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Why do aluminum, magnesium, and calcium form white precipitate when mix with sodium hydroxide? Shouldn't there be no reaction? Because sodium is more reactive than all of those element, it should be impossible for sodium to be displace by those three elements.

YouTube video - "NaOH + Al - sodium hydroxide and aluminum"

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When adding aluminium, calcium or magnesium, you are most likely adding them as chlorides. They dissociate almost fully, similarly to sodium hydroxide.

Then you have a solution with these four elements:

  1. Sodium cations
  2. Calcium (for example) cations
  3. Hydroxide anions
  4. Chloride anions

The precipitate that forms depends on the solubility of the different combinations of anions and cations. Sodium chloride and hydroxide are soluble so they remain in solution. Metal chlorides are also soluble. However calcium, magnesium, and aluminium hydroxides are insoluble so they form the precipitate.

You’re basically neutralising the basic solution, formed a solution of sodium chloride and a “basic” solid precipitate.

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  • $\begingroup$ Thanks for the answer! And why does the precipitation disappear when we add an excess amount of Sodium Hydroxide to solution containing Aluminum as the cation. $\endgroup$ – user28603 May 9 '18 at 11:20

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