Final answer:
The temperature readings will be too low because heat can escape to the air if a calorimeter is used without a lid. This results in an underestimate of the temperature change and therefore the energy change in the process being studied.
Step-by-step explanation:
If a calorimeter is used without a lid, the most likely error would be:
C). Temperature readings will be too low because heat will be absorbed by the air.
The reason for this is that in the absence of a lid, the calorimeter allows heat to escape to the surroundings which in most cases is the air. Given that a calorimeter is supposed to be an isolated system for accurate measurements, any heat loss to the environment will lead to an incorrect measurement of the temperature change, as more heat is required to cause the observed temperature change in the experiment. This can lead to underestimating the energy changes in a chemical reaction or process being studied.
When scientists use calorimeters, they often prefer well-insulated devices that limit the exchange of heat with the environment. The use of a coffee cup calorimeter which is less insulated, as mentioned in the provided references, already allows more heat exchange compared to more precise calorimeters. Therefore, removing the lid would exacerbate this issue, making the calorimeter even less isolated and reducing the accuracy of the heat measurement even further.