Qualitative tests

Chemistry 1110 · CCEA A Level · 7 min read · Updated: 31 Aug 2026

Welcome to Qualitative Tests!

Imagine being a chemical detective. You are handed an unlabelled bottle containing a mysterious white powder or a colourless gas. How do you find out exactly what elements or compounds are inside? That is precisely what qualitative analysis is all about! Unlike quantitative analysis (which measures "how much"), qualitative tests tell us "what substance is present" using distinct colour changes, precipitates, smells, and flame colours.

In this chapter for CCEA AS 1 Chemistry, you will master the standard tests for identifying common gases, metal cations, the ammonium ion, and important anions like halides, sulfates, and carbonates.

1. Testing for Gases

When chemical reactions occur, they often produce gases. Here are the five key gases you need to recognise instantly, along with the precise observations required by CCEA examiners.

A. Hydrogen (\(\text{H}_2\))

Test: Hold a burning (lighted) splint near the mouth of the test tube.
Observation: Burns with a characteristic "squeaky pop" sound.

B. Oxygen (\(\text{O}_2\))

Test: Insert a glowing splint into the test tube.
Observation: Relights the glowing splint.

C. Chlorine (\(\text{Cl}_2\))

Test: Hold damp blue litmus paper (or damp Universal Indicator paper) in the gas.
Observation: Turns red (due to its acidic nature in water) and is then bleached white.

D. Carbon Dioxide (\(\text{CO}_2\))

Test: Bubble the gas through limewater (aqueous calcium hydroxide, \(\text{Ca(OH)}_2\)).
Observation: Limewater turns milky / cloudy white due to the formation of insoluble calcium carbonate (\(\text{CaCO}_3\)).

E. Ammonia (\(\text{NH}_3\))

Test 1: Hold damp red litmus paper in the gas.
Observation 1: Turns blue (ammonia is an alkaline gas).
Test 2: Hold a glass rod dipped in concentrated hydrochloric acid (\(\text{HCl}\)) near the gas.
Observation 2: Dense white fumes / smoke of solid ammonium chloride (\(\text{NH}_4\text{Cl}\)) are formed.
Extra clue: Ammonia has a sharp, pungent, choking smell.

Examiner Tip: Always remember to write damp litmus paper. Dry gases cannot react with dry indicator paper because water is required to form the ions that cause the colour change!

Key Takeaway for Gases:
\(\text{H}_2\) \(\rightarrow\) squeaky pop with burning splint | \(\text{O}_2\) \(\rightarrow\) relights glowing splint | \(\text{Cl}_2\) \(\rightarrow\) bleaches damp litmus white | \(\text{CO}_2\) \(\rightarrow\) turns limewater milky | \(\text{NH}_3\) \(\rightarrow\) damp red litmus turns blue & dense white fumes with conc. \(\text{HCl}\).

2. Identifying Metal Cations: Flame Tests

When metal ions are heated in a hot flame, their electrons absorb thermal energy, jump to higher energy levels (excited state), and then drop back down to lower energy levels, releasing light of specific wavelengths (colours). This allows us to identify metal cations by their unique flame colours.

The Flame Test Procedure: Step-by-Step

Don't worry if experimental methods seem detailed—following this exact routine guarantees full marks:

1. Take a clean nichrome wire (or platinum wire).
2. Dip the wire into concentrated hydrochloric acid (\(\text{HCl}\)).
3. Place the wire into a non-luminous (roaring blue) Bunsen flame until no colour is imparted (this ensures it is completely clean and converts metal residues to volatile chlorides).
4. Dip the clean wire back into concentrated \(\text{HCl}\), touch the solid sample, and place it into the non-luminous Bunsen flame to observe the colour.

