Tests for Positive Ions — GCSE Chemistry Revision
Revise Tests for Positive Ions for GCSE Chemistry. Step-by-step explanation, worked examples, common mistakes and exam-style practice aligned to AQA, Edexcel, OCR, WJEC, Eduqas, CCEA, Cambridge International (CIE), SQA, IB, AP.
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Go to Tests for Negative IonsWhat is Tests for Positive Ions?
Positive metal ions (cations) in a solution can be identified by adding a few drops of sodium hydroxide solution. Many metal hydroxides are insoluble and have a characteristic colour, so a coloured precipitate is formed. This is a key technique in qualitative analysis.
Board notes: The tests for common metal cations (Cu²⁺, Fe²⁺, Fe³⁺, Al³⁺, Mg²⁺, Ca²⁺) using sodium hydroxide are required knowledge for all boards. The flame tests for metal ions are also part of this topic.
Step-by-step explanationWorked example
To test for copper(II) ions (Cu²⁺) in a solution, add a few drops of sodium hydroxide solution. A blue precipitate of copper(II) hydroxide (Cu(OH)₂) will be formed.
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Common mistakes
- 1Confusing the colours of the precipitates. For example, copper(II) hydroxide is blue, while iron(II) hydroxide is green and iron(III) hydroxide is brown.
- 2Adding too much sodium hydroxide at once. You should add it drop by drop to observe the formation of the precipitate clearly.
- 3Forgetting that some precipitates dissolve in excess sodium hydroxide. For example, aluminium hydroxide is a white precipitate that redissolves to form a colourless solution.
Tests for Positive Ions exam questions
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Step-by-step method
Step-by-step explanation
4 steps · Worked method for Tests for Positive Ions
Core concept
Positive metal ions (cations) in a solution can be identified by adding a few drops of sodium hydroxide solution. Many metal hydroxides are insoluble and have a characteristic colour, so a coloured pr…
Frequently asked questions
What is a precipitate?
A precipitate is an insoluble solid that is formed when two solutions are mixed or when a substance becomes insoluble in a solution.
How do you test for ammonium ions (NH₄⁺)?
To test for ammonium ions, add sodium hydroxide solution and gently warm the mixture. If ammonium ions are present, ammonia gas will be produced, which can be identified by its smell or by testing with damp red litmus paper (it will turn blue).