Precipitate Formation

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

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

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

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

H2O H2O 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)

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)

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

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

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

Precipitate Formation