CCEA Required Flame Colours

Memorise these exact colour descriptions:

• Lithium (\(\text{Li}^+\)): Crimson
• Sodium (\(\text{Na}^+\)): Yellow / Orange (yellow-orange)
• Potassium (\(\text{K}^+\)): Lilac (pink through cobalt glass)
• Calcium (\(\text{Ca}^{2+}\)): Brick red / Red
• Barium (\(\text{Ba}^{2+}\)): Green / Apple green
• Copper(II) (\(\text{Cu}^{2+}\)): Blue-green / Green-blue

Memory Trick: Think of Lipstick (Lithium = Crimson), Sunshine (Sodium = Yellow/Orange), and Banana/Apple (Barium = Apple green)!

Key Takeaway for Flame Tests: Use a clean nichrome wire dipped in concentrated \(\text{HCl}\) in a non-luminous flame. Learn your 6 flame colours precisely!

3. Chemical Test for the Ammonium Cation (\(\text{NH}_4^+\))

Ammonium compounds do not give a distinctive flame colour, so we use a chemical test instead.

The Test & Observations

Method: Add aqueous sodium hydroxide (\(\text{NaOH(aq)}\)) to the solid or solution and warm gently.
Observations: A pungent gas (\(\text{NH}_3\)) is produced that turns damp red litmus paper blue and produces dense white fumes when brought near a glass rod dipped in concentrated \(\text{HCl}\).

Ionic Equation (with state symbols):

\(\text{NH}_4^+\text{(aq)} + \text{OH}^-\text{(aq)} \rightarrow \text{NH}_3\text{(g)} + \text{H}_2\text{O(l)}\)

Key Takeaway: Warming any ammonium salt with \(\text{NaOH(aq)}\) releases ammonia gas (\(\text{NH}_3\)).

4. Chemical Tests for Anions (Negative Ions)

A. Carbonate Ion (\(\text{CO}_3^{2-}\))

Method: Add dilute acid (such as dilute \(\text{HCl}\) or dilute \(\text{HNO}_3\)) to the sample.
Observation: Effervescence / fizzing is seen. The colourless gas produced turns limewater milky / cloudy.
Ionic Equation:
\(\text{CO}_3^{2-}\text{(aq)} + 2\text{H}^+\text{(aq)} \rightarrow \text{CO}_2\text{(g)} + \text{H}_2\text{O(l)}\)
(Note: If testing solid carbonate, the state symbol is \(\text{CO}_3^{2-}\text{(s)}\)).

B. Sulfate Ion (\(\text{SO}_4^{2-}\))

Method: Dissolve the sample in water, acidify with dilute hydrochloric acid (\(\text{HCl}\)) (or dilute nitric acid), and add aqueous barium chloride (\(\text{BaCl}_2\)) or barium nitrate.
Observation: A white precipitate of barium sulfate (\(\text{BaSO}_4\)) forms immediately.
Ionic Equation:
\(\text{Ba}^{2+}\text{(aq)} + \text{SO}_4^{2-}\text{(aq)} \rightarrow \text{BaSO}_4\text{(s)}\)

Crucial Examiner Warning: Why do we add dilute \(\text{HCl}\) first? To remove any carbonate ions (\(\text{CO}_3^{2-}\)) that might be present, which would otherwise form an insoluble white precipitate of \(\text{BaCO}_3\) and give a false positive! Never use sulfuric acid (\(\text{H}_2\text{SO}_4\)) to acidify, because sulfuric acid contains sulfate ions and would create a white precipitate automatically!

C. Halide Ions (\(\text{Cl}^-\), \(\text{Br}^-\), \(\text{I}^-\))

Halide ions are tested using silver ions. Because the precipitate colours can look quite similar under lab lighting, we confirm their identities using aqueous ammonia (\(\text{NH}_3\)).

Step 1: Dissolve the solid in deionised water.
Step 2: Acidify with dilute nitric acid (\(\text{HNO}_3\)). (Never use \(\text{HCl}\), as adding chloride ions will ruin the test!).
Step 3: Add aqueous silver nitrate (\(\text{AgNO}_3\)).
Step 4: Add aqueous ammonia (\(\text{NH}_3\text{(aq)}\)) — first dilute, then concentrated.

