Grade: 10
Subject: Physical Sciences
Term: 1
CAPS Type: Informal Experiment
Topic: Atomic Structure, Electron Configuration, Energy Levels and Metal Cations
Aim
To identify selected metal cations and metals by observing the flame colours they produce when heated.
Learning Outcome
- Perform flame tests safely.
- Observe and identify characteristic flame colours.
- Explain why different metal ions produce different colours.
- Describe electron excitation and electron relaxation.
- Relate flame tests to electron energy levels and visible light.
Time Allocation
Approximately 45 minutes.
How Does It Work?
When certain metal salts are heated in a flame, the flame changes colour.
The heat energy from the flame excites electrons inside the metal ions. These electrons absorb energy and move to higher energy levels. This is called the excited state.
The excited state is unstable. The electrons quickly return to their original lower energy levels, known as the ground state.
As the electrons return to lower energy levels, energy is released in the form of visible light.
Electron gains energy → Electron becomes excited
Electron loses energy → Visible light is emitted
Different metal ions release different amounts of energy, producing different colours of light.
This is why sodium produces a yellow flame while copper produces a blue-green flame.
Ground State
The normal energy level of an electron before heat energy is absorbed.
Excited State
A higher energy level reached after the electron absorbs energy from the flame.
Flame tests are commonly used in:
- fireworks,
- chemical analysis,
- mineral testing,
- and forensic science.
Scientific Background
An ionic compound consists of:
- a metal ion (cation),
- and a non-metal ion (anion).
When ionic compounds dissolve in water, the ions separate and move freely in solution.
During a flame test, heat energy from the flame is absorbed by electrons in the metal ions.
The electrons move from the ground state to a higher energy level.
When the electrons fall back to lower energy levels, they release energy as photons of visible light.
The wavelength and colour of the light depend on the amount of energy released during the electron transition.
Because every element has a unique electron arrangement, different metal ions produce different flame colours.
Hypothesis
If different metal salts are heated in a flame, each metal ion will produce a characteristic flame colour.
Variables
Independent Variable
Type of metal salt or metal sample tested.
Dependent Variable
Colour of the flame produced.
Control Variables
- Same heat source.
- Same flame intensity.
- Same testing method.
- Same amount of sample where possible.
- Clean sticks or wires between tests.
- Same observation conditions.
Apparatus
- Watch glasses
- Burner
- Propette or dropper
- Bamboo sticks or flame test wires
- Test tubes or small containers
- Heatproof mat
- Safety goggles
- Matches or lighter
- Waste container
Chemicals / Materials
- Methanol
- Sodium chloride, NaCl
- Copper(II) chloride, CuCl₂
- Calcium chloride, CaCl₂
- Potassium chloride, KCl
- Lithium chloride, LiCl
- Barium chloride, BaCl₂
- Strontium chloride, SrCl₂
- Copper powder
- Magnesium ribbon
- Zinc powder
- Iron powder
Safety Precautions
- Wear safety goggles and protective clothing.
- Methanol is highly flammable and must be handled carefully.
- Keep methanol away from open flames except during controlled testing.
- Use very small quantities of chemicals and methanol.
- Do not inhale fumes directly.
- Do not touch hot apparatus.
- Ensure the burner is switched off when not in use.
- Keep the work area clean and dry.
- Dispose of chemicals according to laboratory safety procedures.
Method
Part 1: Preparing the Samples
- Place small amounts of each metal salt or metal sample onto separate watch glasses.
- Label each sample clearly.
- Add a small amount of methanol where required.
- Prepare clean bamboo sticks or flame test wires for each sample.
Part 2: Performing the Flame Tests
- Light the burner safely.
- Dip a clean bamboo stick or flame test wire into the first sample.
- Hold the sample in the edge of the flame.
- Observe the flame colour carefully.
- Record the colour in the observation table.
- Repeat the process for each sample.
- Use a clean stick or wire for every sample to avoid contamination.
Results
Observation Table
| Sample Tested | Flame Colour Observed | Possible Metal Ion Present |
|---|---|---|
| Sodium chloride | Sodium ion | |
| Copper(II) chloride | Copper ion | |
| Calcium chloride | Calcium ion | |
| Potassium chloride | Potassium ion | |
| Lithium chloride | Lithium ion | |
| Barium chloride | Barium ion | |
| Strontium chloride | Strontium ion |
Typical Flame Colours
| Metal Ion | Typical Flame Colour |
|---|---|
| Sodium | Bright yellow |
| Potassium | Lilac or pale violet |
| Copper | Blue-green |
| Calcium | Orange-red or brick-red |
| Lithium | Crimson red or magenta pink |
| Barium | Bright green |
| Strontium | Bright red |
What Learners Should Observe
- Different metal ions produce different flame colours.
- Some colours are bright and easy to identify.
- Some colours are faint and difficult to observe.
- Flame colours disappear quickly once the sample is removed from the flame.
Potassium often produces a very faint lilac flame that is difficult to observe clearly.
Expected Results
Different metal ions should produce characteristic flame colours that can be used to help identify unknown metal ions.
Conclusion
Different metal ions produce different flame colours because electrons absorb energy from the flame and move to higher energy levels.
When the electrons return to lower energy levels, energy is released as visible light.
The colour of the light depends on the specific electron transitions within the metal ion.
Questions for Learners
- Why do different metal salts produce different flame colours?
- What happens to electrons when the metal salt is heated?
- What is meant by the ground state?
- What is meant by the excited state?
- Why does the atom emit light?
- Which sample produced the brightest flame colour?
- Why must clean sticks or wires be used between tests?
- What could cause inaccurate flame colours?
- Why is potassium difficult to observe?
- Why is methanol handled carefully during this experiment?
Common Mistakes
- Using too much sample.
- Contaminating one sample with another.
- Reusing the same stick or wire without cleaning.
- Holding the sample in the wrong part of the flame.
- Recording colours too slowly.
- Confusing similar flame colours.
- Allowing sodium contamination to overpower weaker flame colours.
- Using excessive methanol near an open flame.
Teacher Notes
- This practical works well as a teacher demonstration if chemicals are limited.
- Sodium contamination is very common and can mask weaker flame colours.
- Potassium produces a faint lilac flame that learners may struggle to observe.
- Use small quantities of chemicals to improve safety and visibility.
- Prepare labelled samples before the lesson starts to save time.
- Discuss electron excitation and energy release after learners have observed the colours.
Teacher Tip
Ask learners to focus carefully on the outer edge of the flame when observing potassium compounds. The lilac colour is often faint and can easily be hidden by contamination from sodium.
Extension Activity
Provide learners with an unknown metal salt sample and ask them to identify the possible metal ion by comparing its flame colour with the known samples.
Learners can also investigate why fireworks produce different colours.
Real-World Application
Flame tests are used in:
- fireworks displays,
- chemical laboratories,
- mineral identification,
- forensic investigations,
- and quality control in industry.
Different chemicals are added to fireworks to produce different colours when heated.