sulfate ions
- BB: A green precipitate is formed upon addition to a solution of iron(II) ions which does not dissolve in excess
- CC: A green precipitate is formed upon addition to a solution of iron(II) ions which dissolves in excess
- DD: An acidic gas is produced when added to ammonium chloride
11.A student analysed a solution and found it to contain ions of silver and barium. What could the identity of the anion be?
Medium
- AA: Nitrate
- BB: Sulfate
- CC: Chloride
- DD: Carbonate
12.Which of the following is the test for carbonate ions?
Easy
- AAdd dilute acid and bubble the gas through limewater; limewater turns cloudy.
- BAdd silver nitrate solution and observe the colour of the precipitate.
- CAdd barium nitrate solution and observe if a white precipitate forms.
- DAdd sodium hydroxide solution and warm; test the gas with damp red litmus paper.
13.What is the colour of the precipitate formed when silver nitrate solution is added to a solution containing bromide ions?
Easy
- AWhite
- BCream
- CYellow
- DBlue
14.Ammonia gas turns damp red litmus paper blue.
Easy
True or false?
15.A solution gives a white precipitate with acidified barium nitrate. Which ion is likely present?
Easy
- AChloride
- BSulfate
- CCarbonate
- DNitrate
16.Which of the following ions produce a white precipitate when aqueous sodium hydroxide is added? (Select all that apply.)
Medium
- ACalcium
- BCopper(II)
- CZinc
- DIron(II)
- EIron(III)
17.In the test for halide ions, hydrochloric acid is used to acidify the sample before adding silver nitrate.
Easy
True or false?
18.Arrange the following steps in the correct order for performing a flame test on a solid sample.
Medium
- Dip the loop into the solid sample.
- Clean the wire loop by dipping in concentrated acid and heating in a blue flame.
- Place the loop in the edge of the blue Bunsen flame.
- Observe the colour of the flame.
19.A white solid gives a yellow precipitate with acidified silver nitrate and a lilac flame in a flame test. What is the solid?
Medium
- ALithium iodide
- BLithium bromide
- CPotassium iodide
- DPotassium bromide
20.Match each gas to its test result.
Medium
- ammonia
- carbon dioxide
- hydrogen
- oxygen
- turns damp red litmus blue
- turns limewater cloudy
- gives a squeaky pop with a lighted splint
- relights a glowing splint
21.Arrange the following steps in the correct order for testing a solution for nitrate ions.
Medium
- Add aqueous NaOH and aluminium foil
- Warm gently
- Test the gas released with damp red litmus paper
- Observe if ammonia is produced
22.Chlorine gas turns damp blue litmus paper red and then bleaches it white.
Easy
True or false?
23.When testing for nitrate ions, the gas released turns damp red litmus paper blue.
Easy
True or false?
24.Match each gas to its correct test result.
Easy
- Hydrogen
- Oxygen
- Chlorine
- Ammonia
- Relights a glowing splint
- Turns damp red litmus blue
- Bleaches damp blue litmus
- Burns with a squeaky pop
25.Arrange the following steps in the correct order for testing a solution for sulfate ions.
Medium
- Add a few drops of barium nitrate solution
- Acidify the sample with dilute nitric acid
- Observe if a white precipitate forms
26.A solution contains a mixture of two anions. When acidified barium nitrate is added, a white precipitate forms. When the precipitate is filtered off and the filtrate is acidified with nitric acid and silver nitrate added, a cream precipitate forms. Which anions are present?
Hard
- ASulfate and chloride
- BSulfate and bromide
- CSulfite and bromide
- DCarbonate and iodide
27.A student adds dilute nitric acid to a solid sample and bubbles the gas produced through limewater. The limewater turns cloudy. Which ion is present in the solid?
Medium
- ASulfate
- BCarbonate
- CNitrate
- DChloride
28.Which gas turns damp red litmus paper blue?
Easy
- AChlorine
- BCarbon dioxide
- CAmmonia
- DHydrogen
29.In the test for sulfate ions, dilute hydrochloric acid is used to acidify the sample before adding barium nitrate solution.
Easy
True or false?
30.What is the colour of the precipitate formed when silver nitrate solution is added to a solution containing iodide ions?
Easy
- AWhite
- BCream
- CYellow
- DGreen
31.Which reagent is used to test for nitrate ions?
Easy
- ABarium nitrate solution
- BSilver nitrate solution
- CSodium hydroxide solution and aluminium foil
- DLimewater
32.Arrange the following steps in the correct order for carrying out a flame test on a solid sample.
Medium
- Dip the loop in concentrated acid and hold in a blue flame until no colour change
- Dip the clean loop into the solid sample
- Hold the loop in the edge of the blue Bunsen flame and observe colour
- Place a small sample of the solid on a metal wire loop
33.Which of the following gases are correctly tested by the method described? (Select all that apply.)
Medium
- AHydrogen - a lighted splint produces a squeaky pop
- BOxygen - a glowing splint relights
- CCarbon dioxide - a lighted splint is extinguished
- DChlorine - damp blue litmus paper turns red then white
- EAmmonia - damp red litmus paper turns blue
