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This question already has an answer here:

This is a follow up from an answer to one of my questions (here is the link)

I tried dissolving aluminum sulfate in pure distilled water. I used the same concentration: 12 mL of aluminum sulfate in 100 g of water, yet I saw a precipitate. Can someone please let me know what is happening. My best guess is that the aluminum sulfate is reacting with water and forming aluminum hydroxide.

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marked as duplicate by paracetamol, Todd Minehardt, Klaus-Dieter Warzecha, Jon Custer, jerepierre Jan 16 '17 at 16:28

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    $\begingroup$ Most likely is that some aluminium sulphate just wouldn’t dissolve. $\endgroup$ – Jan Jan 16 '17 at 2:04
  • $\begingroup$ Explain the origin of the Alum . Post the content of the Sulfate and post in details here. An observation is that you apply 12 mL of Aluminum Sulfate. Therefore, you are using a solvated solution or just measured solid in a volumetric flask? $\endgroup$ – Jose Enrique Calderon Jan 16 '17 at 4:33
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    $\begingroup$ How is it supposed to be different from your original question? $\endgroup$ – Mithoron Jan 16 '17 at 13:06
  • $\begingroup$ @Mithoron It is clarified the used of pure distilled water to discard the possibility of precipitates others that saturation limits. $\endgroup$ – Jose Enrique Calderon Jan 16 '17 at 14:52
  • $\begingroup$ @Mithoron I am using PURE WATER. That's how this question is different. $\endgroup$ – user510 Jan 16 '17 at 18:33
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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.

http://pubs.acs.org/doi/abs/10.1021/ie50430a030

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