Testing for aqueous metal ions

Dilute sodium hydroxide solution reacts with some metal ions in solution, forming metal hydroxides. Some of these metal hydroxides are insoluble, so they appear as precipitates.

For example, copper sulfate solution reacts with a few drops of sodium hydroxide solution:

copper sulfate + sodium hydroxide → sodium sulfate + copper hydroxide

CuSO4(aq) + 2NaOH(aq) → Na2SO4(aq) + Cu(OH)2(s)

Copper hydroxide forms a blue precipitate.

Sodium hydroxide solution is added to copper sulfate solution. Solid copper hydroxide is produced in colourless sodium sulfate solution.Sodium hydroxide solution is added to copper sulfate solution. Solid copper hydroxide is produced in sodium sulfate solution.

The table shows the coloured precipitates formed by five common metal ions.

Metal ionPrecipitate colour
Iron(II), Fe2+Green
Iron(III), Fe3+Orange-brown
Copper(II), Cu2+Blue
Calcium, Ca2+White
Zinc, Zn2+White

Distinguishing between calcium ions and zinc ions

A few drops of dilute sodium hydroxide solution react to form a white precipitate with calcium ions and with zinc ions. However, if excess sodium hydroxide solution is added:

  • the calcium hydroxide precipitate is unchanged
  • the zinc hydroxide precipitate dissolves to form a colourless solution
Question

A green precipitate forms when dilute sodium hydroxide solution is added to a sample in solution. Identify the metal ion present in the original solution.

Iron(II) ions, Fe2+, are present.

curriculum-key-fact
Ions of group 1 metals (Li+, Na+ and K+) form soluble hydroxides. They are identified using flame tests and not by adding sodium hydroxide solution.