Tests for cations (AQA GCSE Chemistry): Revision Notes
Tests for cations
What are cations?
Cations are positive ions that can be identified using special chemical tests. Scientists use two main methods to work out which metal ions are present in a sample: flame tests and reactions with sodium hydroxide.
These identification methods are essential in analytical chemistry because they allow us to determine the composition of unknown samples quickly and accurately.
Flame tests
How flame tests work
Flame tests help us identify different metal ions because each metal produces its own special colour when heated in a flame. This happens because the metal ions get excited by the heat and give off light of particular colours.
The unique colours produced by each metal ion are due to electrons moving between different energy levels when heated. This is why flame tests are so reliable for identification.
How to do a flame test
Step-by-Step Flame Test Procedure:
Step 1: Clean the wire loop Dip a flame test wire loop in acid, then heat it in a blue Bunsen flame until no colour shows
Step 2: Prepare the sample Dip the clean loop in acid again, then into your test compound
Step 3: Test in the flame Hold the loop in the Bunsen flame and watch for the colour
Alternative method: You can dip a damp wooden splint into the solid compound and put it straight into the flame. This avoids using corrosive acid.
Flame test colours to remember
Essential Flame Test Colours:
- Calcium - Orange-red flame
- Sodium - Bright yellow flame
- Potassium - Lilac (purple) flame
- Lithium - Red flame
- Copper - Green flame
Hydroxide precipitates
How the test works
Some metal ions form coloured solid precipitates when you add sodium hydroxide solution to them. The sample goes in a test tube and you add a few drops of dilute sodium hydroxide solution. Different metals make different coloured precipitates.
This test works because metal hydroxides have different solubilities and colours, making it possible to distinguish between various metal ions based on the appearance of the precipitate formed.
Coloured precipitates
These metal ions make coloured precipitates:
- Copper(II) ions (Cu²⁺) - Blue precipitate
- Iron(II) ions (Fe²⁺) - Green precipitate
- Iron(III) ions (Fe³⁺) - Brown precipitate
Copper and iron are transition metals. Many transition metals form coloured compounds, which is why their precipitates have distinctive colours that make identification easier.
White precipitates
Some metal ions form white precipitates that all look the same:
- Magnesium ions (Mg²⁺)
- Calcium ions (Ca²⁺)
- Aluminium ions (Al³⁺)
Since these all look white, you need extra tests to tell them apart.
Distinguishing between white precipitates
Key Difference for White Precipitates:
Aluminium hydroxide precipitate will dissolve if you add more sodium hydroxide solution. The magnesium and calcium hydroxide precipitates will not dissolve in excess sodium hydroxide.
This means if you add lots of sodium hydroxide and the white precipitate disappears, you know it contained aluminium ions.
Practical tips
Important Laboratory Guidelines:
- Always clean your flame test wire properly between tests
- Calcium ions can be detected using both flame tests and hydroxide precipitates
- You cannot tell magnesium and calcium ions apart using just the hydroxide test - they both make white precipitates that don't dissolve in excess sodium hydroxide
Remember!
Key Points to Remember:
- Flame tests identify metals by the colours they produce in a flame
- Sodium hydroxide tests identify metals by the colour of precipitate formed
- Blue precipitate = copper ions, green precipitate = iron(II) ions, brown precipitate = iron(III) ions
- White precipitates need extra testing - only aluminium dissolves in excess sodium hydroxide
- Always clean equipment between tests to avoid contamination