(2) According to life experience, the cooling time of hot water varies with the water quantity of hot water bottle. We guess that "the less water content, the lower the thermal insulation performance";
So the answer is: low.
(3) To verify whether the water temperature is related to the water quality, we should adopt the control variable method, keep other factors unchanged, change the water quality and observe the change of the water temperature. When making the research plan, Zhang Bin thought: To do this experiment well, we must use the same thermos, because different thermos have different thermal insulation properties; Deng Jun and I think that the experiment with the same thermos can't be completed at the same time, although it is in line with Zhang Bin's experimental thinking, because different ambient temperatures will also affect the experimental results.
Therefore, the answer is: different thermos bottles have different thermal insulation properties; Ambient temperature.
(4) As can be seen from the image, the heat preservation records of several thermos bottles are recorded in the experiment, and it can be found that the temperature difference is the smallest when the water content of thermos bottles is 1.9L; That is, when the water content of the thermos bottle is in the range of 2L to 1.9L, the thermal insulation performance increases with the decrease of water content.
In addition, when the water volume is less than 1.9L, the temperature difference becomes larger and larger. That is, when the water amount is less than 1.9L, the thermal insulation performance will be weakened with the decrease of water amount;
Therefore, the following conclusions are drawn: ① When the water volume is in the range of 2L to 1.9L, the thermal insulation performance increases with the decrease of water volume; ② When the water quantity is less than 1.9L, the thermal insulation performance will be weakened with the decrease of water quantity.