Halide Identification Summary:

• Chloride (\(\text{Cl}^-\)):
- Initial precipitate: White precipitate (\(\text{AgCl}\))
- Addition of dilute \(\text{NH}_3\text{(aq)}\): Dissolves / soluble (forms a colourless solution)
- Addition of conc. \(\text{NH}_3\text{(aq)}\): Dissolves / soluble

• Bromide (\(\text{Br}^-\)):
- Initial precipitate: Cream precipitate (\(\text{AgBr}\))
- Addition of dilute \(\text{NH}_3\text{(aq)}\): Insoluble / does not dissolve
- Addition of conc. \(\text{NH}_3\text{(aq)}\): Dissolves / soluble (forms a colourless solution)

• Iodide (\(\text{I}^-\)):
- Initial precipitate: Yellow precipitate (\(\text{AgI}\))
- Addition of dilute \(\text{NH}_3\text{(aq)}\): Insoluble
- Addition of conc. \(\text{NH}_3\text{(aq)}\): Insoluble

General Ionic Equation for Halide Precipitation:

\(\text{Ag}^+\text{(aq)} + \text{X}^-\text{(aq)} \rightarrow \text{AgX(s)}\)
(where \(\text{X} = \text{Cl, Br, or I}\))

Specific examples:
\(\text{Ag}^+\text{(aq)} + \text{Cl}^-\text{(aq)} \rightarrow \text{AgCl(s)}\) (White)
\(\text{Ag}^+\text{(aq)} + \text{Br}^-\text{(aq)} \rightarrow \text{AgBr(s)}\) (Cream)
\(\text{Ag}^+\text{(aq)} + \text{I}^-\text{(aq)} \rightarrow \text{AgI(s)}\) (Yellow)

Mnemonic for Halide Precipitates: Why Can't You? = White (\(\text{AgCl}\)), Cream (\(\text{AgBr}\)), Yellow (\(\text{AgI}\)).

5. Common Exam Pitfalls & How to Avoid Them

1. Always include State Symbols: CCEA mark schemes frequently award a standalone mark for state symbols in ionic equations. Make sure your precipitates have \(\text{(s)}\) and starting ions have \(\text{(aq)}\).
2. "Precipitate" vs. "Cloudy": When describing solid formation in solution, always write "white precipitate", not simply "turns cloudy".
3. Acid Selection:
- Acidifying before silver nitrate? Use dilute \(\text{HNO}_3\).
- Acidifying before barium chloride? Use dilute \(\text{HCl}\).
4. Flame Test Wire: Must be nichrome (or platinum), cleaned with concentrated \(\text{HCl}\), and placed in a non-luminous (roaring blue) flame.

Quick Review Summary Table

• \(\text{Li}^+\): Crimson flame
• \(\text{Na}^+\): Yellow / Orange flame
• \(\text{K}^+\): Lilac flame
• \(\text{Ca}^{2+}\): Brick red flame
• \(\text{Ba}^{2+}\): Apple green flame
• \(\text{Cu}^{2+}\): Blue-green flame
• \(\text{NH}_4^+\): Warm with \(\text{NaOH(aq)}\) \(\rightarrow\) \(\text{NH}_3\) gas (turns damp red litmus blue)
• \(\text{CO}_3^{2-}\): Add dilute acid \(\rightarrow\) effervescence of \(\text{CO}_2\) (turns limewater milky)
• \(\text{SO}_4^{2-}\): Add dilute \(\text{HCl}\) + \(\text{BaCl}_2\text{(aq)}\) \(\rightarrow\) white precipitate (\(\text{BaSO}_4\))
• \(\text{Cl}^-\): Add dilute \(\text{HNO}_3\) + \(\text{AgNO}_3\text{(aq)}\) \(\rightarrow\) white ppt (\(\text{AgCl}\)), soluble in dilute \(\text{NH}_3\)
• \(\text{Br}^-\): Add dilute \(\text{HNO}_3\) + \(\text{AgNO}_3\text{(aq)}\) \(\rightarrow\) cream ppt (\(\text{AgBr}\)), soluble only in conc. \(\text{NH}_3\)
• \(\text{I}^-\): Add dilute \(\text{HNO}_3\) + \(\text{AgNO}_3\text{(aq)}\) \(\rightarrow\) yellow ppt (\(\text{AgI}\)), insoluble in conc. \(\text{NH}_3\)

