Testing for positively charged ions
Dilute sodium hydroxide solution is used in tests for some Shiny element that is a good conductor of electricity and heat, and which forms basic oxides. Electrically charged particle, formed when an atom or molecule gains or loses electrons., and is a test to identify ammonium ions in A substance formed by the chemical union of two or more elements..
Testing for metals
Dilute sodium hydroxide solution reacts with some metal ions in solution, forming metal hydroxides. Some of these metal hydroxides are Unable to dissolve in a particular solvent. For example, sand is insoluble in water., so they appear as A suspension of particles in a liquid formed when a dissolved substance reacts to form an insoluble substance, eg in a precipitation reaction.. For example, copper sulfate solution reacts with a few drops of sodium hydroxide solution:
Copper sulfate + sodium hydroxide 鈫 sodium sulfate + copper hydroxide
CuSO4(aq) + 2NaOH(aq) 鈫 Na2SO4(aq) + Cu(OH)2(s)
Copper hydroxide forms a blue precipitate.
The table shows the coloured precipitates formed by five common metal ions in this test.
| Metal ion | Precipitate colour |
| Aluminium, Al3+ | White |
| Calcium, Ca2+ | White |
| Copper, Cu2+ | Blue |
| Iron(II), Fe2+ | Green |
| Iron(III), Fe3+ | Brown |
| Metal ion | Aluminium, Al3+ |
|---|---|
| Precipitate colour | White |
| Metal ion | Calcium, Ca2+ |
|---|---|
| Precipitate colour | White |
| Metal ion | Copper, Cu2+ |
|---|---|
| Precipitate colour | Blue |
| Metal ion | Iron(II), Fe2+ |
|---|---|
| Precipitate colour | Green |
| Metal ion | Iron(III), Fe3+ |
|---|---|
| Precipitate colour | Brown |
Distinguishing between aluminium ions and calcium ions
A few drops of dilute sodium hydroxide solution react to form a white precipitate with aluminium ions and with calcium ions. However, if excess sodium hydroxide solution is added:
- the aluminium hydroxide precipitate reacts to form a colourless solution
- the calcium hydroxide precipitate is unchanged
Question
A green precipitate forms when dilute sodium hydroxide solution is added to a solution containing a metal ion. Identify the metal ion present.
Iron(II) ions, Fe2+, are present.
Testing for ammonium ions
Ammonium ions, NH4+, react with hydroxide ions to form ammonia and water:
NH4+(aq) + OH-(aq) 鈫 NH3(g) + H2O(l)
This gives a test for ammonium ions:
- add dilute sodium hydroxide solution to the sample
- warm the mixture
Ammonia gas is given off if ammonium ions are present.
Testing for ammonia
Ammonia turns damp red litmus paper blue.
Question
Explain what the test for ammonia tells you about the properties of ammonia.
Ammonia dissolves in water to form an Substance producing more hydroxide ions than hydrogen ions when dissolved in water. solution, because it turns damp red litmus paper blue.