# Solving the Mystery of AlCl3 & NaOH/NH4OH Precipitation

• ChemDunce
In summary, the precipitate in the solution with ammonium hydroxide is because the equilibrium moves to the right and the Al(OH)3 is complexed to create an ion that dissolves in solution.
ChemDunce
Hey Chem geniuses!

I am a struggling high school chem student who couldn't combine water with water without blowing something up so some help would definitley be appreciated.

My chem final has been issued already and it is as follows:

Put 5 mL of 3 M aluminum chloride solution in each of two test tubes. To one test tube add a few drops of 3 M sodium hydroxide solution. Then add more sodium hydroxide solution, with stirring, until the precipitate dissolves. Treat the other test tube similarly, but use 3 M ammonum hydroxide solution to obtain precipitation and then add more ammonium hydroxide solution. Why did the precipitate dissolve in one case and not the other?

I calculated the amounted needed to be 2.00 grams of aluminum chloride. Since I need to put that amount in TWO test tubes, I requested 4.00 grams for my experiment. The 3M of Sodium Hydroxide and 3 M of Ammonium Hydroxide were requested in 2.5 mL each. If anyone sees error in this, please let me know! Thanks

The question remains... why did one case precipitate and not in the other?

Thanks a bunch.

complexing ions...

Hi ChemDunce,

The reason that the precipitate dissolves in one of the solutions is because the equilibrium moves to the right until Al(OH)3(s) is precipitated; this easily reacts with excess NaOH to give the aluminate ion, Al(OH)4-(aq). Basically, the Al(OH)3 is complexed to create an ion that dissolves in solution.

The ammonium hydroxide solution is unable to complex the Al(OH)3 precipitate and, therefore, remains as a precipitate. Hope this information helps. Good luck on your final!

Fernando

Thanks a bunch! Now, for the chemical equations, the AlCl3+ NH4OH is not a double replacement, then what would it be? While the AlCl3+NaOH ---> Al(OH)4-?
Are there any practical uses for this reaction in everyday life?

Last edited:

## 1. What causes the precipitation of AlCl3 and NaOH/NH4OH?

The precipitation of AlCl3 and NaOH/NH4OH is caused by a chemical reaction between aluminum chloride (AlCl3) and sodium hydroxide (NaOH) or ammonium hydroxide (NH4OH). This reaction results in the formation of a solid compound, aluminum hydroxide (Al(OH)3), which is insoluble in water and appears as a precipitate.

## 2. How can the precipitation reaction be used to identify the presence of aluminum?

The precipitation of AlCl3 and NaOH/NH4OH is a specific reaction that only occurs with aluminum ions. Therefore, if a solution contains aluminum ions, the addition of NaOH or NH4OH will result in the formation of a white precipitate, indicating the presence of aluminum.

## 3. Can the precipitation of AlCl3 and NaOH/NH4OH be reversed?

The precipitation reaction between AlCl3 and NaOH/NH4OH cannot be easily reversed. However, the precipitate can be dissolved by adding excess sodium hydroxide or ammonium hydroxide, forming a soluble complex with aluminum ions.

## 4. What other factors can affect the precipitation reaction?

The precipitation reaction can be affected by the concentrations of the reactants and the temperature of the solution. Increasing the concentration of either AlCl3 or NaOH/NH4OH can result in a larger amount of precipitate formed. Additionally, a higher temperature can speed up the reaction and increase the rate of precipitation.

## 5. How is the precipitation reaction used in practical applications?

The precipitation reaction of AlCl3 and NaOH/NH4OH is commonly used in water treatment to remove aluminum ions from drinking water. It is also used in the production of aluminum hydroxide, which is used in the manufacturing of various products such as antacids, cosmetics, and paper fillers.

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