34.A student adds dilute nitric acid to a solution, then adds a few drops of barium nitrate solution. A white precipitate forms. Which ion is present in the solution?
Medium
- ACarbonate
- BSulfate
- CChloride
- DNitrate
35.In the test for halide ions, the sample is acidified with hydrochloric acid before adding silver nitrate.
Easy
True or false?
36.Match each metal ion to the colour of its flame in a flame test.
Easy
- Lithium
- Potassium
- Copper
- Crimson
- Lilac
- Blue-green
37.Complete the sentence.
Easy
The test for oxygen is to use a ____ splint; if oxygen is present, the splint ____.
38.State the colour of the flame produced by lithium ions in a flame test.
Easy
39.A student tests a solution containing an unknown cation. When a few drops of sodium hydroxide are added, a white precipitate forms that dissolves in excess sodium hydroxide. How many different cations from the list (Ca2+, Cu2+, Fe2+, Fe3+, Zn2+) could give this result?
Medium
40.How many different halide ions are tested using silver nitrate solution in the IGCSE syllabus?
Easy
41.A student tests a solution containing sulfate ions. She adds excess barium nitrate solution and collects the precipitate. The mass of the dry precipitate is 2.33 g. Calculate the mass of sulfate ions in the original solution. [Ar: Ba=137, S=32, O=16]
Hard
42.Complete the sentence about the test for carbonate ions.
Easy
To test for carbonate ions, add dilute acid and bubble the gas through ____. If carbonate is present, the solution turns ____.
43.State the colour of the precipitate formed when aqueous sodium hydroxide is added to a solution containing iron(III) ions.
Easy
44.A student adds excess barium nitrate solution to 25.0 cm³ of a solution containing sulfate ions. The white precipitate of barium sulfate is filtered, dried, and weighed. The mass of the precipitate is 0.233 g. Calculate the concentration of sulfate ions in the original solution in mol/dm³. (Molar mass of BaSO₄ = 233 g/mol)
Hard
45.Complete the sentences about the test for nitrate ions.
Medium
To test for nitrate ions, add aqueous ____ and ____ foil, then warm gently. The gas released is ____ which turns damp red litmus paper ____.
46.State the colour of the precipitate formed when aqueous sodium hydroxide is added to a solution containing iron(II) ions.
Easy
47.A 0.500 g sample of a mixture containing only sodium chloride and sodium bromide is dissolved in water. Excess acidified silver nitrate is added, producing a mixture of silver chloride and silver bromide precipitates with a total mass of 1.200 g. Calculate the percentage by mass of sodium chloride in the mixture. (Molar masses: NaCl = 58.5 g/mol, NaBr = 103 g/mol, AgCl = 143.5 g/mol, AgBr = 188 g/mol)
Hard
48.Complete the sentences about the test for ammonium ions.
Easy
To test for ammonium ions, add ____ solution and warm. The gas released is ____ which turns damp red litmus paper ____.
49.Describe the test for nitrate ions, including the reagents used and the expected result.
Medium
50.A student is given a white solid. When dilute hydrochloric acid is added, bubbles of a colourless gas are produced. The gas turns limewater cloudy. The resulting solution is colourless. When aqueous sodium hydroxide is added to the solution, a white precipitate forms that dissolves in excess sodium hydroxide. Identify the white solid and explain the observations.
Hard
51.Describe the test for sulfate ions, including the reagents used and the expected result.
Easy
52.Describe the test for oxygen gas.
Easy
53.Describe the test for ammonia gas.
Easy
54.A student tests a solution containing an unknown cation. When a few drops of sodium hydroxide are added, a white precipitate forms that dissolves in excess sodium hydroxide. How many different cations from the list (Ca2+, Cu2+, Fe2+, Fe3+, Zn2+) could give this result?
Medium
- A2
- B1
- C0
- D3
55.How many different halide ions are tested using silver nitrate solution in the IGCSE syllabus?
Easy
- A5
- B4
- C2
- D3
56.A student tests a solution containing sulfate ions. She adds excess barium nitrate solution and collects the precipitate. The mass of the dry precipitate is 2.33 g. Calculate the mass of sulfate ions in the original solution. [Ar: Ba=137, S=32, O=16]
Medium
- A0.96 g
- B1.96 g
- C-0.04 g
- D2.96 g
57.A student adds excess barium nitrate solution to 25.0 cm³ of a solution containing sulfate ions. The white precipitate of barium sulfate is filtered, dried, and weighed. The mass of the precipitate is 0.233 g. Calculate the concentration of sulfate ions in the original solution in mol/dm³. (Molar mass of BaSO₄ = 233 g/mol)
Hard
- A0.04 mol/dm³
- B1.04 mol/dm³
- C2.04 mol/dm³
- D-0.96 mol/dm³
58.A 0.500 g sample of a mixture containing only sodium chloride and sodium bromide is dissolved in water. Excess acidified silver nitrate is added, producing a mixture of silver chloride and silver bromide precipitates with a total mass of 1.200 g. Calculate the percentage by mass of sodium chloride in the mixture. (Molar masses: NaCl = 58.5 g/mol, NaBr = 103 g/mol, AgCl = 143.5 g/mol, AgBr = 188 g/mol)
Hard
- A91.6 %
- B73.2 %
- C29.3 %
- D8.4 %

