There are soluble metal sulfides in the water and insoluble metal sulfides. Most of the insoluble metal sulfides have colours. In this tutorial, you are going to learn which metal sulfides are insoluble in water and what are the colours of them if they have colours.
inorganic chemistry, Solubility of inorganic compounds
Content
All Metal sulfides dissolve in dilute acids and produce hydrogen sulfide gas and metal salt.
As an example, copper sulfide (CuS) is a black precipitate and dissolve in dilute HCl acid and form blue color Copper chloride (CuCl2) solution.
Colours of metal sulfides are important in identifying compounds in inorganic chemistry.
All alkali metals sulfide and alkaline earth metals sulfides are soluble in water and form colourless aqueous solutions. But due to instability of sulfide ion, sulfide ion can be hydrolyzed and emit hydrogen sulfide gas by giving strong aqueous solution.
Sodium sulfide is soluble in water. But it hydrolysis to sodium hydroxide and hydrogen sulfide gas by forming a strong aqueous solution (Sodium hydroxide is a strong alkali).
Na2S + H2O → 2NaOH + H2S
Some p block metal cations form soluble sulfide compounds and some metal cations form insoluble metal sulfides.
Aluminium sulfide dissolve in water and hydrolysis like sodium sulfide and emits hydrogen sulfide gas. Also a white precipitate, aluminium hydroxide ( Al(OH)3 ) is given.
This is the sulfide of tin's +2 oxidation state. Tin(II) sulfide is insoluble in water and form a yellow brown or dark brown precipitate in water.
Tin(IV) sulfide is insoluble in water and form a yellow precipitate in water.
Lead sulfide is insoluble in water and form a black precipitate in water.
Antimony sulfide is insoluble in water and form a orange color precipitate in water.
Bismuth sulfide is insoluble in water and form a orange brown precipitate in water.
3d metal sulfides are insoluble in water and show colors. 3d metal sulfides are MnS, FeS, Fe2S3, CoS, NiS, CuS and ZnS.
In qualitative analysis of inorganic chemistry, sulfide precipitating is done in two methods due to solubility of different metal sulfide's are different. Also, we can use concept of Solubility Product to categorize which metal cations precipitate in acidic medium and which metal cation precipitate in a basic medium.
Precipitates which has a lower solubility product value are precipitated in an acidic medium.
Following precipitates which has a higher solubility product value are precipitated in an alkali medium.
These insoluble metal sulfides have very low Ksp values and solubility values.
| Metal sulfide | Chemical Formula | Colour |
|---|---|---|
| Mercuric sulfide | HgS | Black |
| Antimony trisulfide | Sb2S3 | red-orange |
| Cadmium sulfide | CdS | yellow |
| Ferrous sulfide / Iron(II) sulfide | FeS | Black |
| Stannous sulfide / Tin(II) sulfide | SnS | Yellow brown or dark brown |
| Tin(IV) sulfide | SnS2 | yellow |
| Copper sulfide | CuS | Black |
| Copper sulfide | CuS | Black |
| Zinc sulfide | ZnS | White |
| Manganese sulfide | MnS | Pink |
| Nickel sulfide | NiS | Black |
| Cobalt sulfide | CoS | Black |
| Lead sulfide | PbS | Black |
| Bismuth sulfide | Bi2S3 | Brown |
Some metal sulfides are soluble in water and some are not. If you need to confirm that which metal sulfide is soluble, check whether it's solubility value. As an example, if you check the solubility of Sodium sulfide (18.6 g/100 mL at 200C), you can understand it is soluble in water.
Copper(II) sulfide is insoluble in water and forms a black precipitate. Also copper(I) sulfide is insoluble in water.
Solubility of CoS is 0.00038 g/100 mL of water at 180C. So CoS is insoluble in water but soluble in dilute acids.
All alkali metal sulfides are soluble in water. But sodium sulfide is not stable in water and it is hydrolyzed to sodium hydroxide by giving strong alkaline solution.
Ksp means solubility product. When Ksp value is low, it indicates that compound has a lower solubility in water. Lets take several metal sulfides to learn this.
You see these metal sulfide's Ksp values are very low and they form precipitates in the water
First of all, I should say, this is an idea. After hydrogen sulfide and other effluent gases are separated in the refining process, it should be sent to aqueous concentrated NaOH solution to produce sodium sulfide. Then other gases are removed. Produced sodium sulfide is sent into another place and heat the solution. Then sodium sulfide will hydrolysis will emit hydrogen sulfide gas.
is CoS soluble in water?
You can see, a black color solid is deposited as CoS precipitate in the water. Therefore, CoS is insolble in water.