Qualitative Analysis: Identification of Ions and Gases

What qualitative analysis reveals

Imagine finding a colourless solution in a labelled practical investigation but not knowing which ions it contains. Its appearance alone may reveal very little. A chemist therefore carries out qualitative analysis: carefully chosen tests that identify substances from observable chemical behaviour.

A useful way to think about every test is:

1. choose and carry out the correct procedure;
2. record the observation directly;
3. infer which species may be present.

For example, “a cream precipitate formed” is an observation. “Bromide ions are present” is the inference drawn from it. Keeping these separate matters because some observations are not unique. A white precipitate with sodium hydroxide, for instance, could indicate more than one metal ion.

OCR includes the use of qualitative reagents and techniques to analyse unknown samples or products among its practical requirements. This work can include gas tests, flame tests and precipitation reactions. In an investigation, several tests may be combined to identify both ions in an unknown ionic compound or to identify products formed by a reaction.

Reliable and safe testing

Wear safety glasses, work with small quantities and follow the hazard information supplied for the particular concentrations being used. Some acids, alkalis and test reagents can harm the eyes or skin. Heating and open flames require careful supervision, and long hair should be secured. Never inhale a gas directly: apply the specified test at the test-tube opening or pass the gas into the required reagent.

Use clean apparatus and divide an unknown into separate, clearly labelled portions before testing. Adding several reagents to the same portion could introduce ions that interfere with later tests. For example, hydrochloric acid adds chloride ions, so it must not be used to acidify a sample before testing that sample for halides.

Record negative results as well as positive ones. “No precipitate” or “the glowing splint did not relight” is still evidence. A results table can contain four columns: sample portion, reagent or procedure, observation and inference.

Identifying gases

Fizzing or effervescence shows that a gas is being released, but it does not establish the gas's identity. A characteristic test is needed.

Limewater is aqueous calcium hydroxide. Carbon dioxide reacts with it to form insoluble calcium carbonate. The suspended particles of this white solid make the liquid look cloudy:

$$\mathrm{CO_2(g) + Ca(OH)_2(aq) \rightarrow CaCO_3(s) + H_2O(l)}$$

The splint tests must also be described precisely. A burning splint is used for hydrogen, whereas a glowing splint is used for oxygen.

Flame tests for metal ions

Some metal ions produce characteristic flame colours when heated strongly. Energy from the flame is transferred to electrons. When the electrons return to lower energy levels, they emit light. Different ions have different electronic structures, so they can produce different colours.

Clean an unreactive wire loop, such as a nichrome loop, with acid and heat it in a blue Bunsen flame until no flame colour is seen. Dip the clean loop into the sample, then place it at the edge of the blue flame and observe the colour. Clean the loop again before testing another sample.

Cleaning prevents contamination from a previous sample. Avoid allowing the wire itself to glow red, as this could be mistaken for the sample's flame colour. Flame tests are most useful for a sample containing one metal ion: in a mixture, colours can overlap and an intense colour such as sodium's yellow can mask another result.

Forming metal hydroxide precipitates

Aqueous sodium hydroxide can help identify metal cations. Hydroxide ions react with many dissolved metal ions to form insoluble metal hydroxides.

Place a fresh portion of the unknown solution in a test tube. Add sodium hydroxide solution a few drops at a time, noting whether a precipitate forms and recording its colour. Then add sodium hydroxide in excess and observe whether the precipitate remains or dissolves.

The general process is:

$$\mathrm{metal\ ion(aq) + hydroxide\ ions(aq) \rightarrow metal\ hydroxide(s)}$$

For copper(II) ions:

$$\mathrm{Cu^{2+}(aq) + 2OH^-(aq) \rightarrow Cu(OH)_2(s)}$$

Adding sodium hydroxide slowly is important. If a precipitate is soluble in excess reagent, adding a large amount immediately could cause it to form and dissolve before it is properly observed. A white precipitate that remains does not by itself distinguish calcium ions from magnesium ions, so the conclusion must reflect that limitation.

Testing for carbonate ions

Add dilute acid to a fresh portion of the sample. Carbonate ions react with hydrogen ions to produce carbon dioxide and water:

$$\mathrm{CO_3^{2-}(aq) + 2H^+(aq) \rightarrow CO_2(g) + H_2O(l)}$$

Effervescence should be observed, but the gas must then be tested. Pass it through limewater; a milky or cloudy result confirms carbon dioxide. The combined evidence supports the presence of carbonate ions.

Testing for halide ions

Halide ions include chloride, bromide and iodide. Acidify a fresh sample with dilute nitric acid, then add aqueous silver nitrate.

Nitric acid removes carbonate interference without adding halide ions. Hydrochloric acid would be unsuitable because it supplies chloride ions and could itself produce a white silver chloride precipitate.

The general ionic equation is:

$$\mathrm{Ag^+(aq) + X^-(aq) \rightarrow AgX(s)}$$

Here, $\mathrm{X^-}$ represents a halide ion. The silver ion and halide ion join to form an insoluble silver halide; its colour helps distinguish the ions.

Testing for sulfate ions

Acidify another fresh portion of the sample with dilute hydrochloric acid, then add aqueous barium chloride. A white precipitate of barium sulfate is a positive result:

$$\mathrm{Ba^{2+}(aq) + SO_4^{2-}(aq) \rightarrow BaSO_4(s)}$$

Acidification removes carbonate ions that could otherwise react with barium ions and create a misleading white precipitate.

Reaching a justified identification

An ionic compound contains positive and negative ions. Use a flame test or sodium hydroxide test to investigate its cation, then test separate portions for possible anions. Gas tests may identify products released during these reactions or during electrolysis and other practical work.

The strongest conclusion combines compatible evidence. A yellow flame supports the presence of sodium ions, while a white precipitate with acidified silver nitrate supports chloride ions. Together, these observations could identify sodium chloride in a suitable unknown-single-compound investigation. If a result leaves several possibilities, state those possibilities and carry out another appropriate test rather than claiming certainty.