Identification of Ions and Gases

Procedure.

1. Take a small amount of the unknown solution.
2. Add NaOH (aq) drop by drop, observing.
3. Add NaOH IN EXCESS — observe whether precipitate redissolves.
4. Repeat with ammonia (NH₃ aq) instead of NaOH.

Reactions.

Cation	+ NaOH	+ NaOH (excess)	+ NH₃	+ NH₃ (excess)
Al³⁺	White ppt	Redissolves	White ppt	Insoluble
Ca²⁺	White ppt	Insoluble	No ppt or slight	—
Cr³⁺	Green ppt	Redissolves	Green ppt	Insoluble
Cu²⁺	Blue ppt	Insoluble	Blue ppt	DEEP BLUE solution
Fe²⁺	Green ppt	Insoluble	Green ppt	Insoluble
Fe³⁺	Red-brown ppt	Insoluble	Red-brown ppt	Insoluble
Zn²⁺	White ppt	Redissolves (colourless)	White ppt	Redissolves (colourless)
NH₄⁺	No ppt; NH₃ released on warming	—	—	—

Distinguishing similar precipitates.

• White ppt (Al³⁺, Ca²⁺, Zn²⁺): use excess. Al and Zn redissolve in excess NaOH; Ca doesn't. Zn redissolves in excess NH₃; Al doesn't.
• Green ppt (Cr³⁺ vs Fe²⁺): Cr³⁺ redissolves in excess NaOH; Fe²⁺ doesn't.

Worked qualitative. Adding NaOH to an unknown solution gives a blue precipitate. Add excess NaOH; precipitate stays blue (insoluble). Conclusion: Cu²⁺ likely.

Halides (Cl⁻, Br⁻, I⁻) — silver nitrate test.

1. Add dilute nitric acid (HNO₃) to acidify (removes carbonates).
2. Add silver nitrate solution (AgNO₃).
3. Observe colour of precipitate.

Halide	Precipitate colour
Cl⁻	WHITE (AgCl)
Br⁻	CREAM (AgBr)
I⁻	YELLOW (AgI)

Confirming with ammonia. Add ammonia to the precipitate:

• AgCl dissolves in dilute NH₃.
• AgBr dissolves in concentrated NH₃.
• AgI doesn't dissolve in ammonia.

Sulfate ($\text{SO}_{4}^{2-}$).

1. Add dilute hydrochloric acid (HCl) to acidify.
2. Add barium chloride (BaCl₂).
3. WHITE PRECIPITATE forms (BaSO₄).

(HCl removes any carbonate that might give a false positive.)

Carbonate ($\text{CO}_{3}^{2-}$).

1. Add dilute acid (HCl).
2. EFFERVESCENCE (CO₂ gas).
3. Bubble gas through limewater → turns CLOUDY (CaCO₃ precipitate).

Nitrate ($\text{NO}_{3}^{-}$).

1. Add NaOH solution.
2. Add aluminium powder; warm gently.
3. Test gas with damp red litmus paper.
4. Litmus turns BLUE → ammonia (NH₃) released → nitrate confirmed.

Cambridge tip. All tests need ACID-FREE conditions or specific acids — wrong acid gives wrong result.

Hydrogen (H₂). Place a LIT splint at the mouth of the test tube. SQUEAKY POP — the burning H₂ + O₂ → H₂O.

Oxygen (O₂). Place a GLOWING splint into the gas. Splint RELIGHTS.

Carbon dioxide (CO₂). Bubble through LIMEWATER. Goes CLOUDY (white precipitate of CaCO₃).

Ammonia (NH₃). Use damp RED LITMUS PAPER. Turns BLUE (NH₃ is alkaline).

Hydrogen chloride (HCl). Hold a stoppered bottle of concentrated NH₃ near the gas. WHITE SMOKE forms (NH₄Cl). Or use damp blue litmus → red.

Chlorine (Cl₂). Use damp BLUE litmus paper — turns RED (acidic), then BLEACHES (becomes white). The bleaching is the diagnostic step.

Sulfur dioxide (SO₂). Bubble through ACIDIFIED potassium manganate(VII) (KMnO₄). Solution turns from PURPLE to COLOURLESS.

Worked qualitative. A student warms a sample with HCl. Gas evolved. Bubbled through limewater — cloudy. Conclusion: CO₂ released → original sample contained a CARBONATE.

Cambridge tip. Cambridge often gives a qualitative analysis question requiring TWO TESTS to distinguish two possibilities. Plan a sequence that distinguishes uniquely.

Procedure.

1. Clean a platinum or nichrome wire by dipping in dilute HCl, then heating in a Bunsen flame until colourless.
2. Dip the wire in concentrated HCl, then into the SOLID being tested.
3. Hold in the EDGE of a non-luminous Bunsen flame.
4. Note the FLAME COLOUR.

Common flame colours.

Cation	Flame colour
Lithium (Li⁺)	RED
Sodium (Na⁺)	YELLOW (intense)
Potassium (K⁺)	LILAC / pale violet
Calcium (Ca²⁺)	ORANGE-RED / brick red
Copper(II) (Cu²⁺)	BLUE-GREEN

Why specific colours? Each metal's electrons absorb heat, jump to higher levels, then drop back releasing photons of specific energies (= specific colours). Like fingerprints.

Cambridge tip. Sodium gives such a strong yellow that even tiny contamination from sweat or fingerprints makes everything LOOK like sodium. Always clean the wire properly between tests.

Worked qualitative. A student does a flame test on an unknown salt. The flame is bright yellow. Identify the cation. Yellow is characteristic of Na⁺.

Worked qualitative. Two solutions both turned bromine water decolourised. To distinguish, do flame tests: A gives red flame (Li⁺); B gives lilac (K⁺).