一次教研活动中,某教师用注射器演示了“水在低压下沸腾”的实验(现行初中课本第二册79页)。 他是这样做的:先将100毫升注射器内吸入一定量的冷水,然后将针咀朝上,并用橡皮塞堵紧针咀,再用力向下拉活塞(如图1所示)。这时可见:针筒内的水“沸腾”了。 不难证明,这样的演示是错误的,我们只要将针咀朝下,用同样的方法来做这个实验(如图2 During a teaching and research activity, a teacher used a syringe to demonstrate the experiment of “boiling water under low pressure” (current junior high school textbook, volume 2, page 79). He does this by first inhaling a certain amount of cold water into a 100 ml syringe, then turning the needle up and plugging the needle with a rubber stopper, and then pulling down the piston (as shown in Figure 1). It can be seen at this time that the water in the syringe “boils”. It is not difficult to prove that such a demonstration is wrong. We simply use the same method to do the experiment with the needle pointing downwards (Figure 2).