240. Preparation. -- Sodium hydrate, caustic soda, or soda by lime, is made by treating a solution of Na2CO3 with milk of lime. CaCO3 is precipitated and al- lowed to settle, the solution is poured off, and NaOH is obtained by evaporating the water and running the residue into molds. 241. Use.--NaOH is a powerful caustic, but its chief use is in making hard soap. 242. Hydrogen Sodium Carbonate.--Hydrogen so- dium carbonate, bicarbonate of sodium, acid sodium carbonate, cooking-soda, etc., HNaCO3, is prepared by passing CO2 into a solution of Na2CO3. Na2CO3 + H2O + CO2 = 2 HNaCO3. Test a solution of it with litmus. Account for the result. Its use in bread-making depends on the ease with which CO2 is liberated. Even a weak acid, as the lactic acid of sour milk, sets this free, and thus causes the dough to rise. 243. Sodium Nitrate.--Sodium nitrate occurs in Chili and Peru. It is the main source of HNO3. Review Experiments 46 and 52. From NaNO3 is also made KNO3, (NaNO3 + KCl = NaCl + KNO3), one of the ingredients of gunpowder. By reason of its deliqcescence NaNO3 is not suitable for making gunpowder, though it is sometimes used for blasting-powder. The action of the latter is slower than that made from KNO3. NaNO3 is cheaper and more abundant than KNO3; this is true of most Na compounds in comparison with those of K. Chapter XLV. POTASSIUM AND AMMONIUM. POTASSIUM AND ITS COMPOUNDS. Examine K, KCl, K2SO4, K2CO3, KOH, HKCO3, KCLO3, KCN. 244. Occurrence and Preparation.--Potassium occurs only in combination, chiefly as silicates, in such minerals as feldspar and mica. By their disintegration it forms a part of soils from which such portions as are soluble are taken up by plants. The ashes of land-plants are leached in pots to dissolve K2CO3; hence it is called potash. Sea-plants likewise give rise to Na2CO3. Wood ashes originally formed the main source of K2CO3. From plants this substance is taken into the animal system, and makes a portion of its tissue. Sheep excrete it in sweat, which is then absorbed by their wool. Large quantities are now obtained by washing wool and evaporating the water. K2CO3 and other compounds of K are mainly derived from KCl, beds of which exist in Germany. In the following list each K compound is prepared like the same Na compound, and the uses of each of the former are similar to those of the latter. K compounds are made in much smaller quantities than those of Na, as KCl is far less common than NaCl. { K KCl { K2SO4 { K2CO3 { KOH KNO3 { { HKCO3 Examine specimens of each, side by side with like Na compounds. Describe in full their preparation, giving the reactions. Also, perform theexperiments given under Na, substituting K therefor. From KOH are made KClO3 and KCN. KOH {KCl03 {KCN 245. Potassium Chlorate.--KCl03 is made by passing Cl into a hot concentrated solution of KOH.