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【简答题】
某同学用右图装置粗略地测定空气中氧气的体积分数，图中烧杯中的生石灰（即氧化钙）与水反应生成氢氧化钙，同时放出大量热．烧杯上方玻璃管（预先固定好）中部有一可左右滑动的活塞，活塞左端管内封有空气，活塞右端的玻璃管口跟外界空气连通．白磷很易燃且燃烧产物与红磷燃烧的产物相同，实验开始前活塞处在刻度5 cm处． （1）向烧杯内加入适量水，与生石灰反应的目的是______，写出反应的文字表达式______． （2）可观察到玻璃管内开始发生的现象：白磷（足量）______；活塞向______（填“左”或“右”）移动． （3）反应结束，恢复到室温后，活塞应停在约______cm处，据此可得出的结论是______．
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