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

Examples: KOH, NaOH



Completely soluble metal 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 metal hydroxides of s block metals

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

Sparingly soluble metal 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 metal 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 metals or ions.

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 - white

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 - white
  • 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).



List of metal hydroxides solubility with colours

Metal hydroxide Solubility Colour of solid state Colour of in the water
LiOH soluble white colourless solution
NaOH soluble white colourless solution
KOH soluble white colourless solution
Be(OH)2 insoluble white a white precipitate with colourless solution
Mg(OH)2 insoluble white a white precipitate with colourless solution
Ca(OH)2 insoluble in higher concentrations of ions white a white precipitate in higher concentrations or colourless solution
Al(OH)3 insoluble white a white precipitate in higher concentrations or colourless solution
Sr(OH)2 soluble white colourless solution
Ba(OH)2 soluble white colourless solution
Cr(OH)3 insoluble green form a green precipitate with green colour solution
Mn(OH)2 insoluble white/pink form a white or pink precipitate
Fe(OH)2 insoluble green form a green precipitate with green colour solution
Fe(OH)3 insoluble brown form a brown precipitate with brown colour solution
Co(OH)2 insoluble light blue form a blue precipitate with blue colour solution
Ni(OH)2 insoluble green form a green precipitate with green colour solution
Co(OH)2 insoluble light blue form a blue precipitate with blue colour solution
Cu(OH)2 insoluble blue form a blue precipitate with blue colour solution
Zn(OH)2 insoluble white form a white precipitate with colourless solution
Pb(OH)2 insoluble white form a white precipitate with colourless solution


This is the summary of solubility of all metal hydroxides.


solubility of metal hydroxides in water

Questions


How do you identify magnesium hydroxide and zinc hydroxide

Both magnesium hydroxide and zinc hydroxide are insoluble in water and form white precipitates.

You know, zinc hydroxide is an amphoteric hydroxide. Therefore it dissolve in aqueous NaOH and form sodium zincate (Na2ZnO2) aqueous solution. But, magnesium hydroxide is not a amphoteric hydroxide. So we can see one white precipitate will dissolve when adding aqueous NaOH.

Learn qualitative analysis of zinc ion testing

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.



Do hydroxides form precipitates?

Some metal hydroxide form precipitates and some are not. Mg(OH)2, 3d metal hydroxides such as Fe(OH)2 are precipitates.



Which of the following hydroxide is insoluble in water?

  1. NaOH
  2. Ba(OH)2
  3. Pb(OH)2
  4. RbOH

Pb(OH)2 is insoluble in water and form a white precipitate. So answer is 3.



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