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I had a question as to what happens when aluminum sulfate is placed into water. After conducting research online, I got varied answers: some say that it dissociates into Al$^{3+}$ and SO$_4$$^{2-}$ ions, while others claim that aluminum sulfate will react with water to form aluminum hydroxide and sulfuric acid.

I even conducted a mini-lab by placing aluminum sulfate in water and sometimes there was precipitate (assumed to be aluminum hydroxide) and sometimes there was none.

What is really happening?

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Aluminum sulfate has special properites and uses. It is true that aluminum sulfate is soluble in water, and it has specific solubility values for certain temperatures:

  • $31.2\ \mathrm g$ per $100\ \mathrm{mL}$ at $0\ \mathrm{^\circ C}$
  • $36.4\ \mathrm g$ per $100\ \mathrm{mL}$ at $20\ \mathrm{^\circ C}$
  • $89.0\ \mathrm g$ per $100\ \mathrm{mL}$ at $100\ \mathrm{^\circ C}$

You stated you used $12\ \mathrm g$ in $100\ \mathrm{mL}$, so it would seem to completely dissolve. However, it this situation, since it isn't being dissolved in very large amounts of water, it doesn't dissociate and form aluminum hydroxide. Aluminum sulfate can be used in water purification in small amounts, because it causes impurities to group up into larger particles and then settle to the bottom of the container (or be filtered out) more easily. The solid you see sometimes are the impurities of the water that have been grouped up and look like precipitate. If you use more pure water, you will see less of these impure solids, or possibly nothing if the water is pure enough (such as if it's distilled water).

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  • $\begingroup$ i added 12g of aluminum sulfate to 100 g of water when there was a precipitate seen and the temperature was room temperature. SO the what is happening? Is the aluminum sulfate going to ions or forming aluminum hydroxide and sulfuric acid $\endgroup$
    – user510
    Mar 10, 2016 at 5:03
  • $\begingroup$ If you adding that little amount, then it has to do with the properties of aluminum sulfate. Check my edited answer. $\endgroup$
    – Abob
    Mar 10, 2016 at 5:18
  • $\begingroup$ where is your edited answer? $\endgroup$
    – user510
    Mar 10, 2016 at 5:20
  • $\begingroup$ ok so the aluminum sulfate is then just being dissociated into individual ions? $\endgroup$
    – user510
    Mar 10, 2016 at 5:33
  • $\begingroup$ Yes, because the ions cause the impurities to group together in smaller amounts of water. $\endgroup$
    – Abob
    Mar 10, 2016 at 6:50
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I am assuming that you are measuring 12 mL in a flask. This is very unusual because Sulfate is in a solid form therefore you are best to weight the sample in a balance on order to determine the solubility component to hold in the water volume.

The density of Alum assumed to be the unhydrate is 2.572 grams per mL therefore you have 32.064 g of the anhydrous form. The solubility i water if 31 to 36 g per 100 mL. You are adding in 100 Ml of water in laboratory analytical conditions. Therefore, you are closing too close to the solubility lists at room temperature.

Another effect to consider is the dramatic increase in specific gravity. The water volume in the solution will shrink to compensate the final specific gravity of 1.33. This will saturate the solution and will precipitate to accommodate specific gravity and normal solution solubility at the temperature.

This chart will help you to evaluate how the ratios of sulfate weight and water volume impacts the specific density the Aluminum Sulfate solution: Ind. Eng. Chem., 1945, 37 (10), 1016–1018

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Aluminium sulfate does dissociate in water, forming ions. That's true. However, most cations with double and triple positive charge bind water very strongly. Strong positive charge make hydrogen atoms of the associated water molecule acidic, so it readily dissociates into hydrogen cation and hydroxide group, associated with aluminium ion. The resulting structure can associate with another aluminium ion, eventually forming large network of aluminium hydroxide.

However, the topic largely avoided in school chemistry for convenience, because the chemistry is large, counterintuitive and is not ruled by simple rules.

In general, it can be safely assumed that at least some hydrolysis occurs for aluminum sulfate unless a strongly acidic solution is involved. The hydrolysis may be complete for at least part of aluminum sulfate until enough free acid produce to stop further hydrolysis.

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