M AGNES fUM. 275 



thetical metal, ammonium. Calcium hydroxide is slightly soluble, 

 less than calcium sulphate, but sufficiently soluble to be employed as 

 a reagent. Hydroxides of the other metals are either insoluble or so 

 little soluble as to be classed insoluble. These are obtained as pre- 

 cipitates by adding a soluble hydroxide (usually of sodium, potas- 

 sium, or ammonium) to a salt of the metals whose hydroxides are in- 

 soluble. The principle involved here is the same as in the case of 

 precipitation of insoluble carbonates, namely, bringing together in 

 solution constituents which by their union can form insoluble prod- 

 ucts and thus be eliminated from the solution, thereby allowing the 

 reaction to go on to completion. This reaction is given as a test under 

 many of the metals. 



Ammonium hydroxide acts, in general, like the alkalies, but 

 toward certain metals it shows a marked difference. For example, 

 calcium hydroxide is precipitated from fairly concentrated solutions 

 of calcium salts by the alkalies, but not by ammonia water. This is 

 explained by the ionic or dissociation theory by the fact that ammo- 

 nium hydroxide is only slightly dissociated. According to this 

 theory, practically all reactions in aqueous solutions take place be- 

 tween ions (see page 195). The alkalies are largely dissociated into 

 metal ions and (OH) ions, which latter unite with the metal ions of 

 the other metals to form the slightly ionized and insoluble hydrox- 

 ides. Now ammonium hydroxide is only slightly ionized in fact, 

 to a less extent than calcium hydroxide so that only a small amount 

 of calcium hydroxide is formed, which remains in solution, because 

 somewhat soluble. The presence of this calcium hydroxide in solu- 

 tion prevents further ionization of the ammonium hydroxide to such 

 an extent that it ceases to act as an alkali or soluble hydroxide. In 

 fact, the slight ionization of ammonium hydroxide accounts for the 

 reverse action, namely, the liberation of ammonia from its salts by 

 the action of calcium hydroxide. 



In the presence of ammonium salts, ammonium hydroxide ionizes 

 only to a very slight extent, so that it loses almost all the character 

 of a hydroxide as far as precipitating other metallic hydroxides is 

 concerned, only the extremely insoluble ones being precipitated. 

 This accounts for the fact that magnesium hydroxide is not precipi- 

 tated by ammonia water when ammonium salts are present. The 

 magnesium hydroxide, although being nearly insoluble, is sufficiently 

 soluble and ionizable not to be precipitated by ammonia water under 

 these conditions. Alkalies, on the other hand, precipitate magnesium 

 hydroxide copiously, because they are almost completely ionized in 



