# Why does Ammonium sulfate cause proteins to precipitate?

I'm trying to understand an observed reaction from a chem practical:

You have a solution of ammonium sulfate, which you add to a bovine serum albumin solution. The proteins precipitate, and the solution becomes cloudy. w/v % of $\ce{(NH4)2SO4}$ at this point is 57%.

When you add water (in a ratio of 2 parts water: 1 part serum solution)to the mix, the process reverses, and the solution becomes clear again.

This can be explained by the ammonium sulfate solution 'stealing' water from the proteins, which causes them to precipitate, and then the process reversing once more water is added. (Right?)

Question: Does the ammonium sulfate 'steal' the water because it wants to reach equilibrium, or because it's supersaturated, or what?

• – Mithoron Feb 12 '15 at 11:26