Core Practical 15: Analyse Organic and Inorganic Unknowns
Objective: To identify unknown substances A–D (inorganic) and X–Z (organic), applying chemical tests for common ions and functional groups.
Safety
- Wear goggles and tie back long hair.
- Corrosive: conc. HCl, NaOH, H₂SO₄
- Irritants: nitric acid, dilute HCl, limewater
- Toxic/Irritant: barium chloride, bromine water
- Flammable: ethanol, cyclohexene, propanal
- Silver nitrate waste must be flushed with water – never stored.
Inorganic Tests
1. Flame Tests
- Dip a clean nichrome wire in conc. HCl and sample.
- Hold in edge of a Bunsen flame.
- Observe flame colour:
| Flame Colour |
Ion |
| Yellow |
Na+ |
| Lilac |
K+ |
| Red |
Li+ or Sr2+ |
| Yellow-red |
Ca2+ |
| Pale green |
Ba2+ |
| Red-violet |
Rb+ |
2. With Sodium Hydroxide (NaOH)
| Ion |
On adding NaOH |
In excess NaOH |
| Cr3+ |
Green ppt |
Dissolves to green sol. |
| Fe2+ |
Green ppt → brown in air |
Insoluble |
| Fe3+ |
Red-brown ppt |
Insoluble |
| Co2+ |
Blue ppt → pink |
Insoluble |
| Cu2+ |
Blue ppt |
Insoluble |
| Mg2+, Ca2+, Ba2+ |
White ppt |
Insoluble |
| Na+, K+ |
No ppt |
— |
3. Halide Test (AgNO₃ after HNO₃)
| Halide |
Colour ppt |
Formula |
In Dilute NH3 |
In Conc. NH3 |
| Cl− |
White |
AgCl |
Soluble |
— |
| Br− |
Cream |
AgBr |
Insoluble |
Soluble |
| I− |
Yellow |
AgI |
Insoluble |
Insoluble |
4. Sulfate Ions
- Add dilute HCl, then BaCl₂.
- White ppt of BaSO₄ confirms SO₄²⁻.
5. Carbonate Ions
- Add dilute HCl to solid.
- Bubble gas into limewater → white ppt = CO₂ = carbonate present.
Organic Tests
1. Alkenes (C=C)
- Add bromine water.
- Orange-brown → colourless if alkene present.
2. Aldehydes (Tollens’ Test)
- Mix AgNO₃, NaOH, then add NH₃ until ppt dissolves.
- Add sample and warm in water bath.
- Silver mirror forms if aldehyde present.
3. Carboxylic Acids
- Mix 1 cm³ sample + 1 cm³ ethanol + few drops conc. H₂SO₄.
- Heat in water bath.
- Pour into Na₂CO₃ → sweet, fruity ester smell = positive.
