Qualitative Analysis: Identification of Ions and Gases

• Oxygen ($O_2$): Place a glowing splint into a test tube of the gas. The splint will relight.
• Carbon Dioxide ($CO_2$): Bubble the gas through limewater. The initially colourless solution will turn cloudy (or milky) as a solid precipitate of calcium carbonate forms:

• Chlorine ($Cl_2$): Hold damp blue litmus paper in the gas. The paper briefly turns red (showing it is acidic) before bleaching to white. The paper must be damp so the gas can dissolve.

Identifying Metal Cations: Flame Tests

If you have ever seen a fireworks display, you have witnessed the colourful chemistry of metal ions in action. Qualitative analysis techniques like flame tests allow us to identify which positively charged cation is present in a sample by the distinct colour of light it emits.

To perform a flame test accurately:

1. Clean a nichrome or platinum wire loop by dipping it in hydrochloric acid ($HCl$). These metals are used because they are unreactive and have high melting points.
2. Hold the loop in the hottest part of a roaring blue Bunsen flame until it burns with no colour.
3. Dip the clean loop back into the acid, then into the solid metal compound.
4. Place the loop at the edge of the blue flame and observe the colour.

The characteristic flame colours for specific metal ions are:

• Lithium ($Li^+$): Red (or crimson)
• Sodium ($Na^+$): Yellow
• Potassium ($K^+$): Lilac
• Calcium ($Ca^{2+}$): Orange-red
• Copper ($Cu^{2+}$): Blue-green

Flame tests can be unreliable for mixtures because the intense colour of one ion can hide another, a phenomenon known as masking. The hydrochloric acid is used because it forms a volatile chloride that evaporates easily to produce a stronger colour.

Identifying Metal Cations: Precipitation Tests

Sometimes, mixing two clear liquids together creates an instant solid suspended in the mixture. We can use aqueous sodium hydroxide ($NaOH$) dropwise to identify metal ions that form insoluble metal hydroxides.

The colour of the resulting precipitate tells you which ion is present:

• Copper(II) ($Cu^{2+}$): Blue precipitate
• Iron(II) ($Fe^{2+}$): Green precipitate (may turn brown at the surface due to oxidation)
• Iron(III) ($Fe^{3+}$): Brown (or orange-brown) precipitate

Both calcium and zinc ions initially form a white precipitate. To distinguish them, you must add excess sodium hydroxide:

• Calcium ($Ca^{2+}$): The white precipitate does not dissolve.
• Zinc ($Zn^{2+}$): The white precipitate dissolves to form a colourless solution because zinc hydroxide is amphoteric.

The general ionic equation for these reactions shows the metal cation reacting with hydroxide ions:

Identifying Anions: Halides and Sulfates

To test for negatively charged ions, known as an anion, chemists use specific chemical reagents that produce characteristic precipitates.

To test for halide ions (chloride, bromide, and iodide), first add dilute nitric acid ($HNO_3$), followed by silver nitrate solution ($AgNO_3$). The acid removes any interfering carbonate ions.

• Chloride ($Cl^-$): Forms a white precipitate of silver chloride ($AgCl$).
• Bromide ($Br^-$): Forms a cream precipitate of silver bromide ($AgBr$).
• Iodide ($I^-$): Forms a yellow precipitate of silver iodide ($AgI$).

Higher Tier: Confirmatory Ammonia Tests You can further distinguish halides by their solubility in ammonia:

• Silver chloride (white) dissolves in dilute ammonia.
• Silver bromide (cream) dissolves in concentrated ammonia.
• Silver iodide (yellow) is insoluble in both dilute and concentrated ammonia.

To test for sulfate ions ($SO_4^{2-}$), add dilute hydrochloric acid ($HCl$), followed by barium chloride solution ($BaCl_2$). A positive result is the formation of a white precipitate of barium sulfate ($BaSO_4$).

Identifying Anions: Carbonates and Test Sequence

The carbonate ion ($CO_3^{2-}$) reacts readily with acids to produce gas bubbles, an observation known as effervescence.

To test for a carbonate, add dilute hydrochloric or nitric acid to the sample. If effervescence occurs, collect the gas and bubble it through limewater. If the limewater turns cloudy, the gas is carbon dioxide, confirming the original sample contained a carbonate.

When testing a single unknown sample for different anions, you must perform the tests in a strict sequence:

1. Carbonate test first: Carbonates interfere with other tests by forming white precipitates with silver and barium ions.
2. Sulfate test second: Once you know there is no carbonate, it is safe to test for sulfates.
3. Halide test last: This is performed only after ruling out both carbonates and sulfates.