Welcome to the World of Chemical Detection!
In Chemistry, we often find ourselves with a mystery substance in a test tube. How do we know what’s inside? Topic 4C: Analysis of Inorganic Compounds is all about the "chemical tests" used to identify specific ions. Think of this as your detective toolkit. By the end of these notes, you will know exactly which reagents to add and what observations to look for to identify carbonates, hydrogencarbonates, sulfates, and ammonium ions.
Note: This topic links closely with Topic 4A (Groups 1 and 2) and Topic 4B (Halogens), which cover flame tests and halide tests. Together, they allow you to identify almost any simple inorganic salt!
1. Testing for Carbonates \( (CO_3^{2-}) \) and Hydrogencarbonates \( (HCO_3^{-}) \)
These two ions behave very similarly when tested. If you add an acid to a solid or a solution containing these ions, they react to produce carbon dioxide gas.
The Test
1. Add dilute nitric acid \( (HNO_3) \) or dilute hydrochloric acid \( (HCl) \) to the sample.
2. If you see effervescence (fizzing/bubbles), a gas is being produced.
3. To prove the gas is \( CO_2 \), bubble it through limewater (calcium hydroxide solution).
The Observation
The limewater will turn cloudy/milky. This happens because the carbon dioxide reacts with the limewater to form a white precipitate of calcium carbonate.
The Ionic Equations
For a carbonate ion reacting with acid:
\( CO_3^{2-}(aq) + 2H^+(aq) \rightarrow CO_2(g) + H_2O(l) \)
For a hydrogencarbonate ion reacting with acid:
\( HCO_3^{-}(aq) + H^+(aq) \rightarrow CO_2(g) + H_2O(l) \)
Quick Tip!
Don't worry if you find it hard to tell these two apart just by looking. In the lab, both will fizz. If the question asks you to "identify the gas," always mention the limewater test.
2. Testing for Sulfate Ions \( (SO_4^{2-}) \)
The test for sulfates relies on forming an insoluble salt. Most sulfates are soluble, but barium sulfate is famously insoluble.
The Test
1. Add dilute hydrochloric acid \( (HCl) \) to the sample. (This is a vital "pre-test" step!)
2. Add barium chloride solution \( (BaCl_2) \).
The Observation
A heavy white precipitate of barium sulfate \( (BaSO_4) \) will form immediately.
The Ionic Equation
\( Ba^{2+}(aq) + SO_4^{2-}(aq) \rightarrow BaSO_4(s) \)
Why do we add acid first?
This is a favorite exam question! We add \( HCl \) to "acidify" the solution. This removes any carbonate ions that might be present. If we didn't add the acid, barium carbonate (which is also a white solid) might form, giving us a "false positive" result for sulfate. The acid reacts with carbonates to turn them into gas, leaving only the sulfate ions to react with the barium.
Quick Review: Sulfate test = Barium Chloride + Acid \( \rightarrow \) White Precipitate.
3. Testing for Ammonium Ions \( (NH_4^{+}) \)
Unlike the tests above, which look for negative ions (anions), the ammonium test identifies a positive ion (cation). This test relies on the fact that ammonium salts react with bases to release ammonia gas \( (NH_3) \).
The Test
1. Add sodium hydroxide solution \( (NaOH) \) to the sample in a test tube.
2. Gently warm the mixture over a Bunsen burner (be careful not to boil it!).
3. Test any gas released using damp red litmus paper.
The Observation
The damp red litmus paper will turn blue. You might also notice a sharp, pungent "choking" smell (the classic smell of ammonia), but the litmus paper is the official proof!
The Ionic Equation
\( NH_4^+(aq) + OH^-(aq) \rightarrow NH_3(g) + H_2O(l) \)
Did you know?
The litmus paper must be damp because the ammonia gas needs to dissolve in the water on the paper to form hydroxide ions, which then cause the color change from red to blue.
Summary Table for Quick Revision
Keep this "Cheat Sheet" in your mind for Paper 1 and Paper 3:
| Ion to Test | Reagents Added | Positive Observation |
|---|---|---|
| Carbonate / Hydrogencarbonate | Dilute Acid (e.g. \( HCl \)) | Effervescence; Gas turns limewater cloudy |
| Sulfate | \( HCl \) then \( BaCl_2 \) solution | White precipitate |
| Ammonium | \( NaOH \) and Warm | Pungent gas turns damp red litmus blue |
Common Mistakes to Avoid
1. Forgetting State Symbols: In ionic equations for precipitates, always use \( (aq) \) for the ions and \( (s) \) for the solid precipitate. For the carbonate test, the gas is \( (g) \).
2. Confusing Reagents: Don't mix up silver nitrate (used for halides in Topic 4B) with barium chloride (used for sulfates).
3. The "Order" of Tests: If you have a mixture of ions, the syllabus usually expects you to test for Carbonates first, then Sulfates, then Halides. This prevents the reagents from reacting with the wrong ions!
You've now mastered the essentials of Topic 4C! Practice writing the ionic equations from memory, as they are high-yield marks in your Pearson Edexcel exams.