 If we take the magnesium oxide that we produced from the synthesis reaction, we notice that it's a very brittle white solid. If we add water to the solid and stir the solid, we see that the compound is not very water soluble. The metal oxide reacts with water to form the metal hydroxide. Most metal hydroxides are not very water soluble. One way to test for the fact that we're producing a metal hydroxide is to add phenethylene to the solution. Phenethylene is an acid-base indicator that is colorless and acidic solution but is pink to purple in basic solution. The formation of the purple color solution indicates that we've converted the metal oxide to some of the metal hydroxide.