Solubility of hydroxides, Metal hydroxides Precipitates, Colours

Some metal hydroxides forms precipitates, and some are solutions. Some hydroxide precipitates dissolve in water when we add more water to the compound. Some hydroxides such as 3d metal's show different colours. Some metal hydroxides are amphoteric compounds and they react with both acids and alkalis to give soluble compounds.

When solubility of metal hydroxide is high, it gives a good alkaline solution due to complete dissociation of hydroxide compound for releasing of hydroxyl ions (OH-).

Examples: KOH, NaOH

Completely soluble hydroxides in water

Alkali Metals

All alkali metals hydroxides are soluble in water.

Alkali earth metals

Only strontium hydroxide ( Sr(OH)2) and barium hydroxides ( Ba(OH)2 ) are completely soluble from alkaline earth metals. Berrylium hydroxide ( Be(OH)2) and magnesium hydroxide ( Mg(OH)2) are completely insoluble in water.

When calcium and hydroxyl ion concentration are high (concentrated), calcium hydroxide is precipitated as a white solid. When ion concentration is low, it become a colourless solution.

Completely soluble hydroxides of s block metals

  • LiOH
  • NaOH
  • KOH
  • Ba(OH)2
  • Sr(OH)2

Sparingly soluble hydroxides in water

These hydroxide are dissolved when excess water is added to the precipitate. When metal hydroxide concentration is increased, it is precipitated.

  • Ca(OH)2

Completely insoluble hydroxides

3d metal hydroxides are insoluble in water. d block metals OH- show colours. Colours of precipitates are noted with respective compound. Colours of these 3d metal hydroxides can be used to identify the 3d metal.

Insoluble 3d metal hydroxides and colours

These 3d metal hydroxides are precipitates and they show different colours.

  • Fe(OH)2 - green
  • Fe(OH)3 - brown
  • Mn(OH)2 - white/pink
  • Cr(OH)3 - green
  • Ni(OH)2 - green
  • Cu(OH)2 - blue
  • Co(OH)2 - light blue
  • Zn(OH)2 - green

Other insoluble metal hydroxides

  • Be(OH)2
  • Mg(OH)2 (white)
  • Cd(OH)2 (white)
  • Pb(OH)3

Amphoteric hydroxides of metals

Some metal hydroxides are amphoteric compounds. These hydroxides are not dissloved in water. But adding excess NaOH(aq) to these precipitates, they dissolve in the water. Also amphoteric hydroxides dissolve in dilute acids such as HCl, H2SO4. Colours of amphoteric metal hydroxides are noted with the compound.

  • Zn(OH)2 - white
  • Cr(OH)3 - green
  • Al(OH)3 - white
  • Be(OH)2 - green
  • Pb(OH)2 - white

Fe(OH)2 and Fe(OH)3

Ferrous hydroxide ( Fe(OH)2 ) and ferric hydroxide ( Fe(OH)3 ) are insoluble in water and form green colour and brown colour precipitates in water. These precipitates do not dissolve in excess aqueous NaOH.

Chromium hydroxide | Cr(OH)3

Chromium hydroxide is green precipitate which is formed when dilute NaOH is added to Cr+3 ion.

  • Cr(OH)3 dissolve in adding excess NaOH to give [Cr(OH)4]- green colour solution.
  • Cr(OH)3 does not dissolve in aqueous ammonia solution.

Nickel hydroxide | Ni(OH)2

Nickel hydroxide is a light green precipitate.

  • Ni(OH)2 does not dissolve in excess NaOH.
  • But in addition of ammonia, light green precipitate dissolve to give blue violet solution [Ni(NH3)]2+.

Solubility of chromium hydroxide

Chromium is a 3d metal and it's +3 ion forms a green colour hydroxide which is a insoluble in water. But with excess NaOH, it dissolves and forms a coordination complex anion, [ Cr(OH)4]-.

Ammonium hydroxide - NH4OH

Ammonium hydroxide is very well soluble in water. But its dissociation in water is partial (incompleteness).

Metal hydroxides solublity

This is the summary of solubility of all metal hydroxides.

solubility of metal hydroxides in water


Are all metal hydroxides soluble?

No. Some metal hydroxides are soluble and some are not. To decide solubility, we have to look solubility product or solubility data from books or any other resource.