Solution: (1)ca(oh)2 solution is alkaline, and its ph value is greater than 7; Ca(oh)2 solution can only react with mgcl2 in crude salt water. The chemical reaction equation is: ca(oh)2+mgcl2═cacl2+mg(oh)2↓, and the purpose of adding excess is to completely remove magnesium chloride; So the answer is: Ca (OH) 2+MgCl2 ═ CaCl2+Mg (OH) 2 ↓;

(2) After ① ②, precipitation occurs, which needs to be removed by filtration experiment, so the name of operation A in step ③ is filtration; Excess calcium hydroxide and generated calcium chloride in step 1 react with excess sodium carbonate in step 2 to precipitate calcium carbonate, sodium chloride and sodium hydroxide, so adding appropriate amount of hydrochloric acid in step 4 can remove excess na2co3 and generated naoh from the filtrate in step 2; So, the answer is: filtering; naoh。

So the answer is: (1) MgCl2+Ca (OH) 2 = mg (OH) 2 ↓+CaCl2; Completely removing magnesium chloride;

(2) filtering; Funnel; Drainage to stabilize the flow; naoh。