Required Practical: Identifying Ions

This lesson covers the required practical on identifying ions in unknown substances, as specified in the AQA GCSE Chemistry specification (8.1.2). You need to be able to plan and carry out a series of tests to identify both the cation (positive ion) and the anion (negative ion) in an unknown ionic compound. This brings together flame tests, NaOH precipitate tests, and anion tests into a systematic analytical procedure.

---

Overview of the Practical

In this required practical, you are given one or more unknown ionic compounds and must identify them by carrying out a series of chemical tests. You will test for:

1. Cations (positive ions) using flame tests and/or sodium hydroxide solution.
2. Anions (negative ions) using the carbonate, halide, and sulfate tests.

Aim

To identify the ions present in unknown ionic compounds using a systematic series of chemical tests.

---

Apparatus and Reagents

Apparatus	Purpose
Test tubes and test tube rack	To carry out reactions in
Bunsen burner	For flame tests
Nichrome wire loop	To hold samples in the flame
Dropping pipettes	To add small quantities of reagents
Safety goggles	Eye protection
Lab coat	Clothing protection

Reagent	Purpose
Concentrated hydrochloric acid	Cleaning the nichrome wire loop
Sodium hydroxide solution (NaOH)	Testing for metal ion precipitates
Dilute hydrochloric acid (HCl)	Testing for carbonates; preparing for sulfate test
Dilute nitric acid (HNO\u2083)	Preparing for halide test
Silver nitrate solution (AgNO\u2083)	Testing for halide ions
Barium chloride solution (BaCl\u2082)	Testing for sulfate ions
Limewater (Ca(OH)\u2082)	Confirming carbon dioxide gas

Exam Tip: In a required practical question, you may be asked to name the reagents needed. Make sure you can list all the reagents above and state what each one is used for. A common mistake is forgetting the acids — remember that dilute nitric acid is needed before silver nitrate, and dilute hydrochloric acid is needed before barium chloride.

---

Method: Step by Step

Step 1: Prepare the Sample

• If the unknown compound is a solid, dissolve a small amount in distilled water in a test tube to make a solution. Some tests require the substance to be in solution.
• Keep some of the dry solid for the flame test.

Step 2: Flame Test for Cations

1. Clean the nichrome wire by dipping in concentrated HCl and holding in a blue Bunsen flame until no colour is produced.
2. Dip the clean wire into concentrated HCl, then into the solid sample.
3. Hold the wire in the Bunsen flame and record the flame colour.
4. Compare with known flame colours:

Flame Colour	Metal Ion
Crimson	Lithium (Li\u207a)
Yellow	Sodium (Na\u207a)
Lilac	Potassium (K\u207a)
Orange-red	Calcium (Ca\u00b2\u207a)
Green	Copper (Cu\u00b2\u207a)

Step 3: NaOH Test for Cations

1. Place about 1 cm\u00b3 of the unknown solution in a test tube.
2. Add a few drops of sodium hydroxide solution.
3. Record the colour of any precipitate formed.
4. If a white precipitate forms, add excess NaOH to see if it dissolves.

Precipitate Colour	Metal Ion
White (does not dissolve in excess)	Calcium (Ca\u00b2\u207a) or Magnesium (Mg\u00b2\u207a)
White (dissolves in excess)	Aluminium (Al\u00b3\u207a)
Blue	Copper(II) (Cu\u00b2\u207a)
Green	Iron(II) (Fe\u00b2\u207a)
Brown	Iron(III) (Fe\u00b3\u207a)

Step 4: Test for Carbonate Ions

1. Place a small amount of the solid (or solution) in a test tube.
2. Add dilute hydrochloric acid.
3. If fizzing occurs, test the gas with limewater.
4. If the limewater turns milky, the compound contains carbonate ions (CO\u2083\u00b2\u207b).

Step 5: Test for Halide Ions

1. Place about 1 cm\u00b3 of the unknown solution in a test tube.
2. Add a few drops of dilute nitric acid.
3. Add a few drops of silver nitrate solution.
4. Record the colour of any precipitate:
   • White = chloride, Cream = bromide, Yellow = iodide.

Step 6: Test for Sulfate Ions

1. Place about 1 cm\u00b3 of the unknown solution in a test tube.
2. Add a few drops of dilute hydrochloric acid.
3. Add a few drops of barium chloride solution.
4. A white precipitate confirms sulfate ions (SO\u2084\u00b2\u207b).

Exam Tip: When writing up this practical, always record your observations in a clear results table. Include the test performed, the reagent(s) used, the observation, and your conclusion. This systematic approach demonstrates good scientific practice and will gain you marks for scientific method.

---

Systematic Testing Flowchart

flowchart TD
    A["Unknown Ionic Compound"] --> B["Dissolve in water"]
    A --> C["Keep some solid for flame test"]

    C --> D["Flame Test"]
    D --> D1{"Flame colour?"}
    D1 -->|Crimson| D2["Li+"]
    D1 -->|Yellow| D3["Na+"]
    D1 -->|Lilac| D4["K+"]
    D1 -->|Orange-red| D5["Ca2+"]
    D1 -->|Green| D6["Cu2+"]
    D1 -->|No colour| D7["Try NaOH test"]

    B --> E["NaOH Precipitate Test"]
    E --> E1{"Precipitate colour?"}
    E1 -->|Blue| E2["Cu2+"]
    E1 -->|Green| E3["Fe2+"]
    E1 -->|Brown| E4["Fe3+"]
    E1 -->|"White (dissolves in excess)"| E5["Al3+"]
    E1 -->|"White (stays)"| E6["Ca2+ or Mg2+"]

    B --> F["Anion Tests"]
    F --> F1["Carbonate: HCl + limewater"]
    F --> F2["Halide: HNO3 + AgNO3"]
    F --> F3["Sulfate: HCl + BaCl2"]

    style A fill:#2c3e50,color:#fff
    style D fill:#e74c3c,color:#fff
    style E fill:#3498db,color:#fff
    style F fill:#27ae60,color:#fff

---

Recording Results

Use a results table like the one below to record your findings:

Test	Reagent(s) Used	Observation	Conclusion
Flame test	Nichrome wire + conc. HCl	e.g., Green flame	Copper ion (Cu\u00b2\u207a) present
NaOH test	NaOH solution	e.g., Blue precipitate	Copper(II) ion (Cu\u00b2\u207a) confirmed
Carbonate test	Dilute HCl + limewater	e.g., No fizzing	No carbonate ions present
Halide test	Dilute HNO\u2083 + AgNO\u2083	e.g., White precipitate	Chloride ion (Cl\u207b) present
Sulfate test	Dilute HCl + BaCl\u2082	e.g., No precipitate	No sulfate ions present
Conclusion			Compound is copper chloride (CuCl\u2082)

---

Example: Identifying an Unknown Compound

Scenario: A student is given an unknown white solid labelled X. They carry out the following tests:

Test	Observation
Flame test	Orange-red flame
NaOH test	White precipitate (does not dissolve in excess)
Dilute HCl added	Fizzes; gas turns limewater milky
Dilute HNO\u2083 + AgNO\u2083	No precipitate
Dilute HCl + BaCl\u2082	No precipitate

Analysis:

• Orange-red flame = calcium ion (Ca\u00b2\u207a)
• White precipitate with NaOH (does not dissolve) = confirms Ca\u00b2\u207a
• Fizzes with HCl; gas turns limewater milky = carbonate ion (CO\u2083\u00b2\u207b)
• No precipitate with AgNO\u2083 = no halide ions
• No precipitate with BaCl\u2082 = no sulfate ions