（1）用化学符号表示：
①缺________会导致甲状腺疾病，②氮分子_________，③氟原子________，④硫酸根离子__________，
⑤过氧化氢中氧元素的化合价__________，⑥相对分子质量最小的氧化物_____________。
（2）将下列物质分类：
石灰石、二氧化硫、汞、碳酸钠、白磷、氧化镁、空气（除混合物之外均写化学式）
①属于混合物的是_______________________，
②属于单质的是_________________，
③属于化合物的是_______________________，
④属于氧化物的是_______________。
（3）化学世界千变万化，请写出下列反应的文字表达式。
①加热氯酸钾和二氧化锰的固体混合物_____________________，
②二氧化碳通入澄清石灰水______________________，
③铁丝在氧气中燃烧____________________________。
（Kate）grandparents usually have a walk around the lake in the morning．
Time spent in a bookshop can be enjoyable．If you go to a good shop，no assistant will come near to you and say．"Can I help you？"You needn't buy anything you don't want．You may just try to find out where the book you want is and read it．But if you fail，the assistant will lead you there and then he will go away．① It seems that he is not interested in selling any books at all．
There is a story which tells us about a good shop．A medical student found a very useful book in the shop．② But it was too expensive for him to buy．He couldn't get it from the school library，either．So every afternoon，he went there to read a little at a time．One day，However，he couldn't find the book from its usual place and was leaving when he saw an assistant singing to him．To his③ ，the assistant pointed to the book in the corner．"I put it there so that it couldn't be sold out．"said he assistant．Then he left and the student went on with his reading．
81．将文中①处画线句子翻译成汉语．
82．将文中②处画线句子改写为同义句．
83．根据短文内容，在文中③处填入恰当的词． ．
84．Where was the book that the student was looking for？
85．What do you think of the assistant？ ．
We shouldn't waste water．Instead，we should （节约） it．
The students are having a （化学） class in the lab．
After working for several hours，he was t and fell asleep soon．