Work out which ions an unknown solution contains. First predict the cation and the anion from the sample's appearance, then run a short sequence of confirmatory tests — flame colours, precipitation reactions, gas tests — to find out, and compare what you found with what you predicted. In each part, make your prediction first, then test it.
Theory — Qualitative Analysis
Qualitative analysis identifies which ions are present in a sample. It does not measure how much is present (that is quantitative analysis). Because cations and anions each give characteristic, repeatable reactions, a planned sequence of simple tests can confirm an ion's identity from what you observe: a flame colour, a precipitate of a certain colour, a gas, or a colour change.
The approach used in this labPredict first, then test. Look at the unknown, predict the cation and the anion, then run the confirmatory tests.
A prediction confirmed by the tests is your evidence; a prediction the tests rule out tells you just as much.
Identifying cations — flame tests
Some metal ions give a characteristic flame colour. The electrons absorb energy in the flame, then release it as visible light of a colour specific to the element.
{{FLAME:#f4d03f}}Na⁺ · yellow
{{FLAME:#b57edc}}K⁺ · lilac
{{FLAME:#e2603b}}Ca²⁺ · brick-red
{{FLAME:#7bbf6a}}Ba²⁺ · apple-green
{{FLAME:#3ea99f}}Cu²⁺ · blue-green
Identifying cations — precipitation tests
Adding sodium hydroxide solution, or aqueous ammonia, forms metal hydroxide precipitates whose colour helps identify the metal. The behaviour in excess reagent is also diagnostic.
Copper(II), Cu²⁺
Pale blue precipitate with sodium hydroxide. With ammonia, the precipitate dissolves in excess to a deep blue solution.
Iron(III), Fe³⁺
Red-brown precipitate with sodium hydroxide or ammonia, insoluble in excess of either.
Ammonium, NH₄⁺
No precipitate, but warming with sodium hydroxide releases ammonia gas, which turns damp red litmus paper blue.
Cation
Flame colour
With sodium hydroxide
With aqueous ammonia
Sodium, Na⁺
Intense yellow
No precipitate
No precipitate
Potassium, K⁺
Lilac (violet)
No precipitate
No precipitate
Calcium, Ca²⁺
Brick-red
White precipitate
No precipitate
Barium, Ba²⁺
Apple-green
No (or faint) precipitate
No precipitate
Copper(II), Cu²⁺
Blue-green
Pale blue precipitate
Dissolves in excess to deep blue
Iron(III), Fe³⁺
No characteristic colour
Red-brown precipitate
Red-brown precipitate
Ammonium, NH₄⁺
No characteristic colour
Ammonia gas on warming
No precipitate
Identifying anions
The test for the halides uses silver nitrate, and the colour of the silver halide tells the three apart:
Add AgNO₃ (acidified)A precipitate means a halide is present. Its colour identifies which one.
WhiteSilver chloride, so the anion is chloride, Cl⁻. Dissolves in dilute ammonia.
CreamSilver bromide, so the anion is bromide, Br⁻. Only partly soluble in ammonia.
YellowSilver iodide, so the anion is iodide, I⁻. Insoluble in ammonia.
Sulfate, SO₄²⁻
Barium chloride with dilute hydrochloric acid gives a white precipitate of barium sulfate that does not dissolve in the acid.
Carbonate, CO₃²⁻
Dilute acid produces brisk effervescence; the carbon dioxide released turns limewater milky.
Nitrate, NO₃⁻
The brown ring test: a brown ring forms at the junction of the layers with iron(II) sulfate and concentrated sulfuric acid.
Why test cations and anions separatelyA dissolved salt splits into its cation and its anion, and each is tested on its own. Confirming one ion never tells you the other, so a full identification needs a cation test and an anion test.
Apparatus
The equipment a real qualitative-analysis experiment uses. In the simulation these are modelled for you, but the readings correspond to what each instrument would measure.
Test tube rack
Holds the samples through the separation scheme.
Reagent bottles
Add the test reagents that identify each ion.
Flame-test wire
Confirms certain metal ions by flame colour.
Filter funnel
Separates precipitates from the solution at each step.
Watch glass
Holds spot tests and evaporates small samples.
pH meter
Sets the pH needed to selectively precipitate ion groups.
Instructions — Running the Virtual Experiment
The simulation has three parts. Predict first in Part 1, then test to confirm in Parts 2 and 3. Choose your unknown sample from the menu at the top of the bench.
Part 1 — Predict
1
Open Simulation and choose an unknown (A through H). Look at the beaker — the colour of the solution is a clue.
2
On the Predict tab, record your predicted cation and predicted anion before you run any test.
Part 2 — Identify the cation
1
Open Identify the Cation. Run the flame test and the reagent tests one at a time and watch the flame and the test tube. Each result is added to the observation log.
2
From your observations, choose the cation you believe is present and check it. Record the tests, the observations, and your reasoning.
Part 3 — Identify the anion
1
Open Identify the Anion. Run the silver nitrate, barium chloride, dilute acid, and brown ring tests, and watch the test tube and the log build up.
2
Choose the anion you believe is present and check it. Then compare your confirmed cation and anion with the prediction from Part 1.