Thẻ ghi nhớ

What is the test for carbonate ions (CO₃²⁻)?

Add dilute acid and bubble the gas through limewater. Limewater turns cloudy if carbonate is present.

What is observed when carbonate ions react with acid?

Fizzing/effervescence occurs as CO₂ gas is produced.

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

Acidify with nitric acid, then add silver nitrate solution. A silver halide precipitate forms.

What colour precipitate does chloride give with silver nitrate?

White precipitate of silver chloride (AgCl).

What colour precipitate does bromide give with silver nitrate?

Cream precipitate of silver bromide (AgBr).

What colour precipitate does iodide give with silver nitrate?

Yellow precipitate of silver iodide (AgI).

Why is nitric acid added before silver nitrate in the halide test?

To remove any carbonate ions that would also form a precipitate and interfere.

What is the test for nitrate ions (NO₃⁻)?

Add aqueous NaOH and aluminium foil, warm gently. Ammonia gas is released, which turns damp red litmus paper blue.

What is the test for sulfate ions (SO₄²⁻)?

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

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

Warm with sodium hydroxide solution. Ammonia gas is released, turning damp red litmus paper blue.

What colour precipitate does Cu²⁺ form with NaOH or ammonia (few drops)?

Light blue precipitate.

What happens to the Cu²⁺ precipitate in excess ammonia?

It dissolves to form a dark blue solution.

What colour precipitate does Fe²⁺ form with NaOH?

Green precipitate.

What colour precipitate does Fe³⁺ form with NaOH?

Brown precipitate.

What colour precipitate does Zn²⁺ form with NaOH or ammonia (few drops)?

White precipitate.

What happens to the Zn²⁺ precipitate in excess NaOH or excess ammonia?

It dissolves to form a colourless solution.

What is the flame colour for lithium (Li⁺)?

Crimson.

What is the flame colour for sodium (Na⁺)?

Yellow.

What is the flame colour for potassium (K⁺)?

Lilac.

What is the flame colour for copper (Cu²⁺)?

Blue-green.

How do you test for ammonia gas?

Damp red litmus paper turns blue.

How do you test for carbon dioxide gas?

Bubble through limewater; it turns cloudy/milky.

How do you test for chlorine gas?

Damp blue litmus paper turns red then bleached white.

How do you test for hydrogen gas?

Hold a lighted splint at the mouth of the test tube; it burns with a squeaky pop.

How do you test for oxygen gas?

Place a glowing splint inside the test tube; it relights.

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