Preliminary analysis
Qualitative analysis finds out which substances are in a sample, not how much. Start with simple tests that tell you what to try next.
| Test | Observation | Possible conclusion |
|---|---|---|
| Appearance | Blue or green crystals | Copper(II) salt |
| Pale green crystals | Iron(II) salt | |
| Brown or yellow-brown solid | Iron(III) salt | |
| White solid | Many salts: sodium, calcium, zinc, magnesium... | |
| Heat a small amount in a dry test tube | Black residue from a green or blue solid | Copper(II) compound (CuO is black) |
| Yellow when hot, white when cold | Zinc compound (ZnO) | |
| Brown gas (NO₂) and oxygen | A nitrate (not of sodium or potassium) | |
| Gas that turns limewater milky | A carbonate or hydrogen carbonate | |
| Water droplets on cool part of tube | A hydrated salt or a hydrogen carbonate |
For the flame test dip a clean wire in concentrated hydrochloric acid, then in the solid, and hold it in a blue Bunsen flame: lithium crimson, sodium golden yellow, potassium lilac, calcium brick-red, barium apple-green, copper blue-green.
Tests in solution
Dissolve the sample in water (or dilute acid). Cations are found by adding sodium hydroxide solution, first a few drops then excess, and ammonia solution in the same way.
| Cation | Add NaOH a few drops | Excess NaOH | Add NH₃ (excess) |
|---|---|---|---|
| Calcium, Ca²⁺ | White precipitate | Stays | No precipitate or a very slight one |
| Magnesium, Mg²⁺ | White precipitate | Stays | White precipitate stays |
| Aluminium, Al³⁺ | White precipitate | Dissolves, colourless solution | White precipitate stays |
| Zinc, Zn²⁺ | White precipitate | Dissolves, colourless solution | Dissolves, colourless solution |
| Lead(II), Pb²⁺ | White precipitate | Dissolves, colourless solution | White precipitate stays |
| Copper(II), Cu²⁺ | Pale blue precipitate | Stays | Dissolves, deep blue solution |
| Iron(II), Fe²⁺ | Green precipitate, turns brown | Stays | Green precipitate stays |
| Iron(III), Fe³⁺ | Red-brown precipitate | Stays | Red-brown precipitate stays |
| Ammonium, NH₄⁺ | No precipitate; warm gives ammonia gas, which turns damp red litmus blue |
| Anion | Test | Positive result |
|---|---|---|
| Chloride, Cl⁻ | Add dilute nitric acid, then silver nitrate solution | White precipitate (dissolves in ammonia) |
| Bromide, Br⁻ | The same | Cream precipitate |
| Iodide, I⁻ | The same | Yellow precipitate |
| Sulphate, SO₄²⁻ | Add dilute hydrochloric acid, then barium chloride solution | White precipitate |
| Carbonate, CO₃²⁻ | Add dilute acid | Fizzing; gas turns limewater milky |
| Nitrate, NO₃⁻ | Add sodium hydroxide and aluminium foil and warm | Ammonia gas, turns damp red litmus blue |
Confirmatory tests: gases
| Gas | Test | Result |
|---|---|---|
| Hydrogen, H₂ | Burning splint | Squeaky pop |
| Oxygen, O₂ | Glowing splint | Relights |
| Carbon dioxide, CO₂ | Bubble through limewater | Turns milky |
| Ammonia, NH₃ | Damp red litmus paper | Turns blue; sharp smell |
| Chlorine, Cl₂ | Damp litmus paper | Bleached white; green gas |
| Sulphur dioxide, SO₂ | Acidified potassium dichromate paper | Orange turns green |
Try itPrecipitating chloride with silver nitrate
A solution contains chloride ions. Excess silver nitrate makes a white precipitate of silver chloride, AgCl (M = 143.5). Find the mass of precipitate.
Key terms
- Qualitative analysis
- Finding out which substances a sample contains.
- Precipitate
- An insoluble solid formed in a solution.
- Cation
- A positive ion.
- Anion
- A negative ion.
Summary
- Qualitative analysis identifies ions and gases; start with appearance, heating and the flame test.
- Add NaOH and ammonia solutions: colour and solubility of the precipitate identify the cation.
- Anion tests: silver nitrate for halides, barium chloride for sulphate, acid for carbonate.
- Know the gas tests: H₂ pop, O₂ relights splint, CO₂ limewater, NH₃ litmus, Cl₂ bleaches.