276 METALS AND THEIR COMBINATIONS. 



dilute solutions, and thus act as strong bases, as we say. Ammonium 

 carbonate behaves very much like ammonia water toward magnesium 

 and some other metals. 



Hydroxides of nearly all metals, when heated sufficiently, lose 

 water and give the oxide. Many oxides are prepared in this way. 

 Only a few oxides unite with water to form a hydroxide. One of the 

 best examples of this is the process of slaking lime. Oxides may 

 also be obtained by heating carbonates or nitrates, or directly from 

 the metals. The method followed in any particular case is deter- 

 mined by the properties of the metal, the question of economy, etc. 



Tests for magnesium. 

 (Use the reagent solution of magnesium sulphate. ) 



1. The addition of an alkali carbonate solution causes a white pre- 

 cipitate of basic magnesium carbonate (see Experiment 25). 



2. Add to the solution some caustic alkali : a white precipitate of 

 magnesium hydroxide, Mg(OH) 2 , is formed, insoluble in excess of alkali. 



Mg" + SO/' + 2Na' + 2(OH)' = Mg(OH), + 2Na' + SO/'. 



3. Add to the solution ammonia water or ammonium carbonate : 

 part of the magnesium is precipitated as hydroxide or carbonate. 

 The latter is increased on heating. If an equal volume of ammo- 

 nium chloride solution is previously added, no precipitate is obtained 

 (for explanation, see Remarks on Tests above). 



4. To the solution add an equal volume of solution of ammonium 

 chloride and some ammonia water. The mixture should be clear. 

 Then add sodium phosphate solution : a white, finely crystalline pre- 

 cipitate of the double salt, ammonium magnesium phosphate, is pro- 

 duced, which increases by shaking (see reactions under test 1 for 

 phosphoric acid). This is a delicate and decisive test for magnesium, 

 when other metals which resemble it are eliminated. This is easily 

 done by adding to a solution some chloride, sulphide, and carbonate 

 of ammonium, which will remove by precipitation all metals except 

 magnesium and alkali metals. 



QUESTIONS. How is magnesium found in nature? By what process is 

 metallic magnesium obtained? Give the physical and chemical properties of 

 magnesium. State two methods by which magnesium oxide can be obtained. 

 What is calcined magnesia? State the composition and properties of the 

 official magnesium carbonate, and how it is made. What is Epsom salt, and 

 how is it obtained? Which compounds of magnesium are insoluble? Give 

 tests for magnesium compounds. How can the presence of magnesium be 

 demonstrated in a mixture of magnesium sulphate and sodium sulphate ? 



