氢氧化钠固体不仅易吸收空气中的______而潮解，而且也能吸收空气中的______，因此必须______保存以免变质．为了检验长期暴露在空气中的氢氧化钠溶液是否变质，可选择三种不同物质类别（酸、碱、盐）的试剂进行实验，写出所选试剂的化学式．______、______、______．
固体氢氧化钠必须密封保存，这是因为氢氧化钠具有吸水性，能吸收空气中的水分发生潮解；还能与空气中的二氧化碳反应生成碳酸钠和水，反应的化学方程式为：CO2+2NaOH═Na2CO3+H2O．为检验氢氧化钠溶液中是否含有碳酸钠，可向溶液中加入适量稀盐酸或稀硫酸，有气泡冒出即可检验含有碳酸钠；可向溶液中滴加某些碱溶液如氢氧化钙或氢氧化钡溶液，有沉淀产生即可检验含有碳酸钠；可向溶液中加入某些盐溶液，如氯化钙或硝酸钙，有沉淀产生即可检验含有碳酸钠；
故答案为：水分；二氧化碳；密封；HCl（或H2SO4等）；Ca（OH）2[或Ba（OH）2]；Ca（NO3）2（或CaCl2等）．
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| It was a Monday morning. As the teacher walked into the classroom, he heard a low voice, "Here's the teacher, I am 1 this boring fellow is going to talk about how to use commas (逗号)." It was 2 voice and the teacher knew who 3 . However, the teacher didn't get 4 . He said 5 about it but really began to talk about how to use commas. And then he wrote the following sentence on the blackboard, "Nick 6 the teacher is a boring fellow." The class 7 and Nick's 8 turned red. "Now," said the teacher, "I'll tell you the 9 of commas." He put two commas in the sentence, and then read, " 10 is a boring fellow." | ||||
C. The teacher, says, Nick D. Nick, says the teacher, |