For your reportInclude your prediction, the full observation log (a screenshot of the bench works well), your reasoning for each ion, and the confirmed identity of the unknown. Add a screenshot of each test you run.
Simulation — Qualitative Analysis Bench
Qualitative Analysis Virtual LabPredict the ions, then confirm them by testing
no sample
Choose an unknown from the menu above to see its appearance.
Your prediction
Tip
Many transition-metal ions are coloured (copper blue, iron(III) yellow), while many main-group ions are colourless. Colour narrows the cation but rarely fixes the anion.
Run a cation test to see the result.
Test
Observation
No tests run yet.
Run a cation test
Identify the cation
Run an anion test to see the result.
Test
Observation
No tests run yet.
Run an anion test
Identify the anion
Team Questions
Question 1. A sample gives an intense, persistent yellow flame. Which cation is indicated? (one word)
Question 2. Sodium hydroxide gives a red-brown precipitate that does not dissolve in excess. Which cation is this?
Question 3. With aqueous ammonia a precipitate forms, then dissolves in excess to a deep blue solution. Which cation is this?
Question 4. Silver nitrate with dilute nitric acid gives a yellow precipitate, insoluble in ammonia. Which anion is present?
Question 5. Barium chloride with dilute hydrochloric acid gives a white precipitate that does not dissolve in the acid. Which anion is present?
Question 6. Dilute acid on a sample gives brisk effervescence and the gas turns limewater milky. Name the gas.
Question 7 — Challenge. Why must a cation test and an anion test both be done to identify a salt? (a phrase)
Example Lab Report
A worked example showing the expected format. Include your prediction, the observation log with labelled screenshots, your reasoning, and the confirmed identity.
To identify the cation and the anion in an unknown solution by first predicting their identity from the sample's appearance, then confirming them with characteristic qualitative tests.
Theory
Each ion gives characteristic, repeatable reactions. Cations are identified by flame colour and by the colour and behaviour of their hydroxide precipitates with sodium hydroxide and ammonia. Anions are identified by precipitation with silver nitrate or barium chloride, by gas released with acid, or by the brown ring test.
Prediction
Unknown A was a clear blue solution. A blue colour suggests a copper(II) salt, so the prediction was the cation copper(II), Cu²⁺, and, with no colour evidence for the anion, sulfate, SO₄²⁻.
Results (worked example, Unknown A)
Test
Observation
Indicates
Flame test
Blue-green flame
Cu²⁺
Sodium hydroxide
Pale blue precipitate, insoluble in excess
Cu²⁺
Aqueous ammonia
Precipitate dissolves in excess to a deep blue solution
Cu²⁺ confirmed
Silver nitrate (acidified)
No precipitate
No halide
Barium chloride (acidified)
White precipitate, insoluble in acid
SO₄²⁻ confirmed
Analysis
The blue-green flame and the pale blue hydroxide that dissolved in excess ammonia to a deep blue solution together confirm copper(II). The white barium precipitate that did not dissolve in acid confirms sulfate, while the absence of a silver halide precipitate rules out chloride, bromide, and iodide. Both confirmed ions matched the prediction.
Conclusion
Unknown A was identified as copper(II) sulfate, containing the cation Cu²⁺ and the anion SO₄²⁻. The prediction made from the blue colour was confirmed by the tests. Predicting first and then testing made each observation a deliberate check rather than a guess.
Practice Questions
Question 1
A colourless solution gives a lilac flame seen through blue cobalt glass, and dilute acid produces brisk effervescence that turns limewater milky. Identify the cation and the anion.
Hint: lilac flame → potassium, K⁺; effervescence with the limewater test → carbonate, CO₃²⁻. The salt is potassium carbonate.
Question 2
Silver nitrate with dilute nitric acid gives a cream precipitate that is only partly soluble in concentrated ammonia. Which anion is present, and how is it told apart from chloride?
Hint: cream precipitate → bromide, Br⁻; chloride is white and dissolves readily in dilute ammonia, while bromide is cream and only partly soluble.
Question 3
A pale yellow solution gives a red-brown precipitate with sodium hydroxide, and acidified silver nitrate gives a white precipitate that dissolves in dilute ammonia. Identify both ions.
Hint: red-brown hydroxide → iron(III), Fe³⁺; white silver precipitate soluble in ammonia → chloride, Cl⁻. The salt is iron(III) chloride.
Question 4
Why is dilute nitric acid added before silver nitrate in the test for halides?
Hint: the acid removes carbonate (and similar ions) that would also form a precipitate, so any precipitate with silver nitrate is due to a halide.
Question 5
A student predicts barium for a colourless unknown, then sees a pale green flame and a white precipitate with dilute sulfuric acid. Was the prediction supported? Explain.
Hint: yes — the apple-green flame and the white barium sulfate precipitate both confirm barium, Ba²⁺.
Question 6 — Challenge
An unknown gives no flame colour and no precipitate with sodium hydroxide, but warming with sodium hydroxide releases a pungent gas that turns damp red litmus paper blue. Identify the cation and explain the test.
Hint: ammonium, NH₄⁺. Warm alkali converts the ammonium ion to ammonia gas, which is alkaline and turns damp red litmus blue.
Virginia Research Institute
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