# Chemical Analysis and Purity
Chemists need to identify unknown substances and check purity. From flame tests that reveal metal ions to gas tests and chromatography, analytical chemistry provides the tools to identify what a substance contains. This guide covers all the tests you need to know for GCSE Chemistry.
1. Purity
In chemistry, a pure substance contains only one element or compound. Pure substances have sharp melting and boiling points.
Mixtures melt and boil over a range of temperatures. Adding impurities:
- Lowers the melting point
- Raises the boiling point
You can test purity by measuring the melting point and comparing it to the known value.
2. Flame Tests
Flame tests identify metal ions by the colour they produce in a flame.
Method
- Dip a clean nichrome wire loop in concentrated HCl (to clean it)
- Dip the loop in the sample
- Hold the loop in the blue Bunsen flame
- Observe the colour
Flame Colours
| Metal Ion | Flame Colour |
|---|---|
| Lithium (Li⁺) | Crimson (deep red) |
| Sodium (Na⁺) | Yellow |
| Potassium (K⁺) | Lilac |
| Calcium (Ca²⁺) | Orange-red |
| Copper (Cu²⁺) | Green |
Remember: The sodium flame (yellow) is very strong and can mask other colours.
3. Tests for Metal Ions Using NaOH
Adding a few drops of sodium hydroxide solution to a solution containing metal ions produces coloured precipitates (insoluble hydroxides):
| Metal Ion | Precipitate Colour | Formula |
|---|---|---|
| Calcium (Ca²⁺) | White | Ca(OH)₂ |
| Magnesium (Mg²⁺) | White | Mg(OH)₂ |
| Aluminium (Al³⁺) | White (dissolves in excess NaOH) | Al(OH)₃ |
| Copper (Cu²⁺) | Blue | Cu(OH)₂ |
| Iron(II) (Fe²⁺) | Green | Fe(OH)₂ |
| Iron(III) (Fe³⁺) | Brown | Fe(OH)₃ |
Distinguishing Ca²⁺, Mg²⁺, and Al³⁺ (all give white precipitates)
- Ca²⁺: Use flame test (orange-red flame)
- Al³⁺: White precipitate dissolves in excess NaOH
- Mg²⁺: White precipitate does not dissolve in excess NaOH
4. Tests for Negative Ions (Anions)
Carbonate ()
Add dilute acid → effervescence (fizzing). Bubble gas through limewater → turns milky (CO₂ produced).
Sulfate ()
Add dilute HCl (to remove carbonates), then add barium chloride solution.
- White precipitate of barium sulfate confirms sulfate:
Halide ions (, , )
Add dilute nitric acid (to remove carbonates), then add silver nitrate solution:
| Halide | Precipitate | Colour |
|---|---|---|
| Chloride (Cl⁻) | AgCl | White |
| Bromide (Br⁻) | AgBr | Cream |
| Iodide (I⁻) | AgI | Yellow |
5. Tests for Gases
| Gas | Test | Positive Result |
|---|---|---|
| Hydrogen (H₂) | Burning splint | Squeaky pop |
| Oxygen (O₂) | Glowing splint | Relights |
| Carbon dioxide (CO₂) | Bubble through limewater | Turns milky/cloudy |
| Chlorine (Cl₂) | Damp litmus paper | Bleaches it white |
6. Instrumental Methods
Modern analysis often uses instruments because they are:
- More accurate and sensitive than chemical tests
- Faster
- Able to detect very small quantities
- Useful for unknown substances
Flame Emission Spectroscopy
- A sample is heated in a flame, and the light emitted is analysed by a spectrometer
- Each element produces a characteristic line spectrum
- Used to identify metals and measure their concentration
- Advantages over flame tests: more accurate, can identify mixtures, can measure concentration
Worked Example: Identifying a Metal Ion
Question: A white compound gives an orange-red flame and produces a white precipitate with NaOH. Identify the metal ion.
Orange-red flame = calcium. White precipitate with NaOH = consistent with Ca²⁺. The metal ion is calcium (Ca²⁺).
Worked Example: Testing for a Halide
Question: Describe how to test for bromide ions in a solution.
- Add dilute nitric acid to the solution (removes carbonates and other interfering ions)
- Add silver nitrate solution
- A cream precipitate indicates bromide ions are present ()
Worked Example: Purity Check
Question: A student measures the melting point of a sample of aspirin as 132–136°C. Pure aspirin melts at 136°C. Is the sample pure?
The sample is not pure. A pure substance would melt at a sharp, fixed temperature (136°C). The sample melts over a range (132–136°C), indicating it contains impurities that lower the melting point.
8. Practice Questions
- State the flame colours for Li⁺, Na⁺, K⁺, Ca²⁺, and Cu²⁺.
- What colour precipitate forms when NaOH is added to a solution containing Fe²⁺ ions? What about Fe³⁺?
- Describe how to test for sulfate ions in a solution.
- How would you distinguish between chloride, bromide, and iodide ions?
- A sample melts at exactly 80.0°C. Is it likely to be pure or impure? Explain.
Want to check your answers and get step-by-step solutions?
9. Common Misconceptions
| Misconception | Reality |
|---|---|
| Flame tests work for all metals | Only some metals produce visible flame colours |
| A white precipitate always means calcium | Mg²⁺ and Al³⁺ also give white precipitates — you need additional tests |
| All gases can be tested with a burning splint | Only H₂ and O₂ use a splint; CO₂ uses limewater; Cl₂ uses litmus |
| Impure substances don't melt | They do melt, but over a range of temperatures |
10. Exam Tips
- Learn all flame colours and precipitate colours — these are commonly tested
- For ion tests, always state the reagent, the observation, and the conclusion
- Know the gas tests perfectly — they appear in practicals and theory questions
- When describing tests, be specific: "add silver nitrate solution" not just "test with silver"
Summary
- Purity: pure substances have sharp melting/boiling points
- Flame tests: identify metal ions by colour (Li=crimson, Na=yellow, K=lilac, Ca=orange-red, Cu=green)
- NaOH test: Cu²⁺=blue, Fe²⁺=green, Fe³⁺=brown precipitate
- Halide test: silver nitrate → Cl⁻=white, Br⁻=cream, I⁻=yellow precipitate
- Sulfate test: HCl then BaCl₂ → white precipitate
- Gas tests: H₂=squeaky pop, O₂=relights splint, CO₂=milky limewater, Cl₂=bleaches litmus