教师遇到有演示实验的化学课,往往对教学时间把握不准,而难以完成课堂教学任务。造成这种情况的主要原因是:有些演示实验的操作时间较长。例如,讲解初中《化学》第一章第二节《氧气的化学性质》时,实验要求课前制好氧气,但由于气体的逸散,演示氧气的化学性质实验时,现象不明显,若在课堂上现制现用,现象虽明显,但收集多瓶氧气耗时长,难以完成一课时 When teachers encounter a chemistry class with demonstrations, they often cannot grasp teaching time, and it is difficult to complete classroom teaching tasks. The main reason for this situation is that some demo experiments take longer to operate. For example, when explaining the chemical properties of oxygen in the first chapter of Chemistry in Chapter 1 of junior middle school, the experiment requires that oxygen is prepared before class. However, due to the gas emission, when demonstrating the chemical properties of oxygen, the phenomenon is not obvious. In the classroom, it is currently used and the phenomenon is obvious. However, it takes a long time to collect more bottles of oxygen and it is difficult to complete a lesson.