12.3.2Experimental Chemistry
Identification of Cations — Combined Science 0653 - Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 10 of 10 Unit quiz
Section 1 of 6Testing for ammonium ions
1
Add sodium hydroxide solution to the sample, which supplies hydroxide ions.
2
Warm the mixture gently.
3
Ammonia gas is released from ammonium ions, NH4+.
4
Hold damp red litmus paper in the gas. It turns blue, which confirms ammonium.
Section 2 of 6Testing metal ions with hydroxide
1Add a few drops
- Add a few drops Add sodium hydroxide or ammonia solution slowly, a few drops at first.
2Precipitate forms
- Precipitate forms The metal ion combines with hydroxide ions to give an insoluble metal hydroxide, and its colour is recorded.
3Add excess
- Add excess Some precipitates redissolve when too much reagent is added, which is why the drops are added slowly.
4Identify
- Identify Compare the colour and behaviour with the table to name the metal ion.
Section 3 of 6Results for metal cations
| Ion | A few drops of NaOH or ammonia | Excess NaOH | Excess ammonia |
|---|---|---|---|
| Calcium, Ca2+ | White precipitate | Does not dissolve | Does not dissolve |
| Copper(II), Cu2+ | Light blue precipitate | Does not dissolve | Dissolves to a dark blue solution |
| Iron(II), Fe2+ | Green precipitate | Does not dissolve | Does not dissolve |
| Iron(III), Fe3+ | Brown precipitate | Does not dissolve | Does not dissolve |
| Zinc, Zn2+ | White precipitate | Dissolves to a colourless solution | Dissolves to a colourless solution |
Section 4 of 6Analysing results
Same start
- Same start Sodium hydroxide and ammonia give the same colours at first because both supply hydroxide ions, OH−.
Telling the white ones apart
- Telling the white ones apart Calcium and zinc both give white precipitates.
- The precipitate from zinc dissolves in excess sodium hydroxide, while that from calcium does not.
Ionic equation example
- Ionic equation example Cu2+(aq) + 2OH−(aq) → Cu(OH)2(s)
- Because a precipitate forms, the metal hydroxide must be insoluble in water.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Cation | A positively charged ion, usually from a metal. |
| Precipitate | An insoluble solid formed in a reaction between solutions. |
| Excess | More of a reagent than is needed to react completely. |
| Ammonium ion | NH4+, which releases ammonia when warmed with alkali. |
| Hydroxide ion | OH−, the ion that makes a solution alkaline and forms metal hydroxide precipitates. |
Section 6 of 6Exam technique
1
Adding too much sodium hydroxide at once, which can redissolve a precipitate and hide the result.
2
Confusing iron(II) and iron(III). Green is iron(II) and brown is iron(III).
3
Claiming copper(II) hydroxide dissolves in excess sodium hydroxide. It dissolves only in excess ammonia, to give a dark blue solution.
4
Using dry red litmus paper for the ammonia test. It must be damp.
5
Calling a white precipitate 'calcium' without testing in excess sodium hydroxide, since zinc also gives white.
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