Final answer:
In a test to determine which thermos keeps water the coolest, the control is the consistent environmental conditions like ambient temperature, and the variables are the different designs and materials of the thermoses. The experiment involves using standardized conditions and measuring the change in water temperature with thermometers inside each thermos in a pre-heated incubator. The efficiency of a Dewar flask, a specialized thermos, is due to its design that reduces all forms of heat transfer.
Step-by-step explanation:
When testing which thermos keeps water the coolest, the key aspect to pay attention to is maintaining a controlled experiment where you keep all variables the same except for the one you are testing. In this case, the control is the environmental condition - such as the ambient temperature, which should remain constant for all the thermoses being tested. The variables include the different thermoses with distinct designs, materials, or insulation properties. To accurately measure how well each thermos maintains the temperature of the water, you could use a standardized amount of water at the same initial temperature for each thermos and measure the temperature change over a fixed period.
To elaborate further using the information given, the experiment might involve setting each thermos in an environment with a uniform temperature, such as the pre-heated incubator at 40°C, and observing the rate at which the water temperature within each thermos approaches that of the surrounding environment. The thermometers inserted through the stoppers of each thermos would allow the monitoring of the internal water temperature, ensuring that the only significant variable affecting the temperature of the water is the thermos itself.
In reference to calorimetry and the properties of the thermos described in Figure 14.30, the Dewar flask is designed to minimize heat transfer. The vacuum between the double-walled container minimizes heat conduction and convection, silvering of the walls reflects radiant heat, and the thin-walled long glass neck reduces the area through which heat can be transferred. The rubber support and air layer provide additional insulation, and the stopper seals the container to further minimize heat transfer.