Use separate portions of an unknown when testing it for carbonate and sulfate. Acid releases carbon dioxide from carbonate, while the sulfate test uses dilute nitric acid followed by aqueous barium nitrate. Separate portions help you avoid transferring reagents or products from one test to the other.
The 2027 SEC G3 Chemistry K324 qualitative analysis notes specify the observations for carbonate, sulfate and carbon dioxide. This guide shows how to report the results when a question describes two positive tests.
- Label two fresh portions before starting.
- Confirm carbon dioxide with limewater in the carbonate test.
- Use nitric acid before barium nitrate in the sulfate test.
- Report observations separately before identifying the ions.
CHAPTER 1 OF 4 · Back to contents
Keep the two observations distinct
In a supervised qualitative analysis, follow the supplied instructions. For carbonate, adding dilute acid can produce effervescence. Carbon dioxide is confirmed when the gas produces a white precipitate in limewater, making it appear milky.
For sulfate, acidify the fresh portion with dilute nitric acid and then add aqueous barium nitrate. The white precipitate is barium sulfate in the standard test.
Adding sulfuric acid before the sulfate test would introduce sulfate ions. A later white precipitate could then come from the reagent rather than the original sample.
CHAPTER 2 OF 4 · Back to contents
Worked example | A sample gives both positive results
An unknown solution is divided into two fresh portions. Portion A gives bubbles with dilute acid. The gas makes limewater milky. Portion B is acidified with dilute nitric acid before aqueous barium nitrate is added. A white precipitate forms.
| Portion | Diagnostic observation | Inference |
|---|---|---|
| A | Gas makes limewater milky | Carbonate is present |
| B | White precipitate after acidification and barium nitrate | Sulfate is present |
In A, the bubbles are the first observation. The limewater result identifies the gas as carbon dioxide. In B, report the white precipitate and the sequence of reagents used.
These tests identify anions. Separate evidence would be needed to identify any cations in the mixture.
Contents · Previous chapter · Next chapter
CHAPTER 3 OF 4 · Back to contents
Practise spotting a contaminated test
A pupil adds sulfuric acid to a portion and later uses that same portion for a sulfate test. A white precipitate appears after barium nitrate is added. The test cannot show whether sulfate was present before the acid was added.
Use a fresh portion and the specified nitric acid. Acidification also removes carbonate interference because carbonate could otherwise form a white precipitate with barium ions.
The net ionic equation for the intended sulfate result is Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).
Contents · Previous chapter · Next chapter
CHAPTER 4 OF 4 · Back to contents
Frequently asked questions
Are bubbles alone enough to identify carbonate?
Confirm the gas as carbon dioxide using limewater when the question asks for evidence.
What does white precipitate mean?
A white insoluble solid has formed in the liquid.
Which acid belongs in the sulfate test?
Use the dilute nitric acid specified in the SEAB qualitative analysis notes.
Contents · Previous chapter · Continue with the related guide
Sources and next reading
SEAB 2027 SEC G3 Chemistry K324 syllabus and qualitative analysis notes