1.1 - Flame Tests (Leaving Cert Chemistry): Revision Notes
📚 Revision Notes
1.1 - Flame Tests
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Experiment Summary
The flame test is used to identify metal ions based on the characteristic colours they emit when heated in a Bunsen burner flame. By observing the flame colour, the presence of metals like lithium, sodium, potassium, barium, strontium, or copper can be determined. This method is particularly useful for identifying unknown metal salts.
Materials and Apparatus Required
Chemicals
- Lithium chloride
- Sodium chloride
- Potassium chloride
- Barium chloride
- Strontium chloride
- Copper(II) chloride
- Concentrated hydrochloric acid
Apparatus
- Platinum or nichrome wire held in a glass rod (or wooden splints)
- Bunsen burner
- Six small beakers
- Test tubes
- Pestle and mortar
Safety Precautions
- Wear safety glasses at all times.
- Handle concentrated hydrochloric acid with care; it is highly corrosive and can irritate the lungs.
- Copper(II) chloride is toxic if swallowed and can irritate the eyes and skin.
- Barium chloride and lithium chloride are harmful by ingestion or inhalation.
- Ensure proper disposal by diluting with excess water and neutralising with sodium carbonate.
Method
Using Platinum/Nichrome Wire:
- Clean the wire by dipping it in concentrated hydrochloric acid.
- Heat the wire in the Bunsen burner flame until no colour is emitted (indicating it is clean).
- Crush a small amount of the salt with a pestle and mortar.
- Dip the clean wire into hydrochloric acid, then into the salt.
- Hold the wire in the flame and observe the colour.
- Record the colour of each metal.
- If testing multiple salts, use separate wires or clean thoroughly between tests to avoid contamination.
Using Wooden Splints:
- Soak wooden splints in water overnight.
- Crush a small amount of salt with a pestle and mortar.
- Dip a soaked splint into the salt.
- Wave the splint gently in the flame and note the colour.
- Use a fresh splint for each salt to avoid contamination.
Results
- Lithium: Deep red flame
- Sodium: Yellow flame
- Potassium: Lilac flame
- Barium: Yellow-green flame
- Strontium: Red flame
- Copper: Blue-green flame
Example Questions with Answers
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Q1: What colours are observed when the following salts are heated in a flame?
- Copper sulfate: Blue-green
- Lithium carbonate: Deep red
- Sodium sulfate: Yellow
- Strontium nitrate: Red
- Barium nitrate: Yellow-green
- Potassium sulfate: Lilac
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Q2: Why do metals produce characteristic colours in a flame?
- Electrons in metal atoms absorb energy from the flame and move to higher energy levels.
- As they return to lower levels, they emit light.
- The colour of the emitted light depends on the energy difference between these levels, which varies for each metal.
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Q3: How can cross-contamination be avoided during flame tests?
Use separate wires for each test or thoroughly clean the wire with hydrochloric acid between tests.
If using wooden splints, use a new splint for each sample.