初中化学实验中经常涉及实验操作的先与后以及反应发生的先与后,这些问题可能涉及物质之间相互反应的规律、化学实验的成败、师生的安全等。因此,记住这些“先”“后”,对做好实验,学好化学有比较重要的意义。一、使用托盘天平　使用托盘天平时,首先要调节平衡。调节平衡时,先? The chemistry experiment in junior high school often involves the first and the later of the experiment operation and the first and the later of the reaction. These problems may involve the law of interaction between substances, the success or failure of chemical experiments, and the safety of teachers and students. Therefore, remember that these “first” and “later” have more important significance for doing experiments and learning chemistry. First, use the tray balance When using the tray balance, first adjust the balance. When adjusting balance, first?