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

What are the tests and positive observations for hydrogen (\(\text{H}_2\)) and oxygen (\(\text{O}_2\)) gases?

\(\text{H}_2\) burns with a "squeaky pop" when tested with a burning splint; \(\text{O}_2\) relights a glowing splint.

How do you test for chlorine gas (\(\text{Cl}_2\)) and what is the observation?

Hold damp blue litmus paper in the gas; it turns red and is then bleached white.

How is carbon dioxide (\(\text{CO}_2\)) gas identified chemically?

Bubble the gas through limewater (\(\text{Ca(OH)}_2\text{(aq)}\)); it turns milky/cloudy white due to the formation of \(\text{CaCO}_3\text{(s)}\).

What are the two laboratory tests used to identify ammonia gas (\(\text{NH}_3\))?

Ammonia turns damp red litmus paper blue and produces dense white fumes of \(\text{NH}_4\text{Cl}\) with concentrated \(\text{HCl}\).

How is a flame test carried out experimentally?

Dip a clean nichrome wire into concentrated \(\text{HCl}\), touch the sample, and heat it in a non-luminous (roaring blue) Bunsen flame.

What are the characteristic flame test colours for \(\text{Li}^+\), \(\text{Na}^+\), \(\text{K}^+\), \(\text{Ca}^{2+}\), \(\text{Ba}^{2+}\), and \(\text{Cu}^{2+}\)?

\(\text{Li}^+\): crimson; \(\text{Na}^+\): yellow/orange; \(\text{K}^+\): lilac; \(\text{Ca}^{2+}\): brick red; \(\text{Ba}^{2+}\): apple green; \(\text{Cu}^{2+}\): blue-green.

How do you test for the ammonium cation (\(\text{NH}_4^+\)) and write the ionic equation for the reaction?

Warm with aqueous \(\text{NaOH}\) to release \(\text{NH}_3\) gas: \(\text{NH}_4^+\text{(aq)} + \text{OH}^-\text{(aq)} \rightarrow \text{NH}_3\text{(g)} + \text{H}_2\text{O(l)}\).

How is the carbonate ion (\(\text{CO}_3^{2-}\)) identified in the laboratory?

Add dilute acid; effervescence occurs and the evolved \(\text{CO}_2\) gas turns limewater milky (\(\text{CO}_3^{2-}\text{(aq)} + 2\text{H}^+\text{(aq)} \rightarrow \text{CO}_2\text{(g)} + \text{H}_2\text{O(l)}\)).

What reagents and observation are used to test for sulfate ions (\(\text{SO}_4^{2-}\)), and why is dilute \(\text{HCl}\) added first?

Add dilute \(\text{HCl}\) followed by \(\text{BaCl}_2\text{(aq)}\) to give a white precipitate of \(\text{BaSO}_4\); \(\text{HCl}\) removes interfering carbonate ions.

How can halide ions (\(\text{Cl}^-\), \(\text{Br}^-\), \(\text{I}^-\)) be distinguished using silver nitrate and aqueous ammonia?

\(\text{Cl}^-\) gives a white precipitate soluble in dilute \(\text{NH}_3\); \(\text{Br}^-\) gives a cream precipitate soluble only in conc. \(\text{NH}_3\); \(\text{I}^-\) gives a yellow precipitate insoluble in both.

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