1. Precipitate Formation
When two clear solutions are mixed a
blue precipitate is produced.

2. Purpose
To observe a double displacement reaction which occurs in solution
To represent the chemical reaction using an ionic equation

3. Demonstration
The clear, colorless calcium chloride solution when mixed with the cobalt chloride solution formed a new solution containing
calcium ions
chloride ions
cobalt(II)hydroxide precipitate
The cobalt(II) hydroxide was blue and appeared as a blue suspension in the solution which sunk to the bottom of the beaker

4. Concepts
1. Solution Chemistry
2. Ionic Equations
3. Solubility

5. 1. Solution Chemistry Each solution contains a different salt
Salts are made of ions
Pink solution
CoCl2(s) Co2+(aq) + 2Cl-(aq)
Clear solution
Ca(OH)2(s) Ca2+(aq) + 2(OH-)(aq)
H2O
H2O

6. 2. Ionic Equations The chemical equation for the mixed solutions is
Co2+(aq) + 2Cl-(aq) + 2Ca2+(aq) + 2(OH-)(aq)
2Ca2+(aq) + 2Cl-(aq) + Co(OH)2(s)
The net ionic equation is
Co2+(aq) + 2(OH-)(aq) Co(OH)2(s)

7. 3. Solubility
When the cobalt ion and the hydroxide ions are present in the same solution, they bond tightly to form a new compound which is insoluble
Co(OH)2(s)
On the other hand, when the calcium and chloride ions are present, the salt that could form is soluble, thus exists as individual ions in solution
CaCl2(s) Ca2+(aq) + Cl-(aq)
H2O

8. Conclusions
If both calcium chloride and cobalt chloride were soluble salts, no reaction would have occurred
Since at least one of the possible products was insoluble, the reaction occurred as shown by the color change and formation of the blue precipitate
The chemical reaction in this demonstration was a double replacement reaction

9. Comments
This reaction is not a redox reaction since the ionic species do not change oxidation number
The hydroxides of many metals are insoluble and form precipitates