Hou, a scientist from China, has made outstanding contributions to the development of soda ash and nitrogen fertilizer industry technology in China, followed by Hou Shi's combined soda production method and the western Soviet Union system. (1) Caustic soda is sodium hydroxide, which reacts with magnesium chloride to generate magnesium hydroxide precipitate and sodium chloride; Filtration is the operation of separating solids insoluble in liquid from liquid, so insoluble impurities in liquid mixture are removed by filtration; Hydrochloric acid can react with excess sodium hydroxide to produce sodium chloride and water, and removing excess sodium hydroxide with hydrochloric acid will not produce new impurities;

(2) Carbon dioxide can be dissolved in water, but its solubility is poor, and the solubility of gas increases with the increase of pressure. In order to promote the dissolution of more carbon dioxide and make the reaction more complete, pressure is needed;

(3) Ammonium salt reacts with alkaline substances to generate ammonia gas, and alkaline substances must be added to ammonium chloride solution to generate circulating ammonia gas; Because the reaction produces calcium chloride, the added substance is hydrated lime; Both sodium chloride solution and ammonium chloride solution contain chloride ions, so adding an appropriate amount of sodium chloride crystals into ammonium chloride solution can reduce the solubility of ammonium chloride and precipitate ammonium chloride crystals.

(4) Ammonium chloride solid does not evaporate and crystallize directly because ammonium chloride is easy to decompose when heated, as can be seen from the information given in the title;

So the answer is:

( 1)MgCl 2 +2naoh═mg(oh)2↓+2 NaCl； Filtering;

Hydrochloric acid; ? naoh+hcl═nacl+h 2o；

(2) increasing the solubility of carbon dioxide to make the reaction more complete;

(3) calcium hydroxide; Solubility;

(4) Ammonium chloride is easy to decompose when heated.