Dissolving Aluminum Hydroxide by Complex Reactions

Problem In order to present students with a cognitive conflict it is possible to precipitate a solid and to dissolve this solid by the same substance. One way is to add a small amount of sodium hydroxide solution to aluminum chloride solution aluminum hydroxide precipitates as a white solid. After that an excess of sodium hydroxide is added the white solid disappears because the soluble tetrahydroxide aluminum complex is formed. If this excess of hydroxide solution is added from the beginning, no precipitation occurs, the complex is just formed. 

Material Test tubes aluminum chloride (AIC13 2 H2Oj, sodium hydroxide solution. 

Procedure Fill a test tube to one-third with aluminum chloride solution, add some drops of sodium hydroxide solution. After observing a precipitate add some more sodium hydroxide solution slowly the white solid dissolves. In a second test tube add to the aluminum chloride solution a large amount of sodium hydroxide at once. 

Observation In the first step a white solid is formed, in the second step the precipitation disappears. In the second test tube no precipitation occurs at all. 

Tip The association of industrially produced aluminum hydroxide from Bauxite can be discussed aluminum hydroxide is separated from the accompanying substances of the Bauxite by mixing the pulverized mineral with hot concentrated sodium hydroxide solution. 

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