4.3.1Electrochemistry
Identifying Metal Ions — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 4 parts Lesson 3 of 20 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 4Ions and Their Tests
Ions are charged particles, and the tests in this section target the positive and negative ions separately.
Cations
- Cations Positively charged metal ions, such as Na⁺, Ca²⁺ and Al³⁺, which are identified by flame tests and hydroxide precipitates.
Anions
- Anions Negatively charged ions, such as CO₃²⁻, Cl⁻ and SO₄²⁻, which are identified by their own specific chemical tests.
Flame test
- Flame test Used for a metal ion that gives a characteristic coloured flame, such as lithium giving a crimson flame.
Hydroxide test
- Hydroxide test Used for metal ions that form coloured precipitates with sodium hydroxide solution, such as copper(II) forming a light blue solid.
Part 2 of 4Choosing the Right Test
A systematic approach narrows down the possibilities before a final identification is made.
1
Start by noting the appearance of the solid or solution, including its colour, because this often gives an early clue.
2
Carry out a flame test on the sample to see whether a characteristic metal colour is produced.
3
Add sodium hydroxide slowly and note any precipitate colour, then check whether the precipitate dissolves in excess.
4
Use a specific anion test, such as the carbonate or halide test, to confirm the negative ion present in the compound.
5
Compare the results of every test with the table of known ion behaviours before deciding which ion is present.
Part 3 of 4Interpreting Results
Interpreting Results
Pattern
- Pattern Each metal ion gives a pattern of results, and a single test rarely identifies an ion on its own.
Cross-checking
- Cross-checking Two independent tests that agree give much stronger evidence than one test alone.
Precipitates
- Precipitates A precipitate that forms and then dissolves in excess sodium hydroxide shows that the hydroxide dissolves in excess alkali,
- as aluminium hydroxide does.
Limits
- Limits Some ions, such as calcium and magnesium, give similar precipitates and need a second test to tell them apart.
Practical link
- Practical link Running the flame test and the sodium hydroxide test on the same sample gives two sets of evidence to compare.
Part 4 of 4Exam Checklist & Tips
4.3.1 – Key Points for the Exam
- Cation A positively charged ion, formed when a metal atom loses electrons.
- Anion A negatively charged ion, formed when a non-metal atom gains electrons.
- Precipitate An insoluble solid that forms when two solutions react together.
- Identification The use of observations from chemical tests to name an unknown substance or ion.
Common Mistakes
- Using a flame test to identify a negative ion, when flame tests only identify metal cations.
- Relying on a single observation, such as a colour, without checking with a second test.
- Confusing the colour of a solution with the colour of a precipitate formed in it.
- Stating that a precipitate is clear or colourless when it is actually white or coloured.
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