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【简答题】
观察比较与归纳是学习化学的重要方法．对于以下三个实验： （1）写出上述三个反应化学方程式 ①______； ②______； ③______． （2）比较上述三个反应，发现它们有许多共同点：请你写出其中两点： ①______；②______． （3）以上化学方程式也有许多不同之处，请你写出其中一个与另两个的一点不同之处______． （4）某同学取一段纱窗网上的细铁丝，在自己收集到的氧气中做“铁丝在氧气中燃烧”的实验，结果没有观察到“火星四射”的现象．请分析此实验失败的两种可能原因：______、______．后经改进实验成功了，在进行该实验时必须在瓶底铺一层细沙或装少量水，目的是______．______．
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