PRECIPITATION ON HEATING. 123 



colourless solutions ; in the second, two pale yellow fluids give a 

 deep Hue precipitate. The colour of a precipitate cannot be in- 

 ferred from the colour of the solutions employed to prepare it ; 

 thus the two colourless fluids obtained by dissolving in water 

 iodide of potassium and perchloride of mercury respectively give a 

 scarlet precipitate on pouring the second into the first ; if now you 

 add more of the first fluid and stir up, the scarlet precipitate again 

 dissolves, forming a colourless solution ; on further adding more of 

 the second fluid the scarlet precipitate reappears; and so on, 

 alternately, as long as you please. If to the solution of perchloride 

 of mercury you add a solution of caustic soda, a bright yellow 

 precipitate is formed. If you mix the perchloride of mercury 

 solution with iodide of potassium solution in such quantity as to 

 dissolve the scarlet precipitate first formed, and then add some 

 caustic soda solution, you obtain a nearly colourless fluid, which 

 forms an orange-brown precipitate on adding to it a very small 

 quantity of weak solution of ammonia. 



A colourless solution of sulphuretted hydrogen, when added to a 

 colourless solution of tartar emetic, gives a beautiful orange pre- 

 cipitate ; if to a colourless solution of sulphate of cadmium, a 

 bright yellow precipitate ; * and if to a colourless solution of acetate 

 of lead, a black precipitate ; whilst it may also be made to form a 

 white precipitate by adding it to the colourless fluid obtained by 

 adding a little solution of ammonia to solution of sulphate of zinc, 

 so as to produce a white precipitate, and then adding more 

 ammonia to this until the liquid becomes clear again. 



Expt. 122. Production of a Precipitate by simply Heating. 

 As already mentioned, in general solids dissolve more freely in hot 

 water than in cold, so that it is a very unusual thing to find that 

 a solution forms a precipitate on heating, without the occurrence 

 of any chemical change, simply owing to the fact that more sub- 

 stance is present than hot water can dissolve, although the same 

 quantity of cold water could keep it permanently dissolved. Such 

 exceptional cases, however, sometimes occur ; for instance, a clear 

 saturated cold solution of the substance termed butyrate of calcium 

 forms a precipitate, and becomes milky on heating, because this 

 salt is more soluble in cold water than in hot, so that on heating 

 a part is necessarily thrown out of solution, just as in ordi- 

 nary cases a solution saturated hot deposits some on cooling 

 (Chapter V.). 



Cases where precipitates form on heating, owing to the occur- 

 rence of chemical changes, are much more common. For example, 



* This yellow substance is the " cadmium yellow " used by artists as one 

 of the best and most permanent yellow coloring matters known. 



