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研究性学习小组的同学为探究空气中氧气的体积分数，设计了如图所示装置．其中U型管左侧容器壁上标有刻度，电阻丝通电后能发热，右侧玻璃口．请根据图示实验回答下列问题： （1）闭合电源开关，可观察到白磷______；白磷所发生反应的化学方程式为______； （2）在上述实验中，下列物质不能代替白磷的是______（填编号） ①木炭 ② ③镁带 ④铜丝 （3）实验结束后，理论上可以观察到U型管内左侧液面______；若实际观察到左侧液面在不到“1”的位置处，则实验结果比理论值______（填“偏大”或“偏小”），可能的原因是______（2点）． （4）此实验可推知反应后剩余气体的两点性质是______、______． （5）通过这个实验得出的结